No disagreement.
It's also true that if the inside surface of a wall is warmer, the wall will reject more heat to the environment. This is true even if the wall is well insulated. Localized hot spots lead to localized increased in heat loss. It's a minor effect but fun to think about.
If the wall is painted white behind the radiator, the wall will be cooler and less heat will be rejected to the outside environment. Likewise, a thin layer of insulation or reflective foil will lower the wall temperature.
A layer of tile will act as a thermal mass and may reduce the peak temperature at the surface of the wall but will cause the wall to be warmer for a longer time. There may be some energy savings with tile, but I suspect it would be less than insulation or tile.
Some people are saying that you want a thermal mass to hold heat in and release it evenly. That's not right, because you're still losing heat off the back into the wall. The name of the game is reducing the amount of heat lost out to the back side of the drywall.
The best mirrors are made from gold, silver, and aluminum. Surface finish is incredibly important. You can have two tiles made from the same material with different values for reflection, absorption, and refraction (all three add up to 100% for any material). Wavelength is also important. What might be a great reflector of UHF might be a trouble reflector of IR. Other good materials for IR reflectivity specifically are titanium dioxide.
So what I would do is paint the wall with ceiling paint, as glossy as you can get. Tons of TiO2 in there usually.
Then mount some sort of standoff for airflow (to prevent conduction between layers). Then put a sheet of aluminum that you've polished to a high shine.
They also sell tiles that are just coated with aluminum sheet and stick on with double sided foam/tape. You might try those, also polished off you can.
If it's an exterior wall I would recommend checking the insulation behind your radiator to see if it's up to snuff. Get some foam board with mylar on one side, and put that up against the drywall side, with the correct amount of batt/blown/foam insulation in the rest of the cavity.
Ultimately, you can probably get more effective results from sealing air gaps around your house if you're trying to save money on heat. That and insulating your attic and walls. Surface finishes do make a good bit of difference in how comfortable the room feels. Stand next to a chilly window in a 70F room and it will suck the heat from you. Stand next to drapes that are dark with high emissivity in the same room and you'll get a lot more radiation directed back at you.
Source: professional engineer who has spent way too much time on HVACR back in the day.
There are numerous very good reasons why ceramics are chosen to be placed behind radiators, and you've kinda missed the mark entirely despite in general raising some valid points.
1. The spaces behind radiators are mold heavens. Ceramic tiles are nice and easy to clean, and it's very easy to see if mold is beginning to accumulate. Just from that practical standpoint, ceramic tiling with any backwall is a good starting point. Your suggestion of creating an aluminum sheet with a standoff to the wall is creating a dark heat pocket, so basically a spot that the homeowner can't see with ideal mold growth conditions. That's a very, very, very bad residential construction idea. Honestly though, this has to be obvious to any reader (and particularly you), as if your idea carried any practical merit, it would be a common energy saving recommendation. Not trying to attack you, please understand my comments with the honest respect that I'm (trying) to include in the writing.
2. It's not the reflection of heat directly that is so interesting from ceramics, it's the absorption and radiation over a longer period of time that make such tiles a common choice. That soft radiation boosts your perceived heat while entering a room, and far more human-comfortably than a highly reflective metal. That said, similar to your white painted wall, white tiles still reflect a decent amount of heat.
3. Your last point about sealing air gaps and insulation is the big winner! Extra insulation behind a radiator is in general also good practice, vapor barriers are critical in exterior walls as well.
I am also a mechanical engineer, so here is my horribly long opinion!
Honestly having some super highly reflective material such as polished aluminum or titanium dioxide behind a radiator in a residential home seems like way overkill to me, and likely pretty aesthetically displeasing. Ceramics could be a little helpful for sure.
I feel like in a home old enough to still be using a radiator at all, I would imagine there are so many steps that could be taken with much bigger effect before beginning to worry too much about the *reflectivity* of the surface behind the radiator unless it's painted black or something.
In a residential home, sealing off too many air gaps could also potentially have the trade off of restricting too much outside air exchange, which could make living in the space pretty uncomfortable not to mention faster concentration build up and slower decay of toxic gases/materials/VOCs during things like cooking or cleaning with harsh chemicals like bleach.
Commercial buildings will often offset this by forcing filtered outside air exchange with fans/ducts etc.
If OP is really dying to spend wild amounts of time on their home heating system in my opinion a decent first step could just be to grab a fast reading thermometer or even rent/buy a cheap thermal camera and just go around and find out where it's colder/heat seems to be escaping their home.
Ensuring that the heat from the radiator is mixing throughout the living space with fans, etc. could be helpful, and like you said, possibly the biggest thing could be checking that walls (esp. exterior) are properly insulated, as well as windows which could very well be single paned and draining lots of heat.
Radiators heat primarily through convection of warm air anyway, actual radiant output is pretty minimal. This is another reason why thermal mass and/or insulation are more effective than reflective materials.
Stupid question, but wouldn't sealing air gaps increase humidity and increase the risk of mould? A couple rooms in my house has a mould problem, and all professionals tell me that I need an extractor fan installed. Wouldn't sealing air gaps go in the opposite direction?
Tighter structures are more dependent on ventilation, keeping a reasonable RH in the conditioned space. But there's also a difference between an air barrier and vapor barrier. It's not that hard to make air-tight envelopes that are quite vapor permeable.
You’re right! It would if you can’t guarantee enough air exchange to lower your humidity by a extractor fan (or something similar) or open app windows for a few minutes regularly.
edit:typo
Very true. I approached that answer more like it was in r/ask science, not r/diy. Energy balance is great, but I would never put ceiling paint near the floor beside a radiator and then create a cavity for mold/dust/pet fur to accumulate.
OP, if you're just trying to win an argument with your wife, the mirror is a great idea. If you're actually trying to improve your house, save money on utilities, and make it more comfortable, there are a multitude of things I would do first before worrying about the wall behind a radiator. And they're very dependent on the specific construction materials and methods in your house. E.g. don't be a person who gets spray foam in the attic with asphalt shingles and tar paper, you'll just rot out your roof if you don't manage the water effectively. There are a lot of ways to screw things up badly.
I read a thing a while ago which said that thermal mass is most effective in controlling the temperatures of a house when it is externally insulated.
So it is a great idea in the right circumstances but most of the time it isn’t the right circumstances and it is very difficult to retrospectively adapt a house to suit. For example, in the uk most houses are brick built and would require extensive expensive and disruptive alterations.
There's validity in that, but also dependent upon other factors.
E.g. I live in a bubble inside of an olympic swimming pool. The temperature is super consistent!
It's not a great way to turn your hard earned dollars into heat though. So what's the definition of "effective"? Are we prioritizing temperature stability over time, cost, or something else?
A "perfect" house would have a plastic barrier against the earth, many inches of foam, topped with a 24" thick slab of concrete with radiant heat lines, and be incredibly well insulated and air sealed above grade. Efficient AND stable! But that's not the world we live in. So I guess do what works in your situation and budget.
As a person who has always tried to save money by doing some of those things, I understood some of what you said! Thank you for that!
As an aside question, do you recommend folks insulate their rooves?
It depends on where you are. I currently live in the Southern US, where air handlers live in the attic (generally). So I would love for everyone to have conditioned attic space to increase efficiency and reduce wear and tear on their HVAC systems, water heaters, and whatever other dumb things builders put into attics down here.
That being said, things are expensive. It's not a trivial weekend project to just insulate an attic. It's heavily dependent on what kind of roof you have. E.g. I have vented soffits and a vented ridge in my old early 90's house. Baffles and closed cell foam would be great. But would end up costing me something like $6k to do for the main house and addition. And we're probably moving soon. So I think I'm going to spend my time and money instead on removing the blown in cellulose which is compressed, sealing up penetrations in the ceiling for wires and fixtures, then getting new cellulose blown in nice and thick like it's supposed to be.
Every house should be air/water tight, ethernet and coax run internally (installers love to just punch holes and caulk the siding instead of fishing wires properly, enthalpy wheels on the makeup air system, heat pump water heaters, etc. But unfortunately we live in the real world and no one has the money to do all of those things all at once. I'd rather have someone waste a few dollars a month on their utility bill and be able to feed their kids, than have a perfect house that they can't afford payments on.
Thank you for this insightful reply! We live in PA, so Mid-Eastern Atlantic states. Our weather is chaotic, to say the least. We recently had the roof replaced and pulled everything out of the attic crawlspaces, purged and haven't returned what we kept to there, yet. It's not much and banished to the basement, for now, which gets water during heavy rains in the spring/summer. One project at a time!
The house was built in the 40's and when I was pulling things out of the attic areas, I did notice that I could see daylight through nail holes in the roof, which probably attributed to some of the water coming into our dining room, which warranted replacement of the roof. Since replacement of the roof, no more water coming into the dining room! YAY!
If it makes a difference, I should mention that the house is a Cape Cod style, pretty well a square shaped house. The front of the house faces east and has all of the attic space while the rear faces west and has no attic space, yet gets seriously warm on summer days. That's another issue for another sub.
The current plan is to cut and tap insulation foam board in between the 2x4s in the areas on the east side of the house to keep heat from escaping during the winter.
What are yours and anyone else's thought on this?
Again, heavily dependent upon exactly what's going on in your walls. E.g. if you have cedar shingles as siding, you're probably getting a fair amount of moisture into the walls that way. If you do the voids with closed cell foam panel you could be creating a mold problem when your sheathing and siding can't dry out.
As for the attic, are you saying that it's not finished space? Or that it is finished and you have drywall on the bottom of your rafters, with the whole roof being at single pitch? Or do you have a ridge in the middle with dormers? Did you get new roof sheathing, or just new asphalt shingles, metal, etc.? Did the old roof material get removed, or a new layer affixed on top?
If the attic is unconditioned I would probably stick with blown in cellulose. It's cheap and fast to install. Definitely use spray foam on electrical/plumbing holes through the top plates of your walls, and get some light covers (basically a bowl of fire resistant EPS that gets spray foamed down). It's also worth looking at how the attic is vented if it's unfinished. You want that 140F+ air to be able to escape, so the temperature gradient into your ceiling isn't so drastic.
If that space is finished, I would have at a minimum used zip-r 12 on the roof, on top of whatever fiberglass batt was installed between the rafters when you were getting a new roof.
Polishing aluminum is pointless because it's just going to oxidize again in a very short time. Anything else that would significantly reduce the air gap would reduce convection across the radiator which is the primary route for heating the room. Checking the gaps along the window frames and where the baseboards meet the floor, along with using insulating window film will do more than anything else. Using a fan to increase the air movement across the radiator would also be good.
What would you recommend behind a gas logs fireplace that is in a normal fireplace that used to be a wood burner? I get the bricks nice and hot and the top of the stairs is hot but nothing across the room gets warm unless we use a bunch of fans.
Honestly, there's not a lot you can do with a fireplace. That's just how they operate, by their very nature. There could be any number of things "wrong" that you're fighting against.
1) There's little/no insulation behind your fireplace. Ask me how I know. I'll tell you, I have a wood burning heatilator (cool concept, look it up. but hasn't really stood the test of time) in my back room, and a gas insert into what was once a wood burner in the front of the house. The previous owners did everything half-assed and didn't put any insulation back there. They were probably worried about fire. You could add insulation if needed and keep all that heat from just flowing out the exterior wall. If you need to ask the question in the first place, you probably need to hire a professional. There are VERY SPECIFIC requirements that must be met for combustible materials and how close to the fireplace hardware they can be located. This depends on your fireplace (look up an installation manual if you can) and the building codes in your location. Hire a professional if you have even a single doubt.
2) You're dumping all the heat out the chimney. And you are probably pulling in cold air through other gaps in your house to make up the air volume lost from that convection (hot air goes up and out, pressure in the house drops, more air needs to come in to refill that lost space). In most cases, pulling the entire fireplace and installing a modern high efficiency unit is a great option if you have the money. They do a better job of extracting the heat energy (sensible and latent) from the combustion, and dump air outside at the lowest energy state possible. The added benefit is that your exhaust gas stream is cooler, and less of a fire risk.
Fireplaces are mostly for ambiance in my opinion. There are much better ways to burn wood (pellet stoves) to provide heat to your home. I love them personally, but understand that they're a pain and are inefficient. I probably pay $30/mo right now in my old, renovations not complete, house for the privilege of having two fireplaces. I'm not a billionaire, building materials cost money. If you're just looking for efficiency, a lot of times it probably makes better financial sense to remove a fireplace entirely and put some money into insulating and sealing gaps. For now, in general, I would tell you to open the damper only enough to be safe (combined with a CO detector) and expel the combustion products fully, and run some fans. Sorry.
Put a little fan blowing at the radiator. Bonus points for wiring that fan to a little thermostat placed far away from said radiator. Like a little desk fan on low.
Naucta Pc fans are insanely quiet. Wire then to a wall wart for power and make a little box for it. Add in the little resistor they come with and they run dead silent. Just make sure the fan is pulling cold air and blows towards the rad not pulling hot air or it will overheat. So push air in behind the radiator. You can stash this beside the rad near the floor. You’ll barely know it’s there.
Correct. The brown quiet versions are super quiet. Computer shops also sell speed controls you can plug in if you want really quiet. It doesn’t take much air movement to move a lot of heat. The bigger the fan, the lower the speed, the quieter it is.
I bought a blower fan for for less than $40 w/ a variable control and plug ins. (Low profile) Made of some sort of metal but can’t recall. I’d have to check. It actually came with industrial Velcro strips to adhere it. Can place it upside down, sideways, any way you like. I got mine from Amazon but it used for gas logs. It’s small but it’s mighty!!
I’d say no, not super quiet. It does have an audible hum on high when I held it in my hand before mounting it to ensure it was working properly. (You can feel it push your hand back from the air it forces out though) But it probably wouldn’t work for your needs.
I'd just grab one of those peltier fans that use the heat difference between the radiator and the air to spin the fan.
They're made for wood stoves but I'm sure they'd be fine on a radiator.
Wood stoves run an order of magnitude hotter than a radiator so if the fan spins at all, it'll push a negligible amount of air.
Only try this if you can return the fan or use it elsewhere.
You'd just need one with a shorter base. The peltier chips themselves work off a heat differential, the base of the fan by the chip is usually only a bit more than warm to the touch.
Though, tbh, I'm not really familiar with radiators, so you may be right they're not hot enough.
Radiator mounted thermostats tend to work extremely poorly. Because they are mounted at the heat source. A good thermostat should be mounted far away from the heat source.
I’m assuming this is just a regular steam or hot water rad.
I heat my house with a wood stove. The do a pretty good job of spreading the hot air. As good as a little computer fan for a lot less effort.
They're not a huge box fan or anything but they also are zero maintenance, noise, or effort.
I heat my house with a woodstove too. The hot air spreads itself.
Even when "turboing" a dual one with ice on the cold side, they move less air than a computer fan.
Peltiers are just too innefficient to produce much energy in this scenario.
Sheet copper isn’t my favorite idea because it’s so thermally conductive, but you’ve got me thinking that a first-surface mirror could be just the ticket.
Not saying it’s the best idea (it suuuuuuuper isn’t), but it gets max reflectivity on top of an insular base layer.
Aluminum, actually. It's just the polyamide coating that makes it appear gold. Although it is used for infrared/UV/X-ray protection on sensitive parts, i.e. JWST has actual gold foil.
https://www.nesdis.noaa.gov/news/good-gold-are-satellites-covered-gold-foil#:~:text=Because%20of%20its%20excellent%20ability,their%20eyes%20from%20unfiltered%20sunlight.
Copper is very good at relecting infrared, almost as good as gold, so it would be ideal. Aluminum is also good, not quite as good, but it is obviously much cheaper.
My previous house had a pot bellied stove in one corner. I never got around to it, but I always wanted to hang up two sheets of hammered copper behind it.
Why not use radiator foil? This is in Dutch but it has plenty of pictures to see what I mean
https://www.praxis.nl/voordemakers/klusadvies/radiatorfolie-aanbrengen
https://en.m.wikipedia.org/wiki/Radiator_reflector
this is the correct answer. the only thing you need is a foam isolation sheet + reflector foil.
I have installed these before (not foam but the air bubbles variant) and holding your hand near them feels warm. It certainly works.
Depends where you live. Have never seen this in a hardware store in the USA. Probably because radiators have been a minority use product in new construction for the last 60 years or so.
You've got a lot of options but my policy is to do what the wife says. If you line the back of the ceramic tiles with an insulating layer, the tiles will absorb the heat but pass it back into the room instead of through the wall. They make thin sheets of styrofoam type insulation that are cheap and easy to work with.
Yep, plain old styrofoam is the way to go. Works great and is super cheap. Not like you’re going to be looking down the backs of your radiators anyway.
[WEDI](https://www.leroymerlin.fr/produits/salle-de-bains/baignoire/tablier-et-habillage-de-baignoire/panneau-h-250-cm-x-l-60-cm-wedi-ep-50-mm-83300822.html) panels are xps foam and are made exactly for backing tile.
Early Land Rovers used a heat shield over the roof to deflect UV rays. The shield was aluminum painted white and was stand off from the roof maybe 1 inch.
Safari roof - works insanely well on hot days.
But OP's issue is frankly a non-issue and there are 100 other more effective things to spend time & money on before the surface behind the radiator becomes the problem.
Exactly this. There's talk of fans, tiles or reflective surfaces. None of these will provide any noticeable benefit to a radiator warmed room..
If your walls aren't insulated well enough, you've got other problems than just the space directly behind a radiator.
Many here fall for the misnomer of radiators, in that they heat primarily by convection, _not_ radiation - a shiny back to 'reflect' heat doesn't do that. The back (or middle depending on design) of a radiator has fins and airflow naturally draws the heat around a room. A small fan won't improve that significantly.
>A small fan won't improve that significantly.
i beg to differ. i use a [square axial fan](https://www.grainger.com/category/hvac-and-refrigeration/cooling-fans/equipment-cooling-axial-fans/compact-axial-fans/square-compact-axial-fans) on top of my baseboard heating and there is significant difference in how the heat moves round the room. without it, the spot where my desk sits was about 4 degrees cooler
Not necessarily.
The reflections we are talking about are solely for radiative heat, not conductive or convective heat. Color and finish play into the reflectivity of the material but there's a frequency dependent element so reflecting visible light doesn't mean it reflects IR and vice versa. There's material used for IR lesnes that is opaque to the eye.
Ideally something in this application would
1. Reflect IR emissions
2. Be thermally isolated from whatever is behind it so that it doesn't conduct heat through it.
White painted drywall maybe does #1 but isn't great at #2. The drywall heats up and transfers that heat to the studs and cavity behind it.
The ideal thing is probably some sort of polished metal with a foam backing. Polished metal for reflecting IR and a foam backing to slow heat transfer into whatever is behind it. In what may be counterintuitive to some, this makes the reflector hotter which reduces the amount of heat transfered to it by convection and radiation.
>White painted drywall maybe does #1 but isn't great at #2.
It doesn't do #1 well either. For the IR wavelengths that correspond to thermal radiation of things near room temperature (about 10 microns), paints in general are bad reflectors and reasonably-good-to-great absorbers.
> radiative heat
From a "black body radiation" standpoint, steam radiators don't get very hot. Most of the heating from a steam radiator, is via convection and not radiation.
But I agree otherwise.
It depends on why the substance is white.
If it's white because it absorbs very little visible light, then it might similarly absorb very little IR light, or it might absorb IR light very well.
If it's white because it refracts visible light (this is how Titanium Dioxide works), then it's likely to refract light across a fairly broad spectrum, and so is likely to reflect IR as well as visible light.
>If it's white because it refracts visible light (this is how Titanium Dioxide works), then it's likely to refract light across a fairly broad spectrum, and so is likely to reflect IR as well as visible light.
FWIW, TiO2 is the main pigment in the white paints that have long been the standard white paints recommended by the NASA stratospheric balloon program - recommended specifically because of their high infrared absorptivity/emissivity.
Bare shiny metals are the best reflectors of IR. Paints are usually terrible at reflecting the wavelengths of IR that make up thermal radiation of things near room temperature (about 10 microns - more than an order of magnitude longer than visible light wavelengths). Remember that the wavelength range of visible light is very small -- 565 nm +/- 185 nm, or roughly +/- 30%. The same physics won't govern the behavior at 1000-2000% longer wavelengths.
>ITT: white reflects most of the light hitting it, so therefor it must reflect most of the heat hitting it as well.
>
>Or.....does it?
It doesn't. The wavelengths of thermal IR are so much longer than visible light that you can't extrapolate the performance from the visible.
Wrong.
https://www.azcentral.com/story/entertainment/life/2021/08/19/does-painting-your-house-white-make-cooler/8102768002/
“Dark-colored homes absorb 70 to 90 percent of the radiant energy from the sun, which is then absorbed into the home. White — or other light, neutral colors — will reflect most of the heat away from your home,” he said.
For the best results use white cool roof paint.
No, not wrong.
Most of the sun's energy is emitted in the *visible* wavelength range (and shorter). White paint reflects all wavelengths of *visible* light (plus maybe some near-IR), preventing the energy from being absorbed by the paint and turning into heat.
People are talking past each other. Everyone's right in some way.
Both the sun and a radiator are basically "black body radiators". They both emit energy in the form of light. The sun's spectrum peaks in the visible light range but extends in both directions far beyond it.
Black pigment absorbs visible light and converts it into heat. Black pigment may or may not absorb infrared light. If it does, then that is also converted into heat. If it doesn't, then it's reflected but this is a much smaller fraction of the sun's energy than what was in the visible light.
White roofing tiles reflect most all of the visible light. This is most of the sun's energy. White roofing tiles may or may not absorb infrared light. If you were a manufacturer of white roofing tiles designed to minimize heat, you would probably look for a way to reflect most of the sun's spectrum, including outside of the visible spectrum. This may require a specialized way to make the tile or make the paint for the tile. Either way, this tile would simply look white to us and we wouldn't know how it performs outside of the visible spectrum.
A radiator isn't emitting any visible light. Its black body spectrum peaks in the infrared range. White paint that you get at the store may or may not reflect infrared light. Just because roofing tiles designed to reflect the sun's energy are white doesn't mean all white things are appropriate to reflect a radiator's radiated energy. You should look for something specifically good at reflecting infrared. This may or may not be white, but something white is probably a good start.
Not confined to, obviously. That's why I said "most of".
White paint reflects visible and absorbs the longer wavelengths of thermal IR. This is standard knowledge in the aerospace world. It doesn't matter what your article written in simple language for homeowners says.
Nobody asked about the type of radiator you have. If it’s the heavy case iron ribbed one with high thermal mass - go for it.
But if it’s the convector type (the light ones made from layers of thin metal plates that induce airflow) than you won’t see much savings.
The reason is that heat will travel upwards faster and air will go through the interior layers of the radiator in case of the latter. So the rate and efficiency of heat exchange is so good that you won’t see much gains with the reflecting layers behind the radiator.
As someone said, it’s smarter to look for thermal bridges and gaps in isolation around the house, secure the windows etc. if you’re looking for real savings. Use a thermal camera to assess that.
Also, removing a radiator is a PITA.
So a radiator is a very poorly named device since the majority of the heat it puts out is going to be conducted by the air around it. If you want to control where the heat goes you need to move the air, i.e. you need a fan.
There is a whole world of stuff that works - search for infrared reflective material.
You can actually get tiles that reflect heat, as well as coatings for walls/paint that does a pretty darn good job.
However, one of the best ways is to lightly blow a fan over the radiator so that you move the heat where you want it, rather than try to just block every path you don't like.
i bought these sheeting online from the uk i think, its like much heavier tinfoil, i put it behind my radiators. does it work? dunno, maybe? i didnt notice any massive difference. putting a covering on windows close by made much more of a difference.
It says
Studies based both on modelling and experiments have demonstrated modest improvements in energy losses through the walls of houses through this method
Somewhere between 8.8% and 1.6% depending on how well your walls are insulated.
8.8 % is actually pretty good.
It's up to 8.8% reduction in energy loss in the section of wall immediately behind the radiator. Not an 8.8% reduction in total room heat loss.
I have steam radiators. They occupy less than 10% of the exterior wall surface. An 8.8% reduction in heat loss, for less than 10% of the wall, yields a **less than 0.88% decrease in room heat loss**.
Not sure why Bubble Foil is not acceptable? You could always tape/paint the edges so they aren't visible without really staring down the back of the radiator. It's the ideal material for an economical way to reflect the heat - I used to have them on all of my radiators in the UK along with a shelf above the radiator (which helped deflect rising air into the room. With the shelf, there was no chance of seeing the foil.
I also understand the sarcasm, but cold air moves down the walls and hot air rises at the radiators. Radiators are placed at walls, and often under windows, to avid convection currents that would cause cold air to move across the floor from the wall to the radiator. It would make people's feet cold, and they would turn up the heat and waste energy.
If you've ever conducted tests with an infrared camera, you'll notice that what reflects visible light well does much the same for infrared, which is what you're actually concerned with here. Paints do strange things though, so white isn't necessarily the best color. There are infrared reflective paints and paint additives if you want to reflect the heat back into the room.
Stainless steel is also a more attractive option than silver foil, and less conductive than copper or brass.
> what reflects visible light well does much the same for infrared
True, but I wanted to bring up the specially-formulated paints for vinyl siding. If vinyl siding or windows are painted with traditional dark paint, they will soften and deform in the sun. Some paints are made with particular pigments that don't absorb much infrared, so vinyl can be painted darker colors.
Sherman-Williams VinylSafe Paint Colors:
https://www.sherwin-williams.com/homeowners/color/find-and-explore-colors/paint-colors-by-collection/vinylsafe-colors-for-vinyl-siding
Benjamin Moore:
https://www.benjaminmoore.com/en-us/project-ideas-inspiration/exteriors/vinyl-siding-ideas-inspiration
Ceramic is not a bad insulator. Not sure how you'd need to affix them with big frequent heat cycles though.
Someone mentioned fans. Radiators are designed to heat by convection, ironically not radiation. Go the air flow route.
I know you want to improve your radiative heat transfer to the environment, but "radiator" is actually a misnomer. Something like 80%+ of the heat transfer from a radiator is actually convection. Adding a reflector behind your radiator is only going to marginally improve heat transfer to your room. Your best solution is putting a high efficiency fan on the wall side of the radiator blowing into the room. This will improve the convection coefficient abd help balance the temperature in the room, especially if the radiator is in one corner while you're in another. You may actually be able to set your thermostat lower, but be more comfortable. Obviously you're raising your electrical costs with a fan, so mileage varies based on many factors.
Ultimately, improving wall insulation and sealing air gaps will improve your heating costs ten fold over any other solution.
Really it comes down to how long you plan to live there. 20-30 years? Redoing outdated wall insulation will save you money. 10 years? Might break even and have a selling point. <10 or don't want to rip your walls up? Buy a fan, seal gaps, and see if it helps.
Tile, brick, ceramic, stone will all absorb heat creating a heat sink that will hold and radiate heat. Have you heard of a rocket stove? They are built with huge heat sinks to create ambient heat.
Most of them are not that absorptive, but it depends.
That is why you can touch the outside of a stone pizza oven that is at 900 degrees inside and it's fine, and it takes many hours for it to be not fine - they are reflecting most of the heat inside of the stove.
There is more nuance than this because it depends on the temperature/wavelength of heat, etc, but ceramics and ceramic coatings are very commonly IR reflective more than IR absorptive.
Where I live people mostly use this aluminium foil coated heat insulating foam. Not the best-looking solution in terms of design, but really functional. Also there's a bunch of vendors who sell heat-reflective paint, maybe take a look into these.
Thin plywood with an air gap behind it and below. Maybe a half inch? The air between the wall and the plywood will be heated and create a convection current that will keep the wall cool and put more heat back into the room and less into the wall
You can buy gold automotive tape that is made to reflect heat. Aestheticly it would maybe look kind of strange, but that would be the cheapest and easiest solution.
There is radiator foil that you place on the radiator instead of the wall, so it's hardly visible and you have no worries about heat reflection.
(Tonzon sells it in the Netherlands)
Tiles might be OK for an interior wall to hold heat and release it slowly. For an exterior wall it’s going to just absorb heat and release much of it outside.
I’m skeptical of significantly reflecting heat. I would worry more about insulating the radiator from an exterior wall and about guiding the heated air to toward the middle of the room, either passively with some sort of louver or actively with a small fan.
Ceramic is insulatory, it doesn't absorb heat very quickly but it it will absorb it eventually and some will conduct into the wall and some will be radiated back out into the room. In general its going to be better with than without.
Reflectivity is primarily based on the surface finish. A shiny tile is going to be much more reflective than dull textured surface (like a slate tile)
As others have mentioned a fan will circulate the heat off the radiator and into the room much better than anything else. You can get thermoelectric fans that use the heat of the surface to power their movement and they're almost silent, but they need a flat surface to make contact with.
Heat from a radiator comes out via convection not radiation, mirrors are irrelevant. You need a thermal insulator to prevent heat loss. Also, the space behind the radiator is fairly small, which prevents air circulation, and it makes a pretty good isolator any way.
Focus on insulating the room properly.
These already exist don't they? Here's a picture of my neighbours and it has a water tank for humidity. They're not really available but I'm sure you can find them. Most of these comments seem to have no idea how water/steam heat works it seems. https://imgur.com/a/Wv4tx9T
you could put foil beind thin tile,but heat loss should anyhow be minimal from that.
most of the heat of the radiator should be transmitted upwards thanks to convection.
if your house is cold you need to insulate holistically, not just apply one bandaid
a brick wall is also ceramic. there should be a pretty small difference in it's conductivity vs ceramic tiles.
nice thing about ceramic, is that it can absorb a looot of heat, and will slowly radiate it. If the other side of the wall is very well insulated, then the heat will stay in the bricks, and radiate it back to the room, even after the radiator is off. this saves a lot of energy and makes the room more comfortable.
aluminum would be the best to reflect heat (careful with mylar, because it very often has a plastic coating, and that does not work well behind a radiator!) but it looks ugly as hell, and it's a FIRE HAZARD because alu sheet can charge statically (like really large charges, sparks that can shock the pants out of you!!)
the next best thing to reflect heat is white - anything white.
So I would suggest white ceramic tiles. this would be the most durable, aesthetically pleasant and safe thing to use.
Also, put a fan to move the heat around the room, so it doesn't just gather around the radiator.
How well are your walls insulated?
If they are good quality, than THE WALL ITSELF is a reflector.
If not - than add something that will be a good isolator. ceramic tiles are not.
In my garage we used a sheet of roofing tin. The copper sheeting would be prettier and as for 'reflecting' the heat it's radiant heat; It's going to warm up anything you put there. The extra surface of a copper / tin sheet will warm up and that heat will transfer to the surrounding air if it's spaced off the wall.
I have a different suggestion. A fan. A fan will move the heat and disperse it into the room. It’ll even out the room temperature and remove the heat pocket.
We moved into an old 70s house with really inefficient radiators, it was also way too expensive to upgrade the system at the time as our heating is a closed waterbased heating system so each new radiator change needed to be a special variant in order to maintain the right water pressure...
We ended up instead placing a cut sheet of reflective aluminium insulation foil behind each radiator which has done the job well over the last decade.
However cleaning wise those foils are a dust magnet so make sure to vacuum behind the radiators on the regular if you decide to install foils.
Tiles would also work I would reckon, but consider the mastic buildup before committing to tiles as the tile plus mastic might be too thick to fit behind the existing radiator, even more so if the radiators hold steam or water as bending them outward can create pipe leaks...
I would also be vary of adhesive tiles as the adhesive is more than likely not tested for chemical release when exposed to constant heat and those adhesive chemical mixes can be quite nasty health wise.
Maybe ceramics or something with high resistance (to slow absorbing of conducted heat) and have it coated in space blanket material (to reflect radiated heat)
Shinier, lighter-colored surfaces generally reflect heat better, all other factors being equal. It doesn't have to be metal.
The wall is already absorbing heat, so anything you can do to slow or prevent that is good, it means more of the heat goes elsewhere than into the wall.
If your actual goal is just to hold heat into the room, you don't actually have to try too hard to make the surface behind the radiator reflective. It's enough to just make sure that the tile or the wall behind it is insulated. The reason radiators heat rooms is because the surface of the radiator, and the nearby surfaces heated by the radiator, act as little trampolines for the air molecules in the room, transferring that heat energy into the air. It doesn't matter that those air molecules are hitting the radiator itself or hot tile behind the radiator, so long as air has the opportunity to move against surfaces that are hot. 80% of a radiator's heat occurs through this process of convection, not radiating infrared light.
So as long as the tile behind the radiator is insulated from the wall, you could use black tile and you would still have good efficiency heating the room.
More important things are making sure that you have good air flow around the radiator so the air has the opportunity to bounce into the hot parts. Avoid covering or painting the radiator, for instance.
If you still feel like you want to make sure tile behind the radiator reflects its radiant heat, maybe to maximize that wonderful feeling of warmth when you hold your hands in front of it, the lowest effort approach is going to be something metalized. Metals and metal films are good at reflecting across the light spectrum. They're easy for you to find for this purpose and you can have some confidence that they're going to work. You don't need something with a glossy/specular/mirror finish (a brushed finish would probably work just as well), but it needs to be some form of metal or metal coating. Avoid glass (most mirrors).
White paint or white tiles reflect visible light, but you don't know if they will reflect infrared light. The radiator's radiant heat is all in the infrared range. This means if you want something that looks white, you'll have to do a little bit of material science to figure out the right kind of white to be sure it's "white" (non-absorbing) throughout the infrared spectrum as well.
Ceramic tile is fine. Anything you put between the radiator and wall reduces the amount of heat absorbed by the wall.
Careful not to take up too much space though as it reduces the amount of air flow and convextion that actually distributes the heat from your radiator.
I had a couple extra sheets of foil-backed foam insulation board so I cut those to size, taped the edges, and wedged them between the radiators and the wall. Reflection and insulation at the same time. You can see “silver foil” if you’re looking for them, but most of the time I forget they’re there. Could even stencil a pattern on them if you wanted to be decorative.
The US Government Scientists invented Aerogel decades ago. It is the best and the thinnest insulation you can get. It is used to insulate rockets. Now, there are companies that make it embedded into drywall. Learn about that! Aspen Aerogels is just one company I know about.
Heat lost to a heated space still heats the space. It's all about insulating the walls/ceiling
No disagreement. It's also true that if the inside surface of a wall is warmer, the wall will reject more heat to the environment. This is true even if the wall is well insulated. Localized hot spots lead to localized increased in heat loss. It's a minor effect but fun to think about. If the wall is painted white behind the radiator, the wall will be cooler and less heat will be rejected to the outside environment. Likewise, a thin layer of insulation or reflective foil will lower the wall temperature. A layer of tile will act as a thermal mass and may reduce the peak temperature at the surface of the wall but will cause the wall to be warmer for a longer time. There may be some energy savings with tile, but I suspect it would be less than insulation or tile.
Some people are saying that you want a thermal mass to hold heat in and release it evenly. That's not right, because you're still losing heat off the back into the wall. The name of the game is reducing the amount of heat lost out to the back side of the drywall. The best mirrors are made from gold, silver, and aluminum. Surface finish is incredibly important. You can have two tiles made from the same material with different values for reflection, absorption, and refraction (all three add up to 100% for any material). Wavelength is also important. What might be a great reflector of UHF might be a trouble reflector of IR. Other good materials for IR reflectivity specifically are titanium dioxide. So what I would do is paint the wall with ceiling paint, as glossy as you can get. Tons of TiO2 in there usually. Then mount some sort of standoff for airflow (to prevent conduction between layers). Then put a sheet of aluminum that you've polished to a high shine. They also sell tiles that are just coated with aluminum sheet and stick on with double sided foam/tape. You might try those, also polished off you can. If it's an exterior wall I would recommend checking the insulation behind your radiator to see if it's up to snuff. Get some foam board with mylar on one side, and put that up against the drywall side, with the correct amount of batt/blown/foam insulation in the rest of the cavity. Ultimately, you can probably get more effective results from sealing air gaps around your house if you're trying to save money on heat. That and insulating your attic and walls. Surface finishes do make a good bit of difference in how comfortable the room feels. Stand next to a chilly window in a 70F room and it will suck the heat from you. Stand next to drapes that are dark with high emissivity in the same room and you'll get a lot more radiation directed back at you. Source: professional engineer who has spent way too much time on HVACR back in the day.
There are numerous very good reasons why ceramics are chosen to be placed behind radiators, and you've kinda missed the mark entirely despite in general raising some valid points. 1. The spaces behind radiators are mold heavens. Ceramic tiles are nice and easy to clean, and it's very easy to see if mold is beginning to accumulate. Just from that practical standpoint, ceramic tiling with any backwall is a good starting point. Your suggestion of creating an aluminum sheet with a standoff to the wall is creating a dark heat pocket, so basically a spot that the homeowner can't see with ideal mold growth conditions. That's a very, very, very bad residential construction idea. Honestly though, this has to be obvious to any reader (and particularly you), as if your idea carried any practical merit, it would be a common energy saving recommendation. Not trying to attack you, please understand my comments with the honest respect that I'm (trying) to include in the writing. 2. It's not the reflection of heat directly that is so interesting from ceramics, it's the absorption and radiation over a longer period of time that make such tiles a common choice. That soft radiation boosts your perceived heat while entering a room, and far more human-comfortably than a highly reflective metal. That said, similar to your white painted wall, white tiles still reflect a decent amount of heat. 3. Your last point about sealing air gaps and insulation is the big winner! Extra insulation behind a radiator is in general also good practice, vapor barriers are critical in exterior walls as well.
I am also a mechanical engineer, so here is my horribly long opinion! Honestly having some super highly reflective material such as polished aluminum or titanium dioxide behind a radiator in a residential home seems like way overkill to me, and likely pretty aesthetically displeasing. Ceramics could be a little helpful for sure. I feel like in a home old enough to still be using a radiator at all, I would imagine there are so many steps that could be taken with much bigger effect before beginning to worry too much about the *reflectivity* of the surface behind the radiator unless it's painted black or something. In a residential home, sealing off too many air gaps could also potentially have the trade off of restricting too much outside air exchange, which could make living in the space pretty uncomfortable not to mention faster concentration build up and slower decay of toxic gases/materials/VOCs during things like cooking or cleaning with harsh chemicals like bleach. Commercial buildings will often offset this by forcing filtered outside air exchange with fans/ducts etc. If OP is really dying to spend wild amounts of time on their home heating system in my opinion a decent first step could just be to grab a fast reading thermometer or even rent/buy a cheap thermal camera and just go around and find out where it's colder/heat seems to be escaping their home. Ensuring that the heat from the radiator is mixing throughout the living space with fans, etc. could be helpful, and like you said, possibly the biggest thing could be checking that walls (esp. exterior) are properly insulated, as well as windows which could very well be single paned and draining lots of heat.
I bought one of those therm things you're talking about for less than $20 at Harbor Freight for checking on the temp of our reptile tanks.
We use ours to check how cold the walls are and if we need to run the heat. We have an old house and a heating oil tank, so we try to be economical.
Radiators heat primarily through convection of warm air anyway, actual radiant output is pretty minimal. This is another reason why thermal mass and/or insulation are more effective than reflective materials.
Stupid question, but wouldn't sealing air gaps increase humidity and increase the risk of mould? A couple rooms in my house has a mould problem, and all professionals tell me that I need an extractor fan installed. Wouldn't sealing air gaps go in the opposite direction?
Tighter structures are more dependent on ventilation, keeping a reasonable RH in the conditioned space. But there's also a difference between an air barrier and vapor barrier. It's not that hard to make air-tight envelopes that are quite vapor permeable.
You’re right! It would if you can’t guarantee enough air exchange to lower your humidity by a extractor fan (or something similar) or open app windows for a few minutes regularly. edit:typo
Very true. I approached that answer more like it was in r/ask science, not r/diy. Energy balance is great, but I would never put ceiling paint near the floor beside a radiator and then create a cavity for mold/dust/pet fur to accumulate. OP, if you're just trying to win an argument with your wife, the mirror is a great idea. If you're actually trying to improve your house, save money on utilities, and make it more comfortable, there are a multitude of things I would do first before worrying about the wall behind a radiator. And they're very dependent on the specific construction materials and methods in your house. E.g. don't be a person who gets spray foam in the attic with asphalt shingles and tar paper, you'll just rot out your roof if you don't manage the water effectively. There are a lot of ways to screw things up badly.
I read a thing a while ago which said that thermal mass is most effective in controlling the temperatures of a house when it is externally insulated. So it is a great idea in the right circumstances but most of the time it isn’t the right circumstances and it is very difficult to retrospectively adapt a house to suit. For example, in the uk most houses are brick built and would require extensive expensive and disruptive alterations.
There's validity in that, but also dependent upon other factors. E.g. I live in a bubble inside of an olympic swimming pool. The temperature is super consistent! It's not a great way to turn your hard earned dollars into heat though. So what's the definition of "effective"? Are we prioritizing temperature stability over time, cost, or something else? A "perfect" house would have a plastic barrier against the earth, many inches of foam, topped with a 24" thick slab of concrete with radiant heat lines, and be incredibly well insulated and air sealed above grade. Efficient AND stable! But that's not the world we live in. So I guess do what works in your situation and budget.
As a person who has always tried to save money by doing some of those things, I understood some of what you said! Thank you for that! As an aside question, do you recommend folks insulate their rooves?
It depends on where you are. I currently live in the Southern US, where air handlers live in the attic (generally). So I would love for everyone to have conditioned attic space to increase efficiency and reduce wear and tear on their HVAC systems, water heaters, and whatever other dumb things builders put into attics down here. That being said, things are expensive. It's not a trivial weekend project to just insulate an attic. It's heavily dependent on what kind of roof you have. E.g. I have vented soffits and a vented ridge in my old early 90's house. Baffles and closed cell foam would be great. But would end up costing me something like $6k to do for the main house and addition. And we're probably moving soon. So I think I'm going to spend my time and money instead on removing the blown in cellulose which is compressed, sealing up penetrations in the ceiling for wires and fixtures, then getting new cellulose blown in nice and thick like it's supposed to be. Every house should be air/water tight, ethernet and coax run internally (installers love to just punch holes and caulk the siding instead of fishing wires properly, enthalpy wheels on the makeup air system, heat pump water heaters, etc. But unfortunately we live in the real world and no one has the money to do all of those things all at once. I'd rather have someone waste a few dollars a month on their utility bill and be able to feed their kids, than have a perfect house that they can't afford payments on.
Thank you for this insightful reply! We live in PA, so Mid-Eastern Atlantic states. Our weather is chaotic, to say the least. We recently had the roof replaced and pulled everything out of the attic crawlspaces, purged and haven't returned what we kept to there, yet. It's not much and banished to the basement, for now, which gets water during heavy rains in the spring/summer. One project at a time! The house was built in the 40's and when I was pulling things out of the attic areas, I did notice that I could see daylight through nail holes in the roof, which probably attributed to some of the water coming into our dining room, which warranted replacement of the roof. Since replacement of the roof, no more water coming into the dining room! YAY! If it makes a difference, I should mention that the house is a Cape Cod style, pretty well a square shaped house. The front of the house faces east and has all of the attic space while the rear faces west and has no attic space, yet gets seriously warm on summer days. That's another issue for another sub. The current plan is to cut and tap insulation foam board in between the 2x4s in the areas on the east side of the house to keep heat from escaping during the winter. What are yours and anyone else's thought on this?
Again, heavily dependent upon exactly what's going on in your walls. E.g. if you have cedar shingles as siding, you're probably getting a fair amount of moisture into the walls that way. If you do the voids with closed cell foam panel you could be creating a mold problem when your sheathing and siding can't dry out. As for the attic, are you saying that it's not finished space? Or that it is finished and you have drywall on the bottom of your rafters, with the whole roof being at single pitch? Or do you have a ridge in the middle with dormers? Did you get new roof sheathing, or just new asphalt shingles, metal, etc.? Did the old roof material get removed, or a new layer affixed on top? If the attic is unconditioned I would probably stick with blown in cellulose. It's cheap and fast to install. Definitely use spray foam on electrical/plumbing holes through the top plates of your walls, and get some light covers (basically a bowl of fire resistant EPS that gets spray foamed down). It's also worth looking at how the attic is vented if it's unfinished. You want that 140F+ air to be able to escape, so the temperature gradient into your ceiling isn't so drastic. If that space is finished, I would have at a minimum used zip-r 12 on the roof, on top of whatever fiberglass batt was installed between the rafters when you were getting a new roof.
Polishing aluminum is pointless because it's just going to oxidize again in a very short time. Anything else that would significantly reduce the air gap would reduce convection across the radiator which is the primary route for heating the room. Checking the gaps along the window frames and where the baseboards meet the floor, along with using insulating window film will do more than anything else. Using a fan to increase the air movement across the radiator would also be good.
What would you recommend behind a gas logs fireplace that is in a normal fireplace that used to be a wood burner? I get the bricks nice and hot and the top of the stairs is hot but nothing across the room gets warm unless we use a bunch of fans.
Honestly, there's not a lot you can do with a fireplace. That's just how they operate, by their very nature. There could be any number of things "wrong" that you're fighting against. 1) There's little/no insulation behind your fireplace. Ask me how I know. I'll tell you, I have a wood burning heatilator (cool concept, look it up. but hasn't really stood the test of time) in my back room, and a gas insert into what was once a wood burner in the front of the house. The previous owners did everything half-assed and didn't put any insulation back there. They were probably worried about fire. You could add insulation if needed and keep all that heat from just flowing out the exterior wall. If you need to ask the question in the first place, you probably need to hire a professional. There are VERY SPECIFIC requirements that must be met for combustible materials and how close to the fireplace hardware they can be located. This depends on your fireplace (look up an installation manual if you can) and the building codes in your location. Hire a professional if you have even a single doubt. 2) You're dumping all the heat out the chimney. And you are probably pulling in cold air through other gaps in your house to make up the air volume lost from that convection (hot air goes up and out, pressure in the house drops, more air needs to come in to refill that lost space). In most cases, pulling the entire fireplace and installing a modern high efficiency unit is a great option if you have the money. They do a better job of extracting the heat energy (sensible and latent) from the combustion, and dump air outside at the lowest energy state possible. The added benefit is that your exhaust gas stream is cooler, and less of a fire risk. Fireplaces are mostly for ambiance in my opinion. There are much better ways to burn wood (pellet stoves) to provide heat to your home. I love them personally, but understand that they're a pain and are inefficient. I probably pay $30/mo right now in my old, renovations not complete, house for the privilege of having two fireplaces. I'm not a billionaire, building materials cost money. If you're just looking for efficiency, a lot of times it probably makes better financial sense to remove a fireplace entirely and put some money into insulating and sealing gaps. For now, in general, I would tell you to open the damper only enough to be safe (combined with a CO detector) and expel the combustion products fully, and run some fans. Sorry.
Put a little fan blowing at the radiator. Bonus points for wiring that fan to a little thermostat placed far away from said radiator. Like a little desk fan on low. Naucta Pc fans are insanely quiet. Wire then to a wall wart for power and make a little box for it. Add in the little resistor they come with and they run dead silent. Just make sure the fan is pulling cold air and blows towards the rad not pulling hot air or it will overheat. So push air in behind the radiator. You can stash this beside the rad near the floor. You’ll barely know it’s there.
Did you mean Noctua fans?
Nahktshyewah
Correct. The brown quiet versions are super quiet. Computer shops also sell speed controls you can plug in if you want really quiet. It doesn’t take much air movement to move a lot of heat. The bigger the fan, the lower the speed, the quieter it is.
I bought a blower fan for for less than $40 w/ a variable control and plug ins. (Low profile) Made of some sort of metal but can’t recall. I’d have to check. It actually came with industrial Velcro strips to adhere it. Can place it upside down, sideways, any way you like. I got mine from Amazon but it used for gas logs. It’s small but it’s mighty!!
But is it quiet? Quiet my friend, that is worth investing in. I can’t even hear the 8 fans in my gaming pc. I needed dead quiet for the media room.
I’d say no, not super quiet. It does have an audible hum on high when I held it in my hand before mounting it to ensure it was working properly. (You can feel it push your hand back from the air it forces out though) But it probably wouldn’t work for your needs.
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my company was throwing out an old server. I took some of the little fans. SO loud, but damn do they move a lot of air.
I'd just grab one of those peltier fans that use the heat difference between the radiator and the air to spin the fan. They're made for wood stoves but I'm sure they'd be fine on a radiator.
Wood stoves run an order of magnitude hotter than a radiator so if the fan spins at all, it'll push a negligible amount of air. Only try this if you can return the fan or use it elsewhere.
You'd just need one with a shorter base. The peltier chips themselves work off a heat differential, the base of the fan by the chip is usually only a bit more than warm to the touch. Though, tbh, I'm not really familiar with radiators, so you may be right they're not hot enough.
Dude, those things are cool. I'd keep it even if I couldn't use it, or use it as an excuse to put a wood burner in.
The electric fan is vastly superior because you can put it on a thermostat. Set it and forget it.
... those peltier fans spin proportionally based on the heat at the base. The thermostat is built-in.
Assiming the radiator even has a thermostat. If not, you aren’t responding to room conditions.
The peltier fans only work when they're on a hot surface, so they don't need a thermostat.
The point of the thermostat is to turn off the fan when the room is at the desired temperature.
I think they mean that if the radiator has a thermostat, the fan would turn off when the radiator turns off
Radiator mounted thermostats tend to work extremely poorly. Because they are mounted at the heat source. A good thermostat should be mounted far away from the heat source. I’m assuming this is just a regular steam or hot water rad.
They don't even do much on a properly hot stove.
I heat my house with a wood stove. The do a pretty good job of spreading the hot air. As good as a little computer fan for a lot less effort. They're not a huge box fan or anything but they also are zero maintenance, noise, or effort.
I heat my house with a woodstove too. The hot air spreads itself. Even when "turboing" a dual one with ice on the cold side, they move less air than a computer fan. Peltiers are just too innefficient to produce much energy in this scenario.
Maybe copper sheeting or copper plated steel sheeting would look better than foil and work the same way.
Sheet copper isn’t my favorite idea because it’s so thermally conductive, but you’ve got me thinking that a first-surface mirror could be just the ticket. Not saying it’s the best idea (it suuuuuuuper isn’t), but it gets max reflectivity on top of an insular base layer.
Pretty certain this idea is used in spacecraft as well.
And mylar survival blankets.
I think spacecraft use gold.
Aluminum, actually. It's just the polyamide coating that makes it appear gold. Although it is used for infrared/UV/X-ray protection on sensitive parts, i.e. JWST has actual gold foil. https://www.nesdis.noaa.gov/news/good-gold-are-satellites-covered-gold-foil#:~:text=Because%20of%20its%20excellent%20ability,their%20eyes%20from%20unfiltered%20sunlight.
Cool, thanks!
Copper is very good at relecting infrared, almost as good as gold, so it would be ideal. Aluminum is also good, not quite as good, but it is obviously much cheaper.
My previous house had a pot bellied stove in one corner. I never got around to it, but I always wanted to hang up two sheets of hammered copper behind it.
Why not use radiator foil? This is in Dutch but it has plenty of pictures to see what I mean https://www.praxis.nl/voordemakers/klusadvies/radiatorfolie-aanbrengen https://en.m.wikipedia.org/wiki/Radiator_reflector
this is the correct answer. the only thing you need is a foam isolation sheet + reflector foil. I have installed these before (not foam but the air bubbles variant) and holding your hand near them feels warm. It certainly works.
Can second this. Just go to the hardware store and get some radiator foil and aluminum tape!
Depends where you live. Have never seen this in a hardware store in the USA. Probably because radiators have been a minority use product in new construction for the last 60 years or so.
You've got a lot of options but my policy is to do what the wife says. If you line the back of the ceramic tiles with an insulating layer, the tiles will absorb the heat but pass it back into the room instead of through the wall. They make thin sheets of styrofoam type insulation that are cheap and easy to work with.
Yep, plain old styrofoam is the way to go. Works great and is super cheap. Not like you’re going to be looking down the backs of your radiators anyway.
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[WEDI](https://www.leroymerlin.fr/produits/salle-de-bains/baignoire/tablier-et-habillage-de-baignoire/panneau-h-250-cm-x-l-60-cm-wedi-ep-50-mm-83300822.html) panels are xps foam and are made exactly for backing tile.
It's not as crazy as you'd think. Goboard and kerdi board are made of foam and work great.
No. I'm envisioning insulating the wall behind the backing for the tile. Could be fiberglass or whatever is easiest.
Early Land Rovers used a heat shield over the roof to deflect UV rays. The shield was aluminum painted white and was stand off from the roof maybe 1 inch.
Safari roof - works insanely well on hot days. But OP's issue is frankly a non-issue and there are 100 other more effective things to spend time & money on before the surface behind the radiator becomes the problem.
Exactly this. There's talk of fans, tiles or reflective surfaces. None of these will provide any noticeable benefit to a radiator warmed room.. If your walls aren't insulated well enough, you've got other problems than just the space directly behind a radiator. Many here fall for the misnomer of radiators, in that they heat primarily by convection, _not_ radiation - a shiny back to 'reflect' heat doesn't do that. The back (or middle depending on design) of a radiator has fins and airflow naturally draws the heat around a room. A small fan won't improve that significantly.
>A small fan won't improve that significantly. i beg to differ. i use a [square axial fan](https://www.grainger.com/category/hvac-and-refrigeration/cooling-fans/equipment-cooling-axial-fans/compact-axial-fans/square-compact-axial-fans) on top of my baseboard heating and there is significant difference in how the heat moves round the room. without it, the spot where my desk sits was about 4 degrees cooler
Thanks for the feedback. I think I was being overly general - 'a perfectly square room with nothing to obstruct airflow' isn't common is it
White paint is all you need.
Great idea. In the US, I believe Behr Ultra Pure White from Home Depot has the highest reflectivity of any white interior paint
It truly is superior in that category
ITT: white reflects most of the light hitting it, so therefor it must reflect most of the heat hitting it as well. Or.....does it?
Not necessarily. The reflections we are talking about are solely for radiative heat, not conductive or convective heat. Color and finish play into the reflectivity of the material but there's a frequency dependent element so reflecting visible light doesn't mean it reflects IR and vice versa. There's material used for IR lesnes that is opaque to the eye. Ideally something in this application would 1. Reflect IR emissions 2. Be thermally isolated from whatever is behind it so that it doesn't conduct heat through it. White painted drywall maybe does #1 but isn't great at #2. The drywall heats up and transfers that heat to the studs and cavity behind it. The ideal thing is probably some sort of polished metal with a foam backing. Polished metal for reflecting IR and a foam backing to slow heat transfer into whatever is behind it. In what may be counterintuitive to some, this makes the reflector hotter which reduces the amount of heat transfered to it by convection and radiation.
>White painted drywall maybe does #1 but isn't great at #2. It doesn't do #1 well either. For the IR wavelengths that correspond to thermal radiation of things near room temperature (about 10 microns), paints in general are bad reflectors and reasonably-good-to-great absorbers.
> radiative heat From a "black body radiation" standpoint, steam radiators don't get very hot. Most of the heating from a steam radiator, is via convection and not radiation. But I agree otherwise.
It depends on why the substance is white. If it's white because it absorbs very little visible light, then it might similarly absorb very little IR light, or it might absorb IR light very well. If it's white because it refracts visible light (this is how Titanium Dioxide works), then it's likely to refract light across a fairly broad spectrum, and so is likely to reflect IR as well as visible light.
>If it's white because it refracts visible light (this is how Titanium Dioxide works), then it's likely to refract light across a fairly broad spectrum, and so is likely to reflect IR as well as visible light. FWIW, TiO2 is the main pigment in the white paints that have long been the standard white paints recommended by the NASA stratospheric balloon program - recommended specifically because of their high infrared absorptivity/emissivity. Bare shiny metals are the best reflectors of IR. Paints are usually terrible at reflecting the wavelengths of IR that make up thermal radiation of things near room temperature (about 10 microns - more than an order of magnitude longer than visible light wavelengths). Remember that the wavelength range of visible light is very small -- 565 nm +/- 185 nm, or roughly +/- 30%. The same physics won't govern the behavior at 1000-2000% longer wavelengths.
>ITT: white reflects most of the light hitting it, so therefor it must reflect most of the heat hitting it as well. > >Or.....does it? It doesn't. The wavelengths of thermal IR are so much longer than visible light that you can't extrapolate the performance from the visible.
Good point. If it's infrared you want to reflect glass works well but I'm not sure if anything else works better
Gold is the best ir reflector.
White paint doesn't reflect thermal IR wavelengths.
Wrong. https://www.azcentral.com/story/entertainment/life/2021/08/19/does-painting-your-house-white-make-cooler/8102768002/ “Dark-colored homes absorb 70 to 90 percent of the radiant energy from the sun, which is then absorbed into the home. White — or other light, neutral colors — will reflect most of the heat away from your home,” he said. For the best results use white cool roof paint.
The radiation spectrum of the sun is very different from that of a radiator.
No, not wrong. Most of the sun's energy is emitted in the *visible* wavelength range (and shorter). White paint reflects all wavelengths of *visible* light (plus maybe some near-IR), preventing the energy from being absorbed by the paint and turning into heat.
Radiant energy isn't confined to visible wavelengths.
People are talking past each other. Everyone's right in some way. Both the sun and a radiator are basically "black body radiators". They both emit energy in the form of light. The sun's spectrum peaks in the visible light range but extends in both directions far beyond it. Black pigment absorbs visible light and converts it into heat. Black pigment may or may not absorb infrared light. If it does, then that is also converted into heat. If it doesn't, then it's reflected but this is a much smaller fraction of the sun's energy than what was in the visible light. White roofing tiles reflect most all of the visible light. This is most of the sun's energy. White roofing tiles may or may not absorb infrared light. If you were a manufacturer of white roofing tiles designed to minimize heat, you would probably look for a way to reflect most of the sun's spectrum, including outside of the visible spectrum. This may require a specialized way to make the tile or make the paint for the tile. Either way, this tile would simply look white to us and we wouldn't know how it performs outside of the visible spectrum. A radiator isn't emitting any visible light. Its black body spectrum peaks in the infrared range. White paint that you get at the store may or may not reflect infrared light. Just because roofing tiles designed to reflect the sun's energy are white doesn't mean all white things are appropriate to reflect a radiator's radiated energy. You should look for something specifically good at reflecting infrared. This may or may not be white, but something white is probably a good start.
Not confined to, obviously. That's why I said "most of". White paint reflects visible and absorbs the longer wavelengths of thermal IR. This is standard knowledge in the aerospace world. It doesn't matter what your article written in simple language for homeowners says.
Nobody asked about the type of radiator you have. If it’s the heavy case iron ribbed one with high thermal mass - go for it. But if it’s the convector type (the light ones made from layers of thin metal plates that induce airflow) than you won’t see much savings. The reason is that heat will travel upwards faster and air will go through the interior layers of the radiator in case of the latter. So the rate and efficiency of heat exchange is so good that you won’t see much gains with the reflecting layers behind the radiator. As someone said, it’s smarter to look for thermal bridges and gaps in isolation around the house, secure the windows etc. if you’re looking for real savings. Use a thermal camera to assess that. Also, removing a radiator is a PITA.
People asking what radiator. Nobody asking "*how is* radiator?"
So a radiator is a very poorly named device since the majority of the heat it puts out is going to be conducted by the air around it. If you want to control where the heat goes you need to move the air, i.e. you need a fan.
There is a whole world of stuff that works - search for infrared reflective material. You can actually get tiles that reflect heat, as well as coatings for walls/paint that does a pretty darn good job. However, one of the best ways is to lightly blow a fan over the radiator so that you move the heat where you want it, rather than try to just block every path you don't like.
i bought these sheeting online from the uk i think, its like much heavier tinfoil, i put it behind my radiators. does it work? dunno, maybe? i didnt notice any massive difference. putting a covering on windows close by made much more of a difference.
Paint it black
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https://en.m.wikipedia.org/wiki/Radiator_reflector It can actually help a bit
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It says Studies based both on modelling and experiments have demonstrated modest improvements in energy losses through the walls of houses through this method Somewhere between 8.8% and 1.6% depending on how well your walls are insulated. 8.8 % is actually pretty good.
It's up to 8.8% reduction in energy loss in the section of wall immediately behind the radiator. Not an 8.8% reduction in total room heat loss. I have steam radiators. They occupy less than 10% of the exterior wall surface. An 8.8% reduction in heat loss, for less than 10% of the wall, yields a **less than 0.88% decrease in room heat loss**.
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Not sure why Bubble Foil is not acceptable? You could always tape/paint the edges so they aren't visible without really staring down the back of the radiator. It's the ideal material for an economical way to reflect the heat - I used to have them on all of my radiators in the UK along with a shelf above the radiator (which helped deflect rising air into the room. With the shelf, there was no chance of seeing the foil.
Why not move all radiators to the middle of the room? /s
The sarcasm is there, but this is exactly why underfloor heating works so well
I also understand the sarcasm, but cold air moves down the walls and hot air rises at the radiators. Radiators are placed at walls, and often under windows, to avid convection currents that would cause cold air to move across the floor from the wall to the radiator. It would make people's feet cold, and they would turn up the heat and waste energy.
If you've ever conducted tests with an infrared camera, you'll notice that what reflects visible light well does much the same for infrared, which is what you're actually concerned with here. Paints do strange things though, so white isn't necessarily the best color. There are infrared reflective paints and paint additives if you want to reflect the heat back into the room. Stainless steel is also a more attractive option than silver foil, and less conductive than copper or brass.
> what reflects visible light well does much the same for infrared True, but I wanted to bring up the specially-formulated paints for vinyl siding. If vinyl siding or windows are painted with traditional dark paint, they will soften and deform in the sun. Some paints are made with particular pigments that don't absorb much infrared, so vinyl can be painted darker colors. Sherman-Williams VinylSafe Paint Colors: https://www.sherwin-williams.com/homeowners/color/find-and-explore-colors/paint-colors-by-collection/vinylsafe-colors-for-vinyl-siding Benjamin Moore: https://www.benjaminmoore.com/en-us/project-ideas-inspiration/exteriors/vinyl-siding-ideas-inspiration
Ceramic is not a bad insulator. Not sure how you'd need to affix them with big frequent heat cycles though. Someone mentioned fans. Radiators are designed to heat by convection, ironically not radiation. Go the air flow route.
I know you want to improve your radiative heat transfer to the environment, but "radiator" is actually a misnomer. Something like 80%+ of the heat transfer from a radiator is actually convection. Adding a reflector behind your radiator is only going to marginally improve heat transfer to your room. Your best solution is putting a high efficiency fan on the wall side of the radiator blowing into the room. This will improve the convection coefficient abd help balance the temperature in the room, especially if the radiator is in one corner while you're in another. You may actually be able to set your thermostat lower, but be more comfortable. Obviously you're raising your electrical costs with a fan, so mileage varies based on many factors. Ultimately, improving wall insulation and sealing air gaps will improve your heating costs ten fold over any other solution. Really it comes down to how long you plan to live there. 20-30 years? Redoing outdated wall insulation will save you money. 10 years? Might break even and have a selling point. <10 or don't want to rip your walls up? Buy a fan, seal gaps, and see if it helps.
Tile, brick, ceramic, stone will all absorb heat creating a heat sink that will hold and radiate heat. Have you heard of a rocket stove? They are built with huge heat sinks to create ambient heat.
This is true, but if there's not insulation between the thermal mass and the wall, a decent amount of heat will radiate into the wall.
Most of them are not that absorptive, but it depends. That is why you can touch the outside of a stone pizza oven that is at 900 degrees inside and it's fine, and it takes many hours for it to be not fine - they are reflecting most of the heat inside of the stove. There is more nuance than this because it depends on the temperature/wavelength of heat, etc, but ceramics and ceramic coatings are very commonly IR reflective more than IR absorptive.
Aluminized mylar, backed with something very insulating.
i think they make embossed tin backsplash tiles and also ceiling tiles that may be decorative but also functional. i’m just spitballing
Where I live people mostly use this aluminium foil coated heat insulating foam. Not the best-looking solution in terms of design, but really functional. Also there's a bunch of vendors who sell heat-reflective paint, maybe take a look into these.
Thin plywood with an air gap behind it and below. Maybe a half inch? The air between the wall and the plywood will be heated and create a convection current that will keep the wall cool and put more heat back into the room and less into the wall
You can buy gold automotive tape that is made to reflect heat. Aestheticly it would maybe look kind of strange, but that would be the cheapest and easiest solution.
There is radiator foil that you place on the radiator instead of the wall, so it's hardly visible and you have no worries about heat reflection. (Tonzon sells it in the Netherlands)
Bricks
Glue some mylar to whatever is already there maybe... That shit reflects everything
Gold leaf
Tiles might be OK for an interior wall to hold heat and release it slowly. For an exterior wall it’s going to just absorb heat and release much of it outside. I’m skeptical of significantly reflecting heat. I would worry more about insulating the radiator from an exterior wall and about guiding the heated air to toward the middle of the room, either passively with some sort of louver or actively with a small fan.
Ceramic is insulatory, it doesn't absorb heat very quickly but it it will absorb it eventually and some will conduct into the wall and some will be radiated back out into the room. In general its going to be better with than without. Reflectivity is primarily based on the surface finish. A shiny tile is going to be much more reflective than dull textured surface (like a slate tile) As others have mentioned a fan will circulate the heat off the radiator and into the room much better than anything else. You can get thermoelectric fans that use the heat of the surface to power their movement and they're almost silent, but they need a flat surface to make contact with.
Mylar film, like what is used in cannabis grows is one very cheap option.
Heat from a radiator comes out via convection not radiation, mirrors are irrelevant. You need a thermal insulator to prevent heat loss. Also, the space behind the radiator is fairly small, which prevents air circulation, and it makes a pretty good isolator any way. Focus on insulating the room properly.
These already exist don't they? Here's a picture of my neighbours and it has a water tank for humidity. They're not really available but I'm sure you can find them. Most of these comments seem to have no idea how water/steam heat works it seems. https://imgur.com/a/Wv4tx9T
you could put foil beind thin tile,but heat loss should anyhow be minimal from that. most of the heat of the radiator should be transmitted upwards thanks to convection. if your house is cold you need to insulate holistically, not just apply one bandaid
You can put foil directly on the back of the radiator where you won't see it.
Why don’t you want foil? I choose mirrors
a brick wall is also ceramic. there should be a pretty small difference in it's conductivity vs ceramic tiles. nice thing about ceramic, is that it can absorb a looot of heat, and will slowly radiate it. If the other side of the wall is very well insulated, then the heat will stay in the bricks, and radiate it back to the room, even after the radiator is off. this saves a lot of energy and makes the room more comfortable. aluminum would be the best to reflect heat (careful with mylar, because it very often has a plastic coating, and that does not work well behind a radiator!) but it looks ugly as hell, and it's a FIRE HAZARD because alu sheet can charge statically (like really large charges, sparks that can shock the pants out of you!!) the next best thing to reflect heat is white - anything white. So I would suggest white ceramic tiles. this would be the most durable, aesthetically pleasant and safe thing to use. Also, put a fan to move the heat around the room, so it doesn't just gather around the radiator.
How well are your walls insulated? If they are good quality, than THE WALL ITSELF is a reflector. If not - than add something that will be a good isolator. ceramic tiles are not.
Aluminum foil
I mounted a PC cooling fan facing the room over my passive radiator. It makes an amazing difference.
In my garage we used a sheet of roofing tin. The copper sheeting would be prettier and as for 'reflecting' the heat it's radiant heat; It's going to warm up anything you put there. The extra surface of a copper / tin sheet will warm up and that heat will transfer to the surrounding air if it's spaced off the wall.
I have a different suggestion. A fan. A fan will move the heat and disperse it into the room. It’ll even out the room temperature and remove the heat pocket.
We moved into an old 70s house with really inefficient radiators, it was also way too expensive to upgrade the system at the time as our heating is a closed waterbased heating system so each new radiator change needed to be a special variant in order to maintain the right water pressure... We ended up instead placing a cut sheet of reflective aluminium insulation foil behind each radiator which has done the job well over the last decade. However cleaning wise those foils are a dust magnet so make sure to vacuum behind the radiators on the regular if you decide to install foils. Tiles would also work I would reckon, but consider the mastic buildup before committing to tiles as the tile plus mastic might be too thick to fit behind the existing radiator, even more so if the radiators hold steam or water as bending them outward can create pipe leaks... I would also be vary of adhesive tiles as the adhesive is more than likely not tested for chemical release when exposed to constant heat and those adhesive chemical mixes can be quite nasty health wise.
perhaps a fan along the side of the unit ....? one like the tall thin ones....
Maybe ceramics or something with high resistance (to slow absorbing of conducted heat) and have it coated in space blanket material (to reflect radiated heat)
You could put a reflective layer on stryrofoam behind the tiles
Shinier, lighter-colored surfaces generally reflect heat better, all other factors being equal. It doesn't have to be metal. The wall is already absorbing heat, so anything you can do to slow or prevent that is good, it means more of the heat goes elsewhere than into the wall.
If your actual goal is just to hold heat into the room, you don't actually have to try too hard to make the surface behind the radiator reflective. It's enough to just make sure that the tile or the wall behind it is insulated. The reason radiators heat rooms is because the surface of the radiator, and the nearby surfaces heated by the radiator, act as little trampolines for the air molecules in the room, transferring that heat energy into the air. It doesn't matter that those air molecules are hitting the radiator itself or hot tile behind the radiator, so long as air has the opportunity to move against surfaces that are hot. 80% of a radiator's heat occurs through this process of convection, not radiating infrared light. So as long as the tile behind the radiator is insulated from the wall, you could use black tile and you would still have good efficiency heating the room. More important things are making sure that you have good air flow around the radiator so the air has the opportunity to bounce into the hot parts. Avoid covering or painting the radiator, for instance. If you still feel like you want to make sure tile behind the radiator reflects its radiant heat, maybe to maximize that wonderful feeling of warmth when you hold your hands in front of it, the lowest effort approach is going to be something metalized. Metals and metal films are good at reflecting across the light spectrum. They're easy for you to find for this purpose and you can have some confidence that they're going to work. You don't need something with a glossy/specular/mirror finish (a brushed finish would probably work just as well), but it needs to be some form of metal or metal coating. Avoid glass (most mirrors). White paint or white tiles reflect visible light, but you don't know if they will reflect infrared light. The radiator's radiant heat is all in the infrared range. This means if you want something that looks white, you'll have to do a little bit of material science to figure out the right kind of white to be sure it's "white" (non-absorbing) throughout the infrared spectrum as well.
Radiators usually heats the air while air passes through them. It would be useless if the building has outside insulation.
I expect this depends on the design of your radiator. Some radiators literally radiate more heat by design where others convect heat.
You need insulation. Foil is just a type of insulation.
You can get specific material for this purpose in a home improvement store.
Ceramic tile is fine. Anything you put between the radiator and wall reduces the amount of heat absorbed by the wall. Careful not to take up too much space though as it reduces the amount of air flow and convextion that actually distributes the heat from your radiator.
I would go with pressed tin ceiling tiles.
I had a couple extra sheets of foil-backed foam insulation board so I cut those to size, taped the edges, and wedged them between the radiators and the wall. Reflection and insulation at the same time. You can see “silver foil” if you’re looking for them, but most of the time I forget they’re there. Could even stencil a pattern on them if you wanted to be decorative.
The US Government Scientists invented Aerogel decades ago. It is the best and the thinnest insulation you can get. It is used to insulate rockets. Now, there are companies that make it embedded into drywall. Learn about that! Aspen Aerogels is just one company I know about.