This comes up in so many places — engine cooling, interior cooling, component heat shielding, exhaust systems, etc. — that it's probably a good idea to summarize it in one place. At least this way I can just link to this thread whenever someone gets the principles seriously wrong. It's a field overflowing with snake oil, so knowing a bit would tell you the right questions to ask the next purveyor of magic heat-stopper to come down the road. Basically, there are only four kinds of thermal insulation: Air-entrapment insulation works by holding bubbles of air, which is a very poor conductor of heat, in position within the thickness of another material, which is preferably not a very good conductor of heat either. Examples are foam plastics, paper pellets, glass wool, mineral wool, and organic wools including actual wool off a sheep. The effectiveness of this kind of insulation depends entirely on its thickness. You're not going to trap enough air in the thickness of a layer of paint to achieve anything, no matter what the sales rep says. Reflective/emission-resistant insulation works through a surface's ability to reflect radiant heat and inability to emit radiant heat — two qualities which generally go together. Important to understand is that it is the surface doing the work here, and that surface has to face air. Also important is that at the range of temperatures we're likely to be dealing with, surface finish plays a much greater role than colour. If we want to block heat we want glossy. Performance is severely reduced if the surface loses its shine, or there is a layer of dust on top of it. And a layer of aluminium foil sandwiched between two other materials isn't going to stop any radiant heat. In fact it's quite a good conductor of heat. Heat-storage insulation works through the ability of certain materials like ceramics to hold heat and not conduct it away. They tend to work best at higher temperatures. Examples are the tiles on the Space Shuttle and probably most of the ceramic header coatings. High-tech versions use zirconium-based materials, though alternatives are under investigation. The limitations are mass and duty cycle. The more mass there is (e.g. the thicker the layer) the more heat it can store, but eventually you'll need to get rid of the heat again, either by conduction via the metal substrate or into the surrounding air. Phase-change insulation works by the principle that changing a material from one phase to another absorbs heat in one direction and releases heat in the other. We see it in boiling water, which stays at the local boiling point of water and doesn't get hotter, until it's all boiled away. It is the principle used in air conditioning, but also in sodium-filled exhaust valves. The temperature of sodium in an internal cavity in the valve doesn't rise above the melting point of sodium until all the sodium is melted. In a way it's another kind of heat storage, and the same duty-cycle consideration applies. I have a suspicion that a lot of the snake-oil thermal products appear to work initially because part of the material actually melts, something it can only repeat so many times before breaking down. We'll likely solve the vast majority of situations we'll find in a car build by either air-entrapment or reflective/emission-resistant insulation. It's important to know which is which, and understand that they work on totally different principles. It's also useful to understand that heat gets from A to B in only a number of ways, i.e. by conduction, by radiation, or indirectly through processes like convection. Air-entrapment insulation aims to stop conduction; reflective/emission-resistant insulation aims to stop radiation. You can use convection for thermal control, either with or instead of insulation, by incorporating scoops, ducts, vents etc. to keep replacing hot air with cool air: there are lots of opportunities for that kind of thing. Just a few things to keep in mind.
I have good experience with a one inch layer of mineral wool, that has one reflective side (basically cheap roof insulation, - cause I'm cheap) at my firewall (inside) - I just put some carpet over it for a nice look. Also it quiets things down a bit. Frank
This is a great contribution. How about examples of each? I do realize there are marketing budgets for each type but our common sense and wallets will help us determine which is suited best for our purposes. Thanks.
In your case the foil layer serves only to keep the carpet from rubbing the mineral wool to pieces. The carpet touching the foil destroys any reflective/emission-resistant functionality it might have had. 25mm is nevertheless a very useful thickness of air-entrapment insulation.
Thank you for posting this. There is SO MUCH misinformation here and all over the net on this subject!
Its the same for every subject ! The world wide web has in many cases , become a disservice instead of a service , sorting right from wrong is time consuming & mentally taxing !
My pickup, to the left, is dark green with a reflective surface on the roof. Inside it has a cardboard headliner with 1/4" air duct foam insulation stuck to the roof. There might be 1" clearance in places and up to 3" in others. What would be my best solution to keep heat off my head? Air entrapment? Something as simple as fiberglass insulation?
I take it the existing insulation is stuck to the sheetmetal, and has a foil layer? If so, that is already doing quite a lot, and the thing that is doing most of the work is the bottom surface of the foil. A ¼" thickness of air entrapped in foam can only do so much. You would probably get the same performance out of ordinary aluminium foil glued direct to the sheetmetal, shiny side down. Interesting that you mention the colour and finish. This is a case where colour does play a role. The sun is much, much hotter than any heat source in a car, and radiant heat emanating from it happens in a frequency range which includes a lot which is sensitive to surface colour. The dark green colour is going to absorb a lot of sunlight, and the glossy finish will reflect only some sunlight, in a different frequency range. What happens is, the dark paint allows the sheetmetal to absorb radiant heat in the visible to near-infrared part of the spectrum, causing it to become hot. The sheetmetal nevertheless remains a lot cooler than the sun, so it reradiates heat in the mid- to far-infrared part of the spectrum, which isn't really sensitive to colour but more to surface finish. A lot of the heat is stopped by the lower surface of the foil's inability to radiate heat in the lower-frequency range. To preserve that functionality, it is important that there is nothing touching the bottom surface of the foil. The foil is an excellent conductor of heat, so wherever contact with something else forms a thermal bridge the foil will readily conduct all the heat it can to whatever is touching it, which defeats the object of the exercise. Whatever you add to the space between the foil and the headliner would have to keep at least about ½" clear of the foil. The strategy I'd recommend is to map the parts of the headliner where there is more than about 1½"-2" of vertical clearance to the foil, and add about 1" of lightweight air-entrapment insulation only there. That could be something like a glass wool blanket, secured to the headliner in spots. It could even be bits cut out of old fleecy blankets or sweaters; it needn't be fancy.
I've got some questions about radiant heat(from the sun).....it seems insulating the cars body is kinda moot as most of the heat comes in through the glass? Maybe if you have A/C there might be some benefit? If the windows are down won't the air temp be the same inside and out? (heat goes to cold?)
Very interesting, thanks for the information. I now have to pull the headliner to see what's up there.
Radiant heat, yes, certainly. Most of our cars' windows are smaller and more vertical than those of more modern cars, so doing something to the roof would have more benefit than it would with later cars. There are obvious exceptions e.g. a lot of c.1959-63 large cars. When the windows are all open we're dealing with ambient air temperature rather than radiant heat. But part of the overall experience is surely that panel hanging over your head and radiating heat at you, while its shade ought to be cooling you down. It would be good if your body could radiate heat at the headliner instead, and radiant cooling is indeed an actual thing. I've speculated about using it in a car before.
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