Eagleshield heat shield says that their product is like that used by NASA, and that’s true. But spacecraft have (has?) the stuff on the outside of the skin. That wouldn’t be very popular with your homies if you tried it on the house. So they lay it in your attic just like regular insulation. And there’s what I wonder about.
First, does the product do a good job of reflecting heat? It does reflect light, but how about infrared?
And even if it does, wouldn’t that double the heating of the air in the attic and the roof structure.
Reynolds Wrap will tell you that it makes absolutely no difference to the food your which side of the foil you face out when you wrap it.
From the RW site:
BTW; the heat does get into the food via conduction. Why not your living room?
Peace,
mangeorge
To me, a casual observer, it would seem that builders and home owners in the US are ***not ***taking heating and air condition (and insulation) serious enough. It would seem to me that a lot of energy goes into our living environment. There has been a tremendous amount of research put into space insulation, heating and cooling. It would seem to this casual observer that technology is being moved only ever so slowly from space research to something useful at the home or office.
What the company is probably referring as the product used by NASA is a thing called multilayer insulation, or MLI. MLI is a generic term for several layers of a highly reflective (low emissivity) material, usually aluminized mylar. These layers have some sort of conductive resistance between them either by a layer of low conductivity material like fiberglass or the layers are crumpled to allow only for point contact. The reasons that this works so well in space are that there is no air for convection, and the temperature differences between objects can be very large (radiation heat transfer is driven by the difference in the fourth power of the temperatures, as opposed to conduction and convection which are linear). This makes radiation a very dominant mode of heat transfer. The usefulness of this sort of insulation in a residence is probably not a whole lot since conduction and convection will typically dominate, though it also wouldn’t hurt either as I’m sure there are cases that would benefit.
To address your other questions:
it probably will do a good job reflecting IR if it reflects visible, a fact all too familiar to anyone involved with infrared thermography
which side of the aluminum foil you wrap food with actually will make a difference to the heat transfer however, it will be unnoticeable without a controlled experiment.
I’m not sure what you’re getting at there. Heat does get into your living room by conduction. It’s all a matter of the least thermally resistive path, be it conduction, convection, or radiation.
Hopefully that helped, I wrote this in a bit of a hurry.
Heat transmission is relative to the difference in temperature between the two sides. That difference is far greater between the roughly room-temperature inside of a spacecraft and the vacuum of space outside it, compared to that on earth between inside & outside your house. Even in the depths of a Minnesota winter here, it’s still a smaller temperature variation.
If you wrap food in foil and put it in the oven or on the grill it will of course get hot. The foil doesn’t appear to have a dramatic effect there. The heat in your attic isn’t much different than in an oven, is it?
To tell the truth, no insulation would make a lot of difference where I live because of the mild weather. In fact it would make no difference at all in the summer because I don’t use an air conditioner.
The sun heats up your roof, which radiates into your attic, and from there into the rooms below. But if you have insulation in the attic, that slows down the movement of the heat down into the rooms below, so they stay cooler.
Actually, what counts is whether I would feel any cooler, which I wouldn’t. Just numbers, no measurement involved, but I’d be hard put to feel any dufference between, say, 75 deg and 70 deg. I use a fan to move my body heat away, but that doesn’t effect the air temperature in the room.
There would likely be a difference in heating in colder weather.
I really don’t like the idea of an air-tight house. Guess I’d better not move to North Dakota, huh!
Still, the idea of having a spacecraft in my attic does have some appeal.
I’d like to see a cite, or at least an explanation, for that. A material which allowed heat to pass more easily in one direction than the other would violate the Second Law.
I’m not sure I would word it that it allows heat to pass more easily in one direction - it just depends on what each side is exposed to. Take, for example, some thin material which is aluminized (reflective) on one side and painted black on the other. If this material is laid on a hot, isothermal surface with the black side out it will radiate a large amount of heat due to the high emissivity (which, due to Kirchoff’s Law, means low reflectivity, assuming an opaque material). If the reflective side is facing out, it will emit much less heat due to the low emissivity, acting as a radiative insulator. This is for an isothermal, i.e. constant temperature, surface. For a constant heat flux the black side exposed would result in a much lower temperature than the reflective side exposed when steady state is reached. This also assumes that the inner surface is in contact with the hot surface therefore the radiative heat transfer is negligible compared to the conductive heat transfer (typically a very good assumption). This violates the second law no more than does a material that has a low thermal conductivity.
Another example would be an emergency blanket - the shiny, reflective surface needs to be exposed to the sky because that is where the temperature difference is greatest - between the person and outer space (this reminds me of how water can freeze while ambient temperatures are above 0 C). Reflective surface on the inside won’t do much because the temperature difference between the blanket and the person isn’t that much, and other modes of heat transfer dominate.
Hopefully your attic isn’t as hot as an oven. And that is what really makes the difference. For heat transfer from a 473 K (200 C) surface, such as an oven heating element, to a room temperature object at 295 K (22 C) the heat transfer would be 2409 W/m^2. From a really hot roof, say at 100 C to the same room temperature surface would be 668 W/m^2. Note that this assumes a view factor of 1 and perfect blackbodies, neither of which is very accurate. Another factor for the food in a grill is that it has a low thermal mass, and thus heats relatively quickly compared to an entire house.
There are many factors that would determine how much effect radiative insulation would have. The size of the attic - a large open area would benefit more due to the higher convective thermal resistance, compared to a small crawlspace which wouldn’t benefit much. The more extreme temperatures involved would also utilize radiation shielding better. So being in a mild climate you probably would have a very long payback period from an investment in that insulation.
Sorry, I deal with this kind of stuff all day and am a terrible teacher without the use of pictures :p. A blackbody is an ideal radiator and absorber, basically the least resistance possible to radiation heat transfer (as according to Planck’s Law). Things are often approximated as blackbodies, like the sun’s surface having a blackbody distribution (intensity versus wavelength) of an object at 5777 K. I think I’ll hold off explaining gray bodies and real surfaces…
Not on one interpretation. If it allowed heat to pass more easily in one direction when, say, both sides were at the same temperature, that would of course violate the second law. But if it conducted well when side A was hotter than side B, but not so well when the temperatures were reversed, there would be no violation. It would be kind of like a diode, but for heat. Similarly, some cells have what are referred to as rectifying ion channels. These in one sense let ions pass more easily in one direction than the other, but of course they don’t let ions spontaneously flow against a concentration gradient.
Whether anything like this applies to aluminum foil I cannot say.
OK, different types of coupling on the two sides. I get that, then. It intuitively looks like a Maxwell’s demon, but I guess my intuition was wrong there.
Back on the subject of reflective insulation for the house, it exists. It’s called a radiant barrier. We have it on our house, and keeps the hot desert sun from heating our house too much in the summer. It was only an extra $1500, and it will pay for itself in less than two years. You can install it yourself as well, and would reduce the heat coming in the house through radiation inside the attic. For those of you who have hot summers and outrageous electrical bills from A/C, I’d look into this. Might save you a bunch of money.
