Quote:
Originally Posted by Mikesolar
Just make sure that the reflective bubble material is at least 3/4" below the subfloor or it will have next to no effect. Radiant effect requires an air space for the wavelength of heat to reflect. otherwise it just conducts through.
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In the link that michael pointed to, there was the sentence, "
The layers of MLI can be arbitrarily close to each other, as long as they are not in thermal contact."
That material was designed for use in space, where, except for conduction thermal radiation is the only mode of heat gain or loss.
The nature of thermal radiation on earth is the same, but heat loss will have the additional mode of convection. Convection depends on the flow of fluids (air can be considered a fluid, since it flows) and fluid flow is inhibited by friction.
So, if the gap between layers, radiant insulation layers that is, is very small, the fluid friction will inhibit convection losses. The larger the space, the lower the fluid friction, the greater the convective losses.
So, the "at least 3/4 inch" idea might have a problem.
Also, regarding wavelengths, check this out:
Quote:
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Infrared (IR) light is electromagnetic radiation with longer wavelengths than those of visible light, extending from the nominal red edge of the visible spectrum at 700 nanometers (nm) to 1 mm.
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So, a long wavelength is 1mm.
If your wavelength idea is correct, you need a maximum space of 1mm to allow the reflective surface to do it's reflecting magic. As your space becomes larger than that, fluid resistance drops, convective currents form... the larger the space, the greater the convection.
I believe I have heard this referred to as "convective pumping".
Best,
-AC