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When reflection occurs from thin layers of material, internal reflection effects can cause the reflectance to vary with surface thickness. Reflectivity is the limit value of reflectance as the sample becomes thick; it is the intrinsic reflectance of the surface, hence irrespective of other parameters such as the reflectance of the rear surface.
Wood is a poor insulator and so it conducts heat from the radiant barrier to lower surfaces of said wood, where it, in turn, sheds heat by emitting IR radiation. According to the US Department of Energy, “Reflective insulation and radiant barrier products must have an air space adjacent to the reflective material to be effective.” [9]
The basis in physics of a high emittance is quite questionable, since it merely describes a material which easily radiates infrared wavelength heat to the environment, contributing to the greenhouse effect. Highly reflective, low-emittance materials are much better at reducing energy consumption.
Finally, high index materials have higher Fresnel reflection coefficients, so the efficiency of coupling of the light from the ambient into the sphere decreases as the index becomes higher. Commercial retroreflective beads thus vary in index from around 1.5 (common forms of glass) up to around 1.9 (commonly barium titanate glass).
Reflective aluminum foil is the most common material used as a radiant barrier. It has no significant mass to absorb and retain heat. It also has very low emittance values "E-values" (typically 0.03 compared to 0.90 for most bulk insulation) which significantly reduces heat transfer by radiation.
A few materials, like liquids and glasses, lack the internal subdivisions which produce the subsurface scattering mechanism described above, and so give only specular reflection. Among common materials, only polished metals can reflect light specularly with high efficiency, as in aluminum or silver usually used in mirrors.