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Reflective insulation systems are fabricated from aluminum foils with a variety of backings, such as kraft paper, plastic film,polyethylene bubbles, or cardboard. Emittance (or emissivity) refers to the ability of a material’s surface to emit radiant energy. All materials have emissivities ranging from zero to one. The lower the emittance of a material, the lower the heat (infra-red radiant energy) radiated from its surface. Aluminum foil has a very low emittance, which explains its use in reflective insulation and radiant barriers. This study indicates the thermal resistance of the reflective air space in 'building energy saving guideline' should be changed to reflect its realistic value which is 3 times higher that in the guideline. The surface emittance of reflective insulation should also be under 0.05,which is examined by authorized organizations or laboratories and checked by construction supervisors.