Radiant-barriers: inhibit heat transfer by thermal radiation. Thermal energy may also be transferred via conduction or convection and radiant barriers do not necessarily protect against heat transfer via conduction or convection. For window coverings the radiant-barrier material is usually a thin aluminized film. Radiant-barrier performance is measured by the emmissivity (E) of the surface. Emissivity is expressed as a number between zero (0) and one (1) at a given wavelength. The higher the emissivity, the greater the emitted radiation at that wavelength, so radiant-barriers must have low emmissivity (usually less than 0.10, and often 0.05 or less). Thus, 90 to 95% of radiant energy can be reflected by a radiant-barrier. Multiple layers of radiant-barrier material does not decrease the heat flow by compounding E^n, but by something closer to E^(n/2). One of the big drawbacks to radiant-barriers is the increase in emmissivity if the surface gets dusty.