What does emissivity measure in a material?

Emissivity is a measure of a material's ability to emit thermal radiation. It is a dimensionless value ranging from 0 to 1, where 0 indicates a perfect reflector (not emitting any thermal energy) and 1 indicates a perfect emitter (emitting all the heat energy that it absorbs). Materials with high emissivity are efficient at radiating heat away, which is particularly important in applications involving thermal management, such as roofing. In the context of built-up roofing (BUR), understanding a material's emissivity can help in selecting appropriate roofing materials that manage heat effectively, contributing to energy efficiency in buildings.

The other choices refer to different physical properties. Heat resistance pertains to how well a material can withstand heat without degrading or conducting it, insulation relates to a material's effectiveness in reducing heat transfer, and reflectivity is about how well a surface can reflect sunlight rather than absorbing it. Each of these concepts is pertinent in their own right, but they do not define emissivity.