Thermal Emittance

Thermal emittance (also called emissivity) describes how efficiently a surface cools itself by radiating heat as far-infrared energy. It is rated on a scale from 0 to 1. A surface with high emittance releases absorbed heat readily; a surface with low emittance holds on to heat and stays hotter.

Role in cool roofs

A roof heats up by absorbing sunlight and cools by convection (air moving over it) and radiation. Lawrence Berkeley National Laboratory defines a cool roof as one with both high solar reflectance and high thermal emittance. The two properties are independent because they involve different parts of the spectrum: solar reflectance concerns sunlight, while thermal emittance concerns the much longer infrared wavelengths a warm surface emits. The US EPA notes that thermal emittance matters most in warm, sunny climates.

Typical values

Most non-metallic roofing materials, including membranes, shingles, tile, coatings and painted metal, have high emittance. Bare, shiny metal surfaces, such as unpainted aluminium or galvanized steel, have low emittance. That is why a bright bare metal roof can reflect sunlight well yet still run warmer than expected: it absorbs less heat but releases what it does absorb slowly.

Measurement and rating

Thermal emittance is measured with standard laboratory methods and reported, along with solar reflectance, by rating programs such as the Cool Roof Rating Council. Values are often reported both when new and after outdoor aging.

Night-time effects

High emittance helps a roof shed heat to the night sky. On clear nights this can cool the roof surface below air temperature. That benefits cooling in hot climates but can contribute to condensation on the underside of roof decks in some climates if moisture is present.

Combined index

Solar reflectance and thermal emittance are combined in the Solar Reflectance Index (SRI), which estimates how hot a surface will get in the sun relative to standard black and white surfaces.