The new standards and regulations use terms and concepts less familiar than those traditionally used to describe the cooling energy-saving attributes of roofing systems. Resistance to heat flow, and energy savings, have been typically quantified by the R-values of a structure's component building materials. The R-value of a roof system's components measures how well it resists the transmission of heat into a building in the summer or out of a building in the winter. A roofing system's R value does not, however, account for how well the roofing membrane reflects the sun's heat away from the exterior envelope of the building.

The effect of a membrane's solar reflectivity can be measured by making adjustments to the U-value of the roof assembly. Mechanical engineers use the U-value (the heat transfer coefficient) to determine how much heat energy is absorbed, reflected and transmitted through the building envelope (roof, walls, glass and slab). Low U values are energy efficient as are high R-values.

The principle features for an energy efficient membrane are its solar reflectance and its emissivity. Solar reflectance, sometimes referred to as albedo, measures how much of the solar energy striking a roofing membrane's surface is reflected away. Emissivity measures how efficiently absorbed heat energy radiates away from a roof membrane. See how some common building materials measure up.

Energy efficient roof systems have three characteristics in common:

1. Good reflectance. A high percentage of solar energy striking the membrane surface is reflected back into space before being converted into heat energy.

2. Sufficient insulation. The roof insulation resists the flow of heat into the structure from the small amount of energy that is absorbed by the membrane.

3. Good emissivity. The roof surface efficiently radiates the absorbed energy away from the structure, rather than retaining it and causing the roof assembly to get hotter.