Emittance Impacts Energy Use

In a recent study, the Heat Island Group analyzed the relative impact of roof reflectance and emittance on heat and cooling energy use. Link to: Emittance.

ISSUE: If highly reflective roofs reduce cooling-energy use, should low-emittance roofs reduce heating-energy use?

To address this question, the Heat Island Group carried out DOE-2 simulations of four prototype buildings (an old home, a new home, an old small office, and a new small office) in all climate regions for a wide range of roof absorptivity (1 minus roof reflectivity) from 0.2 to 0.8 and roof emissivity from 0.25 to 0.9.

For an old home in a hot climate, decreasing the absorptivity (increasing the reflectivity) of the roof decreased annual energy bills by about $1.00 to $1.40 per square meter of roof area. In a cold climate, the savings were about $0.15 to $0.20. Also in a hot climate, decreasing the emissivity of the roof increased annual energy bills by about $0.30 to $0.70 per square meter of roof area. In a cold climate, the impact was nil. The same results were obtained by simulating a new small office building.

While increasing the reflectivity of a roof reduces annual cooling-energy use in all climates, the impact of decreasing the emissivity of a roof depends on the climate. In a hot climate, decreasing the roof emissivity increases energy use by up to 10%. In a temperate climate, decreasing the roof emissivity essentially has no effect on energy use. Finally, in a cold climate (no cooling energy use), decreasing the roof emissivity decreases energy use by up to 3%.