Reflective roofs reduce the flow of heat into the building by reflecting most of the incident solar radiation during hot summer days. Having a well-insulated roof will also reduce the heat gains during the day. During those hours of the day when the ambient temperature is lower than the inside temperature, having high insulation in the roof would block the path of heat flow out of the building.
During the winter when the days are short and cloudy and the sun angle is low, a reflective roof may add a heating penalty. Therefore, we analysed the impact of the roof reflectance in terms of a trade-off with roof insulation. On that basis, the Envelope Subcommittee directed us to perform comprehensive simulations to analyse cooling energy savings and heating energy penalties of several prototypical buildings over a wide spectrum of climatic conditions.
The DOE- 2.1E building energy simulation program was selected as the tool to perform this analysis. We used a residential building prototype that ASHRAE has used extensively in support of developing criteria for Standard 90.2. The details of the prototypical building are summarized in Table 1. The building was simulated with electric cooling, electric heat pump, electric resistance heating, and gas heating systems.
Our simulations included prototypes with and without attics. These building were simulated for a variety of roof insulation and roof reflectance’s. The roof insulations included ceiling insulation levels: R-1, R-11, R-19, R-30, R-49.
Parametric for roof reflectivity included reflectance of 0.10, 0.25, 0.50, and 0.75. In addition, we modelled distribution system configurations with ducts in the attics with three levels of duct insulation (R-2, R-4, and R-6) and ducts in the conditioned space. For the prototypical buildings with an attic, a fractional leakage area of 1:300 for the attic was assumed.