During winter nights, field data revealed that night sky temperatures were much lower than the surface temperatures of the test roof systems, a situation that drives radiation heat loss to the sky. Good roof design should ideally limit heat gains during the summer while also limiting heat losses in the winter, which is why insulation works better than cool roof systems in cold weather.
ASTM C1340-04, “Standard Practice for Estimation of Heat Gain of Loss through Ceilings under Attics Containing Radiant Barriers by Use of a Computer Program,”  was benchmarked against field data for the insulated and ventilated shingle roof system (Figure 4 and 5). Simulations were made for the hot climates of Miami; Austin, TX; Atlanta and the cold climate of Baltimore with and without air-conditioning ducts in the attic. An attic of 1539 square feet (143 m2) with a roof slope of 18 degrees was modeled with and without a cool color shingle roof; the cool color shingle’s solar reflectance was 0.25. The supply duct surface area was set at 304 square feet (28.7 m2). The return duct assumed 176 square feet (16.4 m2) of surface area exposed in the unconditioned attic. Energy Plus  estimated the hourly run times for an air-conditioner certified with a Seasonal Energy Efficiency Ratio (SEER) of 13 and for an 85 percent efficient gas furnace that heated the home. The hourly indoor air temperature for the house and the run time for the HVAC were estimated by Energy Plus and read by AtticSim to better estimate the roof and attic load as coupled to the building envelope.