STEP 6: Calculate the heat loss through the above-grade envelope q (Btu/h) = U (Btu/h ft 2 °F) * A (ft 2 ) * DT (°F) q (Btu/h) = A (ft 2 ) * D T (°F) - Walls R (ft 2 °F/Btu/h) A (southwall) = 150’ x 12’ – 7 windows = 1,800 ft 2 – 7*(12 ft 2 ) = 1,716 ft 2 (south) = 1,716 ft 2 *(65⁰F – (-15.1⁰F)) = 7,466 Btu/h (18.41 ft 2 ⁰F/Btu/h) (north) = 1,716 ft 2 *(65⁰F – (-15.1⁰F)) = 7,042 Btu/h (19.52 ft 2 ⁰F/Btu/h) q (west) = 456 ft 2 *(65⁰F – (-15.1⁰F)) = 1,871 Btu/h (19.52 ft 2 ⁰F/Btu/h) (east) = 456 ft 2 *(65⁰F – (-15.1⁰F)) = 1,871 Btu/h (19.52 ft 2 ⁰F/Btu/h)
