Passive Solar Design Calculator

South Glazing -- the recommended window area -- and the Thermal Mass sized to absorb it both depend only on Floor Area and Heating Degree Days (HDD); Latitude and Wall Thickness never move either figure, since the glazing percentage formula (7% base, plus 0.8% per 1,000 HDD, capped at 20%) reads climate severity, not geographic position, and thermal mass is a flat 5.5x multiple of glazing area. Latitude does drive Overhang Depth, the summer-shading dimension, through a solar-altitude formula, but that relationship isn't smooth across the full 20-65 degree range: near latitude 23.5 degrees (the Tropic of Cancer), the underlying sun-angle math approaches a mathematical singularity, so this calculator clamps Overhang Depth to a practical 6-60 in band rather than reporting the unbounded raw result -- expect a coarse, non-monotonic jump in that output specifically for latitudes in the low-20s to mid-20s range, and treat the clamped value as a sign to consult a solar shading chart for your exact site rather than as a precise dimension.

Wall R-value and Thermal Lag both come from Wall Thickness alone (roughly R-0.2 and one hour of thermal lag per inch), independent of everything else on the form. HDD 5,500 and 40 degrees latitude (roughly Denver) size a 1,000 sq ft home to 114 sq ft of south glazing and 627 sq ft of thermal mass.