How the check works
Title 24 sets three prescriptive limits for glazing in low-rise residential buildings. First, total window area is capped at roughly 20% of conditioned floor area — anything above that has to be carried by the performance compliance path. Second, the window U-factor (heat-loss rate) must be at or below the prescriptive value for your climate zone — 0.30 in most of the state, 0.27 in cool-marine zones (CZ6, CZ7, CZ8, CZ9, CZ10, CZ15). Third, the solar heat gain coefficient (SHGC) must be at or below the prescriptive value, which tightens in hot inland zones (CZ8, CZ9, CZ10 drop to 0.20, CZ12 to 0.18, CZ15 to 0.20) and loosens in cool-marine zones that benefit from passive solar (CZ1, CZ3, CZ5, CZ16 → 0.35).
Exceeding the prescriptive ratio
The 20% window-to-floor cap is a prescriptive trigger, not an absolute ceiling. Plenty of California homes carry more than 20% glazing — they just have to demonstrate compliance through the performance path, which models the proposed building against a standard reference building and offsets extra glass with envelope or HVAC upgrades. The other two metrics (U-factor, SHGC) are also adjustable on the performance path. See how Title 24 compliance works for the prescriptive-vs-performance trade-off.
Why the SHGC value changes by zone
Hot inland zones (CZ8 through CZ15) penalize summer cooling load. Low SHGC glass blocks solar heat gain — critical when peak cooling demand drives both equipment sizing and TDV cost. Cool marine zones (CZ1, CZ3, CZ5, CZ16) reverse this: passive solar gain offsets winter heating load, so the prescriptive baseline lets in more sun (SHGC 0.35). Hand-picking windows for the wrong zone is one of the most common Title 24 plan-check corrections.