What you're looking at
A horizontal section through a double-pane window with a low-emissivity coating on surface 2 (the inside face of the outer pane) and an argon-filled gap between the panes.
- U-factor (heat-loss arrow, pointing out) measures how much heat the whole window assembly loses per square foot per degree F per hour. Lower is better for winter performance. - SHGC -- the Solar Heat Gain Coefficient -- is the fraction of incoming solar radiation that ends up as heat in the space. Lower SHGC keeps summer cooling loads down; higher SHGC lets winter sun warm the room.
Why the prescriptive values vary by zone
The Title 24 standard design tightens U-factor in hot zones (where window conduction is a problem all summer) and varies SHGC dramatically across CA:
| Zones | Window U | SHGC | |---|---|---| | CZ1, CZ3, CZ5, CZ16 | 0.27 | 0.35 (let sun in) | | CZ2, CZ4 | 0.27 | 0.23 | | CZ6–10, CZ15 | 0.30 | varies 0.20–0.23 | | CZ12 | 0.27 | 0.18 (tightest) | | CZ11, CZ13, CZ14 | 0.27 | 0.23 |
The CZ12 / CZ15 / CZ8–10 tightenings exist because peak cooling load fails the standard otherwise -- inland summer sun through average glazing overwhelms even oversized HVAC.