# §150.1(c)3: Prescriptive fenestration
What it requires
Every window and skylight on a newly constructed residence must meet two NFRC-labelled values:
- U-factor — heat-transfer coefficient (lower is better). Prescriptive maximum is 0.30 for vertical glazing across most California climate zones, 0.28 in CZ1 and CZ16, and 0.55 maximum for skylights.
- SHGC — Solar Heat Gain Coefficient (lower means less solar heat through the glass). Prescriptive maximum varies: 0.25 in CZ1, CZ3, CZ5, CZ16 (cool, low-sun zones), 0.20 in CZ8, CZ9, CZ10 (cooling-driven), 0.18 in CZ12 (the tightest cell), and 0.23 elsewhere.
Doors with > 25% glazing count as fenestration and meet the same thresholds. Solid doors are covered separately under §150.0(g).
When it applies
- Newly constructed single-family homes and ADUs.
- Additions, on the new addition's windows (per §150.2(a)).
- Alterations, when ≥ 50% of the existing windows are being replaced (per §150.2(b)).
How we use it
The Confirm step extracts the U-factor and SHGC values from your plan's window schedule. If the schedule doesn't list NFRC ratings, we fall back to the §110.6 default values — which are intentionally pessimistic and almost always fail the prescriptive limits, forcing either a window upgrade or a performance-path model.
The High-Performance Envelope option keeps the same window ratings as prescriptive (the trade-off lives in the wall and HVAC values).
Per-CZ exact numbers live on the climate-zone landing pages — see Climate Zone 12 for the tightest cell, or Climate Zone 3 for a coastal-mild example.
Common gotchas
- "Generic" or stock windows on the schedule with no NFRC label — the §110.6 defaults apply and prescriptive almost always fails.
- Skylights that meet U-factor (0.55 is generous) but blow past the SHGC limit because they're horizontal and tinted glass costs more.
- French doors and sliders with glazing > 25%, designers sometimes spec them as "doors" thinking the U-factor doesn't apply, but §150.1(c)3 catches them.