# §150.1(c)1: Prescriptive envelope baseline
What it requires
The prescriptive package sets fixed insulation R-values for every major opaque surface on a newly constructed residence. They change by climate zone but not by house size or design. Meet all of them and the envelope passes without a performance trade-off model.
The headline numbers, summarised:
- Ceilings. R-38 in most California climate zones; R-30 in CZ3, CZ5, CZ6, CZ7 (milder coastal climates where the cooling load doesn't justify the extra inch of attic insulation).
- Walls. R-15 cavity + R-4 continuous insulation in 2x4 construction, or R-21 cavity + R-5 CI in 2x6, depending on climate zone. The continuous-insulation sheathing is the expensive part.
- Floors over crawl / unconditioned. R-19 minimum, R-30 in colder zones (CZ1, CZ2, CZ16).
- Slab-edge insulation. R-7 for 24 inches in CZ16 (the only zone where slab-edge is prescriptive).
- Radiant barrier. Required in CZ2–CZ15 (suppressed in CZ1, CZ16, and indirectly via the cool-roof rule in CZ10–CZ15 — see §110.10).
When it applies
Newly constructed single-family homes and detached ADUs. Additions reference this section as their own baseline under §150.2(a). Alterations only trigger it on the specific component being altered.
How we use it
Your project's climate zone (resolved from the project ZIP) sets the prescriptive numbers automatically. The Confirm step's three design cards display the envelope each one carries:
- Prescriptive — the exact §150.1(c)1 numbers for your CZ.
- High-Performance Envelope — tightens ceiling and wall R-values where the calibration sweeps show it pays for itself.
- Hybrid — keeps the §150.1(c)1 cavity insulation but strips the continuous-insulation sheathing, buying it back with HVAC and DHW upgrades on the performance path.
Per-CZ values are visible on the climate-zone landing pages, e.g. Climate Zone 12.
Common gotchas
- Builders pricing "R-21 2x6" walls without realising the prescriptive package also wants R-5 CI on top — the CI is what drives the cost.
- R-30 vs R-38 ceiling confusion: the cutoff is climate-zone-based, not house-size-based.
- Slab-edge insulation gets value-engineered out early in design and then a CZ16 project has to model around it on the performance path.