# §150.0(a): Ceiling and roof insulation
What it requires
Every California residential ceiling assembly must carry a minimum insulation R-value:
- Below-attic ceilings (the most common case — drywall ceiling with vented attic above): R-22 mandatory minimum.
- Cathedral / vaulted ceilings (no attic space — insulation between rafters): R-19 mandatory minimum.
- Roof assemblies with insulation above the deck (insulated panels, foam-on-deck, etc.): R-13 mandatory minimum at the deck plus assembly-specific extras.
The mandatory floor is below the prescriptive numbers in §150.1(c)1 (R-30 or R-38 depending on climate zone). The two work as a stack: §150.0(a) sets the absolute floor that even performance trade-offs cannot go below; §150.1(c)1 sets the prescriptive target that's typically higher.
When it applies
Every California residential project — new construction, additions on the new ceiling, and alterations whenever the ceiling assembly itself is opened up.
How we use it
When you confirm your project on the Confirm step, we set the ceiling construction in the model from the per-climate-zone prescriptive values (R-30 in CZ3, CZ5, CZ6, CZ7; R-38 elsewhere) — which already clear the §150.0(a) mandatory floor. The High-Performance Envelope option drops to R-30 only for ADUs, additions, and large SFR (≥2,500 ft²); smaller homes stay at R-38 because the calibration sweeps showed the drop wasn't safe in that range.
The compliance verification report lists the ceiling R-value used in the model on the construction summary, and the AHJ field inspector verifies the installed batt or blown-in insulation against that value at rough-in.
Common gotchas
- Attic-access doors and hatches that are uninsulated — they're part of the ceiling assembly and the missing R-value drags the assembly average below the mandatory floor.
- Blown-in cellulose that settles 10–15% over the first year — the spec'd inches must include settling allowance to keep the installed R-value above the floor.
- Cathedral ceilings squeezed into 2x10 rafters with R-30 batts — the batts get compressed below their rated R-value if the cavity is too tight. R-30 needs a 2x12 rafter or a strip of foam.