The insulation subcontractor is on the roof, the framing inspector has already signed off, and somebody asks the question that stops the job: is this enough? Not enough for comfort, or for the utility bill — enough for the inspector. In California that question has a published answer, assembly by assembly.
Here is the short version for a newly built single-family home. Ceilings and rafter roofs must hit an area-weighted U-0.043, which the code says can be met with R-22 between wood framing. Walls must hit U-0.095 in two-by-four framing, meetable with R-15, or U-0.069 in two-by-six, meetable with R-21. Raised floors must hit U-0.037, meetable with R-19 (§ 150.0).
Key numbers
| Where | New construction | The remodel-friendly alternative |
|---|---|---|
| Ceilings and rafter roofs | U-0.043, or R-22 between framing | Alterations: U-0.054, or R-19 |
| Walls, two-by-four framing | U-0.095, meetable with R-15 | Existing walls already at U-0.110 or R-11 are excepted |
| Walls, two-by-six or greater | U-0.069, meetable with R-21 at 16 inches on center | — |
| Raised floors | U-0.037, meetable with R-19 | Crawl-space exception, with conditions |
| Roof decks in new attics (certain climate zones) | U-0.184 | Excepted where ducts are inside, ductless, or 12 feet or less of supply duct is outside |
All of the above: § 150.0
Two numbers, one idea
Insulation gets described two ways in the same paragraph, which is where most confusion starts.
R-value measures resistance to heat flow through the insulation itself — higher is better, and it is the number printed on the batt at the store. U-factor measures heat flow through the whole assembly, framing included — lower is better, and it is the number the code actually sets. Wood studs conduct heat around insulation, so a wall stuffed with R-15 does not perform like R-15 overall. That gap is exactly why the code writes the requirement as a U-factor and then offers the R-value as a shortcut for ordinary wood-framed construction.
Practically: if your assembly is conventional, use the R-value. If it is unusual — steel framing, thick exterior foam, masonry — the U-factor is the one that governs and someone will need to calculate it.
The rules that catch people
The attic hatch. A small item with an outsized failure rate:
"Attic access doors shall have permanently attached insulation using adhesive or mechanical fasteners. The attic access shall be gasketed to prevent air leakage" — § 150.0(a)3
Loose-fill depth. Blown insulation is sold by coverage, and it settles. The code requires the installed weight per square foot to meet the manufacturer's design weight for the labeled R-value — so the bags-used count on the certificate is the evidence, not a visual glance at the depth.
Ducts outside the thermal envelope. In certain climate zones the code requires roof decks in newly constructed attics above conditioned space to be insulated to U-0.184 — with exceptions when the air handler and ducts sit entirely inside conditioned space, when the system is ductless, or when 12 linear feet or less of supply duct (including the air handler and plenum) is in unconditioned space. Read from the other direction, that is the code paying to fix a problem it would rather you designed away: ductwork running through a hot attic.
Insulation has to touch what it insulates. It must be installed in direct contact with the roof or ceiling, and that surface has to be sealed against air moving through it. A gap between batt and drywall is a bypass, and air leakage defeats insulation faster than a slightly lower R-value does.
New homes, additions, and everything else
The mandatory features above are written for newly constructed buildings. Which of them reach an addition or an alteration is decided separately, in the code's provisions for additions and alterations to existing residential buildings. Some carry over unchanged; some, like the ceiling rule, carry over at a lower level; some do not apply at all. Assuming a remodel is held to the new-construction standard is a common and expensive mistake — and so is assuming the opposite.
There is also more than one route to compliance. The code sets out both performance and prescriptive approaches for single-family homes (Chapter 8). Prescriptive means meeting each listed number. Performance means modelling the whole house and showing it uses no more energy than a compliant version would — which is how a project trades a weaker wall for better windows or a better heating system. The mandatory measures in this article apply either way; that is what "mandatory" means here.
And a separate layer of requirements applies to systems, equipment and building components across all occupancies (Chapter 2) — the manufacture and installation standards that sit underneath the house-level rules.
Local rules can go further
Your city can require more. As the code's own preface states, a city or county may adopt more restrictive building standards where local climatic, geological or topographical conditions justify them, with findings filed with the state — and local standards adopted against an earlier edition do not carry forward automatically (preface). The state number is a floor. Nobody may go below it; plenty of jurisdictions sit above it.
Guides to the rest of California's code parts, each linked to the current text, are on our building codes hub.
Sources
- 2025 California Energy Code § 150.0 — mandatory features and devices for single-family homes
- 2025 California Energy Code, Chapter 9 — additions and alterations to existing residential buildings
- 2025 California Energy Code, Chapter 8 — performance and prescriptive compliance approaches
- 2025 California Energy Code, Chapter 2 — mandatory requirements for systems, equipment and components
- 2025 California Energy Code — preface on publication and more restrictive local standards
