Do skylights count toward the window area limit?

Yes. Total fenestration under §150.1(c)3B includes skylights, and a skylight pitched at less than 1:12 also counts as west-facing.

What U-factor does a replacement skylight need?

A U-factor of 0.40 or lower and an SHGC of 0.30 or lower, under Exception 2 to §150.2(b)1B.

Is a tubular skylight regulated?

Yes, as fenestration. Up to 3 square feet of tubular skylight with a dual-pane diffuser per dwelling unit is excused from the prescriptive U-factor and SHGC; beyond that it needs an NFRC rating or a default value.

Does adding one small skylight need Title 24 compliance?

It is an alteration that adds fenestration, so the new skylight must meet the U-factor and SHGC limits, though up to 16 square feet is exempt from the area limits. The building department decides what documentation it wants.

Title 24 Skylight Requirements for Homes

A skylight is fenestration under Title 24, held to the same U-factor and SHGC as the home’s windows and counted in its glazing area. Two allowances matter most: in zones 2, 4 and 6 to 15 a new home may have up to 16 square feet of skylight at U-0.40 and SHGC 0.30, and a replacement skylight needs only U-0.40 and SHGC 0.30.

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What counts as a skylight

Section 100.1 defines a skylight as fenestration installed on a roof less than 60 degrees from horizontal, and skylight area as the rough opening. A skylight is not a separate category with its own looser rules; it sits inside the fenestration requirements. The mandatory cap in §150.0(q) says so in terms: fenestration, including skylight products, separating conditioned space from unconditioned space or outdoors must have a weighted average U-factor of 0.40 or lower, with up to 10 square feet or 0.5 per cent of the floor area exempt. Buildings built to the California Wildland-Urban Interface Code, Title 24 Part 7, in designated fire areas are excepted from that cap.

The values come from an NFRC rating under §110.6. A product without one takes a default from Table 110.6-A, and the defaults are punishing: the compliance manual’s tubular daylighting device, treated as a metal-frame, fixed, single-pane skylight, defaults to U-1.19.

New homes

On the prescriptive path, skylights join windows and glazed doors in the area-weighted U-factor and SHGC of Table 150.1-A: U-0.27 in zones 1 to 5, 11 to 14 and 16, U-0.30 in the rest, and SHGC 0.23 in most zones. Section 150.1(c)3A then gives two allowances:

Exception 3: in zones 2, 4 and 6 to 15, each dwelling unit may have up to 16 square feet of skylight with a U-factor of 0.40 and an SHGC of 0.30. The exception adds that zones 1, 3, 5 and 16 have no SHGC requirement.
Exception 2: up to 3 square feet of tubular skylight with a dual-pane diffuser per dwelling unit need not meet the U-factor and SHGC values.

Skylight area also counts toward the 20 per cent total fenestration limit, and a skylight pitched below 1:12 counts as west-facing whichever way it faces, which matters in the zones with a 5 per cent west-facing limit. Additions apply the same values within their own glazing caps, set out on does a room addition need Title 24.

Adding a skylight to an existing home

Cutting a new skylight into an existing roof is an alteration that adds fenestration. Under §150.2(b)1A the new skylight meets the U-factor, SHGC and area limits of §150.1(c)3A and Table 150.1-A. Two concessions apply: adding up to 16 square feet of new fenestration or skylight area is exempt from the total and west-facing area limits, and in zone 15 an alteration that adds glazing may use an SHGC of 0.23.

Replacing a skylight

Replacement is treated more gently. Exception 2 to §150.2(b)1B lets a replaced skylight meet a U-factor of 0.40 and an SHGC of 0.30 instead of the Table 150.1-A values. Replacing only the glass in an existing sash and frame, or a sash in an existing frame, is a repair rather than an alteration, provided the replacement performs at least as well as the original.

On the plans and on site

The plans examiner looks for the skylight U-factor and SHGC from the CF1R in a window and skylight schedule on the drawings, and the inspector checks the installed values. Where products above and below the target are averaged, the area-weighted worksheet CF1R-ENV-02-E shows the result. Commercial skylights, including the daylighting rule for large enclosed spaces, are a different set of requirements, covered on skylights in large spaces.