Skylight Replacement: Title 24 Requirements
Replacing a whole skylight is an alteration, and Exception 2 to Section 150.2(b)1B lets it meet U-0.40 and SHGC 0.30 instead of the window values. Swapping only the glass in the existing frame is a repair. The catch is the rating: every default value for an unrated skylight is above 0.40.
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Which of three jobs is it
- New glass in the existing sash and frame. A repair, provided the replacement performs at least as well as the original.
- A whole unit in the same roof surface. An alteration, listed in the manual as replacing an existing skylight, held to the Exception 2 values.
- A larger or additional skylight. Added fenestration, which meets Section 150.1(c)3A and Table 150.1-A; zone 15 uses SHGC 0.23, and up to 16 square feet of new glass is exempt from the total and west-facing area limits.
What counts as a skylight
Section 100.1 defines a skylight as fenestration installed on a roof less than 60 degrees from horizontal. Skylight area is the area of the rough opening for the skylight. A glazed unit on a slope of 60 degrees or more is a window in a wall, which changes the target values. The mandatory cap of Section 150.0(q) names skylights: the weighted average U-factor of all fenestration is 0.40 or lower, with the first 10 square feet or 0.5 percent of floor area exempt.
Why the label decides it
An unrated skylight takes its U-factor from Table 110.6-A. The double-pane defaults are 1.30 for a metal frame, 1.11 with a thermal break and 0.84 for nonmetal; single pane is 1.98 for metal and 1.47 for nonmetal. Every one is above the 0.40 that Exception 2 allows, so a manufactured replacement skylight in practice needs an NFRC rating. Check the figures on the carton against the permit set before ordering; NFRC labels and window approval shows what the label carries.
The usual mistake
Treating a bigger skylight as a like-for-like swap. The extra rough opening is new area, and in a roof it can also tip the west-facing tally, because a skylight pitched below 1:12 counts as west-facing whichever way it faces.
The full rules for new homes and additions are on Title 24 skylight requirements; window numbers are on new windows and Title 24. See also the no-label case. Our residential reports cover roof and glazing alterations.
Common Title 24 Questions
- When is Title 24 required for a home?
- Does a remodel need Title 24?
- Does a room addition need Title 24?
- What plans are needed for a residential Title 24 report?
- How much does a residential Title 24 report cost?
- How long does a residential Title 24 report take?
Frequently Asked Questions
Glass in an existing sash and frame, or a sash in an existing frame, is a repair if the replacement is at least equivalent in performance to the original.
U-0.40 or lower and SHGC 0.30 or lower under Exception 2 to Section 150.2(b)1B, and it must carry an NFRC rating or a default label.
Explore Skylights
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- Solar Tubes (Tubular Daylighting Devices): Title 24 Rules
- Title 24 Skylight Requirements for Homes
- How Many Square Feet of Skylight Can I Add Under Title 24?
View all Skylights guides → · Windows and Building Envelope · Title 24 guide