ADU Window U-Factor Requirements
A new detached ADU needs windows and glazed doors with an area-weighted U-factor of 0.27 or lower in climate zones 1 to 5, 11 to 14 and 16, and 0.30 or lower in zones 6 to 10 and 15. Default table values start at 0.53, so the units need an NFRC rating.
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Which number your ADU takes
The table figure governs a new detached ADU and, through the addition rules, an attached ADU or garage conversion that adds conditioned space. A new unit of 500 square feet or less in zone 5 may use 0.30 in place of 0.27. In a room that is already conditioned, replacing up to 75 square feet of glass only has to reach 0.40, while windows you add meet the table value.
The 0.40 ceiling sits underneath
Section 150.0(q) caps the weighted average U-factor of all fenestration at 0.40 in every ADU. Up to 10 square feet or 0.5 percent of floor area, whichever is greater, is exempt. Dual-glazed garden windows get 30 square feet. Buildings meeting the Wildland-Urban Interface Code in designated fire areas are excepted from this cap only; the wildfire windows page sets out the limits.
What the label and the default table say
The compliance value is the whole product, frame included, rated to NFRC 100 or taken from Table 110.6-A. Every double-pane default in that table is 0.53 or higher, and the compliance manual says no default product meets 0.40 alone. Add 0.05 for true divided lites, or for dividers between the panes with a spacer under 7/16 inch. An unrated unit carries a label that begins with the words CEC Default U-factor, then the value, and the temporary label stays on until the inspector has seen it.
Glass doors count as windows
An exterior door with 25 percent or more glass is a glazed door and joins the window schedule at the same limit. Up to 3 square feet of door glass per dwelling unit is exempt. A solid door is a separate test, covered on the exterior door page.
Next step
List the NFRC U-factor of every unit on the window schedule. Solar gain has its own limit on the ADU window SHGC page, frame questions are on aluminum windows, and your zone is on the climate zone page. To order, use the ADU reports page.
Common Title 24 Questions
- Do ADUs need Title 24 in California?
- What plans are needed for an ADU Title 24 report?
- How much does an ADU Title 24 report cost?
- How long does an ADU Title 24 report take?
- Does a garage conversion ADU need Title 24?
- Does a JADU need a Title 24 report?
Frequently Asked Questions
It follows the climate zone: 0.27 in zones 1 to 5, 11 to 14 and 16, and 0.30 in zones 6 to 10 and 15. A new ADU of 500 square feet or less in zone 5 may use 0.30.
On the prescriptive path the U-factor is an area-weighted average of the windows and glazed doors, so yes, while the 0.40 mandatory weighted average still holds.
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View all ADU and JADU guides → · Permits and Compliance · Title 24 guide