Title 24 U-Factor vs SHGC: Which Number Matters?
Both, but not equally everywhere. The prescriptive table caps U-factor in all 16 climate zones, at 0.27 or 0.30. It caps SHGC at 0.23 only in zones 2, 4 and 6 to 14, and at 0.20 for new homes in zone 15. In zones 1, 3, 5 and 16 the U-factor is the only window number the table sets.
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Where each limit applies
- Zones 1, 3, 5 and 16: U-0.27 and no SHGC limit.
- Zones 2, 4 and 11 to 14: U-0.27 and SHGC 0.23.
- Zones 6 to 10: U-0.30 and SHGC 0.23.
- Zone 15: U-0.30 and SHGC 0.20 in a new home, 0.23 for replacement and added glass.
These come from Table 150.1-A. The U-factor also has a mandatory ceiling in every zone, a weighted average of 0.40 under Section 150.0(q), while no mandatory SHGC exists.
What each number measures
U-factor is heat flow through the whole product, in Btu per hour per square foot per degree Fahrenheit, counting glass edge, frame and mullions; lower is better. SHGC runs from 0 to 1 and is the share of the sun's heat that gets through; lower blocks more. A manufacturer's centre-of-glass figure is not the one the code uses.
Where a low SHGC backfires
In zones with no SHGC limit, the manual warns that a window with an SHGC of 0.35 or lower can raise heating loads against the standard design, because solar gain helps in winter. In warmer zones, such as 8 to 15, a lower SHGC cuts cooling loads instead.
Replacement shortcuts and trades
Exception 1 to Section 150.2(b)1B sets the 75 square foot relief at U-0.40 in all zones and SHGC 0.35 in zones 2, 4 and 6 to 14, with Exception 3 holding zone 15 to 0.23. U-factor and SHGC are each area-weighted separately on the prescriptive path, so a window that beats one number does not rescue the other. Exterior shading can meet the SHGC requirement under Section 150.1(c)4 but never the U-factor. The performance method scores both against a standard design and lets a strong feature offset a weak one.
VT, the third number on the label, is informational only. Zone values are on the climate zone page, and the rest of the window rules are on new windows and Title 24. See also exterior shading credit and aluminum windows. Our residential reports run both methods.
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
Not for compliance. Zones 1, 3, 5 and 16 set no SHGC limit in Table 150.1-A, and a very low SHGC can raise heating loads there.
No. The mandatory Section 150.0(q) limit is a U-factor of 0.40. SHGC appears only in the prescriptive package and in performance modelling.
Explore U-Factor and SHGC
- Where Window U-Factor and SHGC Appear: CF1R, Plan Set, Label and Installer Certificate
- Are Double-Pane and Low-E Windows Required in California?
- Dynamic Glass and Electrochromic Windows Under Title 24
- Mixing Window Types: How the Area-Weighted Average Works and What to Do When One Misses
View all U-Factor and SHGC guides → · Windows and Building Envelope · Title 24 guide