Mixing Window Types: How the Area-Weighted Average Works and What to Do When One Misses
No single window has to hit the number. On the prescriptive path, U-factor and SHGC are each averaged by area across windows, glazed doors and skylights on worksheet CF1R-ENV-02-E, with a few named carve-outs. If the average still misses, swap the weakest products, use an exception, or move to the performance method.
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Work the average yourself
Multiply each product's area by its U-factor, add the results, and divide by total area (an illustration, not from the code). Take 150 square feet at U-0.26 and 30 square feet of fixed glass at U-0.34: (39 + 10.2) divided by 180 gives 0.273. That passes a 0.30 limit (zones 6 to 10 and 15) and misses the 0.27 limit that applies in zones 1 to 5, 11 to 14 and 16. SHGC is averaged on its own, so a good U-factor never rescues a poor SHGC.
What the exceptions allow
- Up to 3 square feet of door glass per dwelling unit need not meet the table values.
- Neither must up to 3 square feet of tubular skylight with dual-pane diffusers.
- Up to 16 square feet of skylight may be as weak as U-0.40 and SHGC 0.30 in zones 2, 4 and 6 to 15, and zones 1, 3, 5 and 16 set no SHGC limit.
- Chromogenic glazing is considered separately and never averaged with ordinary glass.
Custom and site-built products
Per the manual's mixed-type example, every window meets Sections 110.6 and 110.7, and field-fabricated windows take defaults. To simplify data entry, the highest default U-factor of any planned window may be applied to all, but each type keeps its own SHGC. Unrated site-built windows within the area limit may use the Appendix NA6 method or the default tables.
The separate 0.40 ceiling
Section 150.0(q) is mandatory and apart from the prescriptive average: a weighted average U-factor of 0.40, with 10 square feet or 0.5 percent of floor area exempt. Homes under the California Wildland-Urban Interface Code, Title 24 Part 7, in designated fire areas are excepted.
If the average misses
Replace the worst product, check whether an exception above fits, or use the performance method, which trades glass against other measures. Exterior shading can fix an SHGC miss but never a U-factor miss.
Background is on U-factor versus SHGC, windows and Title 24 and unlabeled windows. Special cases: glass block and garden windows. Our residential reports include the worksheet.
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
No. Windows, glazed doors and skylights are area-weighted together, so some products may exceed the prescriptive value if the average meets it.
The manual points to worksheet CF1R-ENV-02-E for the area-weighted average, with CF1R-ENV-03-E for the SHGC worksheet.
Explore U-Factor and SHGC
- Title 24 U-Factor vs SHGC: Which Number Matters?
- 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
View all U-Factor and SHGC guides → · Windows and Building Envelope · Title 24 guide