Does the 75 square foot relief apply to the whole house?

It applies to vertical replacement glass totalling no more than 75 square feet. A whole-house job usually exceeds it, so the Table 150.1-A values are the target.

Can I replace every window with different brands?

Yes in principle. The U-factor and SHGC are averaged by area, so each product needs its own rating and the average must meet the table.

Whole-House Window Replacement: Title 24 Requirements

Replacing every window is an alteration under Section 150.2(b)1B. The 75 square foot relief is used up quickly, so the climate zone values normally apply, averaged by area across the new windows. When the average falls short, the performance method can trade the shortfall against other upgrades.

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Count the glass wall by wall

Glass that replaces an equal or smaller area in the same existing wall or roof is "altered", even when the new openings are not exactly where the old ones were. Area beyond that is "new" and must meet the added-glass rules. The Commission's 2025 manual still sends readers to its 2022 edition for this definition.

Past 75 square feet, the zone table applies

Exception 1 to Section 150.2(b)1B covers vertical replacement glass of no more than 75 square feet. Only the replaced glass counts: in the manual's example, 50 of 85 square feet of a south wall is replaced, so the 50 is what is tested. A whole-house job is usually far larger; the manual's zone 12 example replaces about 300 square feet. Section 150.1(c)3A then judges the area-weighted average U-factor and SHGC against Table 150.1-A, which means a weaker window can be offset by stronger ones.

When the average falls short

The manual's example of an all single-pane metal house replacing every window with nothing else changing gives two routes: meet Section 150.2(b)1B, or use the Existing plus Alteration performance method. Table 150.2-G sets the comparison. Without verification the standard design is a code-level window. With verification by an ECC-Rater (the former HERS rater, renamed on 1 January 2026) it is the existing window's U-factor and SHGC as verified, which can turn the old glass into a credit.

Mandatory limits sit on top

Section 150.0(q) caps the weighted average U-factor of all fenestration at 0.40, with the first 10 square feet or 0.5 percent of floor area exempt. The manual's window examples still use some 2022 names (Table 150.2-D, and the added-glass exception numbered 1); the 2025 Standards use Table 150.2-G and number it Exception 2.

For the single-pane case in depth, see remodels with existing single-pane windows. A smaller job fits replacing one window, the zone numbers are on new windows and Title 24, and the whole-project view is remodel Title 24. Our residential reports model both routes.

Common Title 24 Questions

Frequently Asked Questions

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