Can Existing Windows Stay During a Home Addition?
Yes. Windows in the existing house that you do not touch are not required to change because an addition is built beside them. Regulated windows are the ones in the addition, windows you replace or enlarge, and windows moved or reused in the new space. A window removed with its wall drops out of the model.
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Three fates for an existing window
- It stays in place. Under the existing + addition + alteration approach, unaltered components are not brought into compliance, and nothing requires alterations to the existing house.
- It goes with a demolished wall. The removed wall and window are ignored by the software, area included (Example 9-50).
- It moves into the addition. A relocated or reused window is treated as new, with its actual or default U-factor and SHGC, so an old unit may miss the prescriptive limits and push the job to performance. It also counts toward the 0.40 weighted-average U-factor, with 10 square feet or 0.5 percent of floor area excused (Example 9-10).
What the new windows are held to
Section 150.2(a) applies the new-home glass limits to the addition: U-0.27 in zones 1 to 5, 11 to 14 and 16, U-0.30 in zones 6 to 10 and 15, SHGC 0.23 in zones 2, 4 and 6 to 14 (zone 15 is 0.20, or 0.23 for additions over 700 square feet), and a total area cap set by size. A 280 square foot addition may carry the greater of 75 square feet or 30 percent, which is 84 square feet. A 400 square foot addition gets 30 percent when modeled with the existing house but only 20 percent as an addition alone, the manual notes.
When replacing the old ones pays off
Swapping existing windows during the project makes them altered components under Section 150.2(b)1B. If an ECC-Rater verifies the old windows before construction, the baseline is what is really there, and a verified single-pane metal operable window at U-1.28 leaves room for a trade.
The junction question
An existing glass door opening onto a new sunspace can stay if the space is truly unconditioned, so only its lighting must comply, though a building official may require habitable space to be conditioned (Example 9-1). Otherwise the room is an addition: see sunroom Title 24.
More: windows, room additions, a 500 square foot addition, prescriptive or performance and the residential report.
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. Nothing in the addition rules requires altering the existing building, and unaltered components are not required to be brought into compliance.
Yes, but it is treated as a new window using its actual or default U-factor and SHGC, which may only be workable through the performance approach.
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View all Windows and Doors guides → · Windows and Building Envelope · Title 24 guide