Bay, Bow and Garden Windows Under Title 24
Three separate rules can meet in one bay, bow or garden window. Dual-glazed garden windows get up to 30 square feet of relief from the 0.40 U-factor average, a bay window roof and floor are insulated like a wall, and the job is an addition only if the bay adds floor area as well as volume.
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Garden and greenhouse windows: 30 square feet of relief
Section 100.1 describes a greenhouse or garden window as a three-dimensional, five-sided unit that generally protrudes from the wall. Its default U-factor misses the mandatory 0.40 (the manual's example, a metal-framed unit, defaults to 1.40), so Exception 2 to Section 150.0(q) excuses up to 30 square feet of dual-glazed units, measured at the rough opening. Any area beyond that is averaged with other windows. In an alteration a dual-glazed unit also satisfies the prescriptive U-factor automatically, but the prescriptive SHGC limit for your zone still applies. Zones 1, 3, 5 and 16 have none.
Bay and bow windows: roof and floor like a wall
A bay window is three or more windows joined side by side or set in opaque assemblies, projecting from the wall with the end panels angled to the center. Section 150.0(c)4 requires its roof and floor to meet the wall insulation requirements of Table 150.1-A, which for framed walls is U-0.048 in most zones and U-0.065 in zones 6 and 7. The glass itself is ordinary fenestration.
Addition or alteration
Classification turns on floor area. The manual lists a bay that extends from floor to ceiling, increasing both floor area and conditioned volume, as an addition. A greenhouse window adds volume but no floor area, so it is an alteration. Additions follow the size bands and glass caps of Section 150.2(a), explained on the room addition page, while alterations are judged component by component. The department makes the final call on any odd projection.
What to put on the plans
- The rough-opening area of each unit and whether it is dual-glazed.
- The NFRC or default U-factor and SHGC on the window schedule.
- The insulation in a bay roof and floor, drawn in section.
The 30 square foot allowance is mentioned briefly on windows and Title 24. How U-factor and SHGC trade against each other is on which number matters, and averaging mixed products is on mixing window types. Our residential reports handle both.
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. Exception 2 to Section 150.0(q) relaxes only the maximum U-factor. The prescriptive SHGC limit for the zone remains.
No. The definition says dual-glazed greenhouse windows add conditioned volume but not conditioned floor area.
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