How Does Window Area Affect Title 24 Calculations?
Glass area is measured against conditioned floor area, and 20 percent is the line. Above it, the reference house in the performance model stops counting your extra glass, so each added square foot must be recovered with better windows, insulation or equipment. A 2,000 square foot house reaches the line at 400 square feet of glass.
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The numbers behind the 20 percent line
The prescriptive package caps total fenestration at 20 percent of conditioned floor area in all 16 climate zones. Fenestration here means windows, glazed doors and skylights together. West-facing glass is also capped at 5 percent in climate zones 2, 4 and 6 through 15, which on the same 2,000 square foot house is 100 square feet.
What the software does with glass past the line
In a performance run, the reference house copies your window area up to 20 percent of floor area. Beyond that it stops at 20 percent, spreads the glass evenly over the four compass points, uses the prescriptive U-factor and SHGC, and has no overhangs. Your extra glass is compared with a house that never had it. A glass-heavy plan therefore usually moves to the performance method.
What gets counted
- A door with 25 percent or more glass is treated as a glazed door and counted as a window.
- Skylights count toward the total, and a skylight tilted in any direction counts as west-facing when its pitch is under 1:12.
Two rules that apply either way
Every house needs a weighted average window U-factor of 0.40 or better, apart from a small allowance of 10 square feet or 0.5 percent of floor area, whichever is greater. Windows without an NFRC rating fall back on default tables, and the double-pane defaults sit far above the 0.27 to 0.30 the package asks for, so an unlabelled window costs you in the model.
If the house already exists
An alteration that adds glass must meet the area limits, but adding up to 16 square feet of new window or skylight area is excused from them. Replacing glass of the same total area is judged on U-factor and SHGC only, as the single-window page shows.
Draw first, then model
Settle the window schedule first, because front orientation and west-facing glass are covered separately. Our residential reports start from the plans, and a preliminary set works if the glass is already drawn.
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 by itself. Glass past 20 percent of floor area is not allowed prescriptively, but the performance method accepts it when the rest of the design makes up the difference.
Yes. Total fenestration includes skylights, and a skylight with a pitch under 1:12 counts as west-facing.
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