Do New Windows Need Title 24?

Usually yes. Fenestration is one of the components the Energy Code regulates directly, so replacing or adding windows brings a project into scope even when nothing else changes.

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The numbers that apply

On the prescriptive path, windows have to meet the 2025 component package. The maximum U-factor is 0.27 in climate zones 1 to 5, 11 to 14 and 16, and 0.30 in zones 6 to 10 and 15. Solar heat gain coefficient is capped at 0.23 in most zones and 0.20 in zone 15, the low desert. Zones 1, 3, 5 and 16 carry no SHGC requirement: in the cool coastal and mountain zones, keeping solar gain out is not the problem it is in the valleys and the desert.

Every house also has a mandatory ceiling: the weighted average U-factor of its windows and skylights cannot exceed 0.40. Up to 10 square feet of glazing, or 0.5 per cent of the floor area if that is larger, is exempt from that ceiling, and so are up to 30 square feet of dual-glazed greenhouse or garden windows.

Replacing windows in an existing house

Replacement windows of the same total area normally have to meet the same U-factor and SHGC limits. Small jobs get relief: replacing up to 75 square feet of windows only needs a U-factor of 0.40 or lower, and an SHGC of 0.35 or lower in zones 2, 4 and 6 to 15. In zone 15 a replacement still has to reach an SHGC of 0.23. New glass in an existing sash or frame counts as a repair, as long as it performs at least as well as the original.

Adding window area is treated differently: the new glazing has to meet the U-factor, SHGC and area limits of the new-home package, although adding up to 16 square feet is exempt from the area limits.

What the values mean when buying windows

U-factor measures heat flow through the assembly — lower is better, and 0.27 to 0.30 is a good double-glazed low-emissivity unit rather than anything exotic. SHGC measures how much solar energy comes through; 0.23 is a fairly selective coating. Both are published on the manufacturer's label for the whole unit, frame included, and it is the whole-unit figure the code uses, not the centre-of-glass number that sometimes appears in marketing.

Area matters as much as performance

On additions over 700 square feet, glazing is capped at the greater of 175 square feet or 20 per cent of the addition's floor area, and smaller additions have their own caps (see Does a room addition need Title 24?). A design can therefore fail on how much glass it has even when every unit specified meets the performance numbers. Where that happens, the performance path allows the extra glazing to be offset elsewhere — but it has to be modelled rather than asserted.

On a straight replacement in an existing opening, the test is normally just the performance of what goes in. Enlarging the opening changes that, because it changes the envelope. For frame materials, see whether aluminum windows can comply.

Common Title 24 Questions

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