What window U-factor and SHGC does a new California ADU need?

On the prescriptive path, a new detached ADU's windows and glazed doors need a maximum area-weighted U-factor of 0.27 in most climate zones (0.30 in zones 6 through 10 and 15). The maximum Solar Heat Gain Coefficient (SHGC) is 0.23 in most zones and 0.20 in zone 15. Total window area is capped at 20 percent of conditioned floor area.

Prescriptive values from Table 150.1-A

• Maximum U-factor 0.27: climate zones 1 to 5, 11 to 14, and 16.
• Maximum U-factor 0.30: climate zones 6 to 10, and 15.
• Maximum SHGC 0.23: climate zones 2, 4, and 6 to 14.
• Maximum SHGC 0.20: climate zone 15.
• No SHGC requirement: climate zones 1, 3, 5 and 16.
• Window area: no more than 20 percent of conditioned floor area in total, and no more than 5 percent west-facing in climate zones 2, 4, and 6 to 15.

Exceptions that help small ADUs

• New dwelling units of 500 square feet or less in climate zone 5 may use a maximum U-factor of 0.30.
• Up to 3 square feet of glazing in doors, and up to 3 square feet of tubular skylights with dual-pane diffusers, are exempt per dwelling unit.
• Up to 16 square feet of skylight per dwelling unit may have a U-factor up to 0.40 and an SHGC up to 0.30 in climate zones 2, 4 and 6 to 15.

The mandatory floor

Whatever path you use, Section 150.0(q) caps the weighted average U-factor of all fenestration at 0.40. Small exceptions cover the greater of 10 square feet or 0.5 percent of floor area, dual-glazed garden windows, and buildings in designated fire hazard or wildland-urban interface areas built to that code.

ADU additions are different

Attached ADUs and garage conversions follow Section 150.2(a). It uses the Table 150.1-A U-factor and SHGC values but swaps in size-based area limits. Additions over 700 square feet can have the greater of 175 square feet or 20 percent of floor area. Additions of 401 to 700 square feet can have the greater of 120 square feet or 25 percent, and additions of 400 square feet or less the greater of 75 square feet or 30 percent. West-facing glass is also limited in climate zones 2, 4 and 6 to 15.

When to use the performance path

If you want a big glass wall, a lot of west-facing glass or a specific product that misses the prescriptive values, the performance path can trade window performance against insulation, equipment efficiency or other measures.

What this means for your MEP plans and Title 24 report

The CF1R lists each window's U-factor, SHGC and area, so the window schedule on the architectural plans has to match. Window performance also feeds the heating and cooling load calculation behind the mechanical plan. A late window substitution can change both. See why your climate zone matters, Title 24 in climate zone 9, and Title 24 reports.

Sources: energy.ca.gov, energy.ca.gov, energy.ca.gov.