Does finishing a basement need Title 24?

Only where it changes what the Energy Code regulates. Conditioning an unconditioned basement makes it an addition. Finishing one that is already conditioned is an alteration, and only altered components such as lighting, windows or equipment must comply.

What insulation do basement walls need under Title 24?

Below grade, Table 150.1-A calls for continuous insulation of R-13 on the inside or R-5 on the outside in zones 1 to 13, more in zones 14 to 16. Above grade, the ordinary mandatory wall values apply.

Is a basement ADU an addition?

If the basement was unconditioned, yes. The Energy Commission treats converting an existing conditioned basement into an ADU or JADU as an alteration.

Does a Basement Conversion Need Title 24?

If the basement is unconditioned today, conditioning it is newly conditioned space and complies as an addition, with its walls below grade taking the Table 150.1-A values of R-13 on the inside or R-5 on the outside in zones 1 to 13. A basement that is already conditioned is an alteration instead, where only the altered components have to comply.

Choose your report
Upload your plans
Pay securely
Permit-ready report by email

First question: is it conditioned now?

The answer changes the whole project. Converting an unconditioned basement into heated or cooled living space adds conditioned floor area and volume, and §100.1 says newly conditioned space must comply with the requirements for an addition. The Energy Commission treats the opposite case just as clearly: converting an already conditioned basement, even into an ADU or junior ADU, is an alteration, and only the components altered, such as a new water heater, a mini-split, lighting or envelope changes, have to comply.

Whether the space counts as conditioned turns on the §100.1 definitions, not on whether it is finished. It is directly conditioned if it has heating above 10 Btu/h per square foot or cooling above 5 Btu/h per square foot, and indirectly conditioned if, for example, air from conditioned rooms passes through it at more than three air changes an hour.

Below-grade walls

This is where a basement differs from every other addition. The compliance manual says the mandatory wall insulation in §150.0(c) applies to heated basement foundation walls only where they are above grade; the portion below grade is governed by the prescriptive requirements. Table 150.1-A sets those as continuous insulation on one face of the wall, or an equivalent assembly U-factor:

Interior insulation: R-13 (U-0.077) in zones 1 to 15, R-15 (U-0.067) in zone 16
Exterior insulation: R-5 (U-0.200) in zones 1 to 13, R-10 (U-0.100) in zones 14 and 15, R-19 (U-0.053) in zone 16

Footnote 7 to the table defines interior as insulation on the inside surface of the wall and exterior as insulation on the outside. In an existing basement the inside face is usually the only one within reach, so the interior column tends to govern. Section 100.1 defines a below-grade wall as the portion of a wall enclosing conditioned space that is below the grade line, so a wall that is partly exposed is treated in two parts.

Floors

Table 150.1-A requires slab perimeter insulation only in zone 16, at R-7. A heated slab, such as radiant floor heating, is different: §110.8(g) and Table 110.8-A require insulation at its edge in every zone, R-5 in zones 1 to 15 and R-10 in zone 16, and for a below-grade slab the vertical insulation runs from the top of the foundation wall to the bottom of the foundation, or to the frost line if that is deeper.

The rest of the addition rules

Size sets the prescriptive package at the same 700 and 400 square foot lines as any addition. Points that come up in basements:

new egress or light-well windows are the addition’s fenestration, inside its glazing caps and U-factor limits
over 700 square feet QII applies, but existing headers need not be insulated and air sealing is not required where the existing air barrier stays (§150.2(a)1Av)
at 1,000 square feet or less the whole-dwelling ventilation airflow is excused, but a bathroom needs local exhaust
heating extended from the existing system brings its ducts under the sealing and testing rules, and the load calculation should cover everything the system serves

Lighting in the new rooms meets §150.0(k). Thresholds and caps are on does a room addition need Title 24, and the same addition-or-alteration split for ADUs is explained on garage conversion ADU Title 24.