What Title 24 actually requires

Mandatory measures first, then one path or the other

A single-family project in California has to meet the Energy Code’s mandatory measures and then, on top of those, either the prescriptive requirements or the performance requirements. It is not a choice among three: the mandatory measures apply under both paths. That one distinction explains most of what gets misread about what Title 24 “requires.”

Everything below is the 2025 code, which applies to buildings whose permit application is submitted on or after January 1, 2026, and to single-family work — one or two dwelling units, plus townhouses, which the code counts as single-family however many are in the row. Three or more dwelling units in one building, townhouses aside, is low-rise multifamily: a different set of sections entirely, and see multifamily.

The three layers, in order

Mandatory measures (Sections 110.0 through 110.10, and 150.0) are a floor you cannot trade away. The CEC states it plainly: a building may comply on the performance path by modeling only R-7 in a raised floor, but R-19 still has to be installed, because that is the mandatory minimum.

The prescriptive path is a component-by-component checklist in Table 150.1-A, organized by climate zone 1 through 16. NR in that table means there is no requirement for that component in that zone. Prescriptive values are frequently stricter than the mandatory ones for the same component — the CEC’s own example is a mandatory R-22 ceiling where the prescriptive package calls for R-38 depending on zone.

The performance path models the building in approved software against a calculated energy budget and allows tradeoffs between components. More work and far more design freedom — the CEC’s manual notes it is popular with builders for exactly that reason. It still does not lower the mandatory floor.

Insulation and air sealing

The mandatory envelope minimums are assembly U-factors, each with an R-value the code accepts as one way to show them: 2×4 framed walls at U-0.095 and 2×6 or greater at U-0.069, which R-15 and R-21 cavity insulation at 16 inches on center will satisfy; ceilings and rafter roofs at U-0.043, or R-22 between wood framing, with U-0.054 or R-19 allowed in an alteration; raised floors at U-0.037, or R-19 in a wood-framed assembly. Every joint, penetration and opening in the envelope that could leak air also has to be sealed.

People ask me for “the Title 24 roof R-value” and there is no such number. Above the mandatory floor, the prescriptive ceiling and roof-deck requirement is a climate-zone-and-option package, so it depends on the zone and on how the assembly is built. Quality Insulation Installation is prescriptive, not mandatory — called for in all 16 zones on new construction and on additions over 700 square feet, not on additions of 700 square feet or less. Wherever it is required, a third-party rater verifies the air barrier and the insulation on site. On the performance path it can be traded away, but a model that takes credit for it still has to have it verified.

Windows and skylights

The mandatory cap is a weighted average U-factor of 0.40, down from 0.45 under the 2022 code, with up to 10 square feet or 0.5 percent of conditioned floor area — whichever is greater — exempt from that average. Prescriptively it is much tighter: U-factor 0.27 in most zones and 0.30 in the rest, SHGC 0.23 in most zones (0.20 in zone 15, no requirement in zones 1, 3, 5 and 16), total fenestration area capped at 20 percent of conditioned floor area everywhere, west-facing area at 5 percent where a requirement applies. Either way the numbers come from an NFRC rating or the code’s default tables, and the temporary label stays on the glass until the inspector has seen it.

Heating, cooling and ducts

Prescriptively, the space-conditioning system in a new building must be a heat pump in every climate zone. Section 150.1(c)6 puts it in one sentence: the space conditioning system shall be a heat pump, or shall meet the performance compliance requirements. Table 150.1-A marks electric resistance as not allowed in all 16 zones and carries no prescriptive gas AFUE value in any of them. Gas is not banned. A gas furnace is still legal; it just has to make up the difference through the model. In an existing home, an altered or replacement space heating system generally may not use electric resistance as the primary heat source — the exceptions are narrow and mostly cover houses that already heat that way.

Ducts are mandatory on either path: R-6.0 minimum installed insulation, and sealing confirmed by field verification and diagnostic testing. In new construction the target is total leakage no greater than 5 percent of air handler airflow, or 4 percent at rough-in with the air handler not yet installed. An alteration triggers duct sealing above 25 feet of new or replacement duct, and separately whenever altered ducts, air handlers, coils or plenums sit in a garage, whatever the length — and altered systems have their own leakage targets, looser for an extension of existing duct than for a full replacement. Refrigerant charge verification is prescriptively required for heat pumps in all 16 zones and for air conditioners in nine of them.

Water heating

Prescriptively a new building gets a single 240-volt heat pump water heater, or solar water heating with electric backup at a minimum annual solar savings fraction of 0.7 — or it goes the performance path. Two exceptions: point-of-use electric for a new dwelling unit of 500 square feet or less, and a 120-volt heat pump water heater for a new dwelling unit with one bedroom or less. Replacing a water heater in an existing home is an alteration, and gas or propane is still allowed there. New for 2025, a heat pump water heater carries a mandatory ventilation requirement and needs backup heat where the inlet air is unconditioned.

Ventilation, the one that surprises people

Every dwelling unit has to meet ASHRAE 62.2 as amended by the Energy Code. The total required rate is Qtot = 0.03 × floor area + 7.5 × (bedrooms + 1) cfm, and the code says outright that opening a window is not a permissible way to provide it. What the fan itself has to deliver is Qtot less a credit for infiltration, measured or assumed, so do not size equipment off Qtot alone. Either way the installed airflow is field verified — measured on site, not just drawn. Local mechanical exhaust is required in each kitchen and each bathroom on top of it: nonenclosed kitchens demand-controlled, bathrooms at 50 cfm demand-controlled or 20 cfm continuous, and range hood ratings that scale with dwelling floor area and range fuel.

Lighting and JA8

No wattage allowance, no high-efficacy percentage to calculate. All installed luminaires and light sources must comply with Reference Joint Appendix JA8 and be certified and marked as JA8 requires, subject to a short exception list. That is the whole residential lighting requirement, and it is the one designers most often carry forward from an older cycle out of habit. The exception list, the controls and the recessed-can rules are on the lighting page.

Where solar PV applies

Solar is prescriptive, not a mandatory measure — Section 150.1(c)14, not 150.0. Every newly constructed single-family building needs PV unless an exception applies, sized as the smaller of Equation 150.1-C or the solar access roof area times 18 for steep-sloped roofs and 14 for low-sloped. Two exceptions do most of the work: none required if the calculated minimum comes in under 1.8 kWdc, or if the solar access roof area is under 80 contiguous square feet. A qualifying battery of at least 7.5 kWh cycling capacity cuts the required array by 25 percent.

PV sets the standard-design budget on the performance path and has no effect on the Efficiency LSC metric, so it cannot be substituted for efficiency measures — though a more efficient building does earn a smaller array. The performance path is the formal way out of the PV requirement, but the CEC’s own manual says it is unlikely you design a home where a PV system is not required. PV is not required for additions. Solar readiness is a separate mandatory measure and narrower than people assume: for single-family it reaches residences in subdivisions of ten or more that do not already have PV installed, so a one-off house outside such a subdivision is not covered by it. ADUs have their own answer. Separately, a one- or two-dwelling-unit residence with electrical service greater than 125 amps must be battery ready: interconnection equipment with at least 60 amps of backed-up capacity and four supplied branch circuits, or a dedicated raceway to a subpanel serving them. Readiness, not a battery. Cycle-to-cycle deltas are on 2025 code changes; the acronyms are in the glossary.

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