Prescriptive vs Performance Compliance

The Energy Code offers two ways to show that a building complies, and both start from the same mandatory measures. Choosing well can be the difference between a design that gets approved as drawn and one that has to change.

The performance method

The performance method, also called the performance approach, proves compliance with an energy model. CEC-approved software models your proposed design, builds a standard design of the same size that meets the prescriptive requirements, and compares the two. If the proposed design uses no more energy than the standard design's budget, the building complies, even where individual components fall short of the prescriptive values. That is what allows trade-offs: more glazing offset by better insulation or more efficient equipment, and it is why the Energy Commission's single-family compliance manual calls the approach popular with builders.

Standard design vs proposed design

The proposed design is your building as drawn: its real orientation, windows, overhangs, insulation and specified equipment. The standard design is generated by the software from your own inputs. It keeps your conditioned floor area, volume, wall, roof and floor areas, but uses the prescriptive package for your climate zone: its insulation levels, window U-factor and SHGC, and minimum-efficiency heating and cooling, plus solar PV and a heat pump water heater on a new home. Its window area matches yours up to 20 per cent of the floor area, spread evenly on all four sides with no overhangs.

For a new home the two are compared on source energy and on long-term system cost (LSC), which values energy by when it is used, so a saving at a statewide peak counts for more. Additions and alterations are compared on LSC alone. Lighting and appliance energy cannot be traded.

The software

The model has to be built in software the Energy Commission has approved for the current code: CBECC, the Commission's own public-domain program, or an approved third-party program. The result is a performance certificate, the CF1R-PRF-01-E for a house, the LMCC-PRF-01 for low-rise multifamily or the NRCC-PRF-01 for nonresidential buildings. See what software Title 24 compliance is done in.

When the performance method is the better path

• Generous glazing. The prescriptive package caps windows at 20 per cent of the conditioned floor area in every climate zone. A model can carry more, offset by better windows, insulation or equipment.
• Gas heating or water heating in a new home. Under the 2025 prescriptive package a new home is heated by a heat pump, and its water by a heat pump water heater or solar water heating. A gas furnace or gas water heater can still comply, but only through the performance method, with the difference made up elsewhere.
• One item that misses. A wall that cannot take the prescriptive insulation fails the checklist outright. In a model it is one input among many.
• Additions. An addition can be modelled together with the existing house, so upgrades there, such as new attic insulation, can count towards it.

Two cautions. Mandatory measures still apply: a model may pass with R-7 under a raised floor, but the mandatory R-19 must still be installed. And where compliance depends on a feature better than the prescriptive minimum, such as higher-efficiency cooling equipment or quality insulation installation, an ECC-Rater has to verify it during construction.

The prescriptive method

The prescriptive method, or prescriptive approach, is a checklist. Each component meets a set requirement for its climate zone (insulation levels, window U-factor and SHGC, glazing area, equipment types and efficiencies) and the building complies without an energy model. It is simple and predictable, with a little flexibility, such as averaging wall insulation by area, but rigid: miss one item and the design does not comply prescriptively, however well the building performs overall.

It suits work with a narrow scope. Additions and alterations generally use it, with performance as an option, on shorter forms: the CF1R-ADD for an addition, the CF1R-ALT-01 for an alteration and the CF1R-ALT-02 for an HVAC changeout.

Zone-specific budgets

The prescriptive package is set climate zone by climate zone, and the standard design is built from it, so the budget a performance model is measured against changes with the zone as well as the building. This is a core reason a report cannot be reused across cities.

Which path will your report use?

Whichever gets your design approved as drawn, at the same price either way. If neither works, see why a Title 24 design does not comply.

Explore Performance Calculations

1. How Much Compliance Margin Should I Keep Before Construction?
2. Title 24 Trade-Offs: A Worked Example of What Can and Cannot Move
3. What Does Compliance Margin Mean on a Title 24 Report?
4. What Is the Title 24 Energy Budget? LSC, Source Energy and the Old TDV
5. Which Title 24 Performance Credits Need an ECC-Rater Test?

View all Performance Calculations guides → · Reports and Calculations · Title 24 guide