Title 24 requires certain minimum features to be installed in all new construction projects, called the Mandatory Measures. This then allows the builder to choose either a prescriptive or performance path to complete the energy budget portion of the efficiency requirements in construction.
Section 1.6 of the Residential Compliance Manual offers further explanation of the Mandatory Measures and Compliance Approaches.
The prescriptive approach is simple but offers relatively little design flexibility; some building envelope components, such as walls, where portions of the wall that do not meet the prescriptive insulation requirement may still comply as long as they are area-weighted with the rest of the walls, and the average wall performance complies.
The performance approach allows trade-offs between different building components (wall, roof, windows, infiltration, equipment, etc.), as long as the overall proposed building energy consumption is shown to be better than the baseline building defined by the standards. These trade-offs are determined either through hourly computer simulations or derived from simulation results. The performance approach may be used for envelope or mechanical compliance; envelope and mechanical compliance; envelope and indoor lighting compliance; or envelope, mechanical and indoor lighting compliance. It is not applicable to outdoor lighting, or to indoor lighting in the absence of envelope compliance.
For additions and alterations, CLICK HERE to refer to Chapter 9 of the Residential Compliance Manual for details of available compliance approaches