Multifamily Envelope

Proposal Description

The goal of the Multifamily Envelope CASE Report is to propose code changes for cool roofs, mandatory wall insulation, and high-performance windows in multifamily buildings. These measures would align more closely with the 2022 Title 24, Part 6 nonresidential or single family residential cool roof requirements, and with concurrent proposals for single family residential wall insulation requirements. It would also align multifamily fenestration requirements with the existing and proposed prescriptive requirements for similar single family residential and nonresidential vertical fenestrations, where possible, and with ENERGY STAR^® Version 7 specifications.

These three measures work in conjunction to reduce the total amount of heat transfer that occurs in a building from the conditioned inside air to the outside ambient air:

• Cool Roof Improvements
• Improved Wall Performance
• High Performance Windows

Cool Roof Improvements

The cool roof measure would increase prescriptive aged solar reflectance (ASR), thermal emittance (TE) and solar reflectance index (SRI) value requirements for multifamily roof materials. Cool roofs reduce the heat absorption into the roof materials, reducing the cooling loads in the building.

The cool roof measure proposes:

• Increase ASR, TE, and SRI for multifamily Option B steep-sloped roof requirements in climate zones, where cost effective.
• Extend Title 24, Part 6 multifamily Option D low-sloped cool roof requirements to more climate zones, where cost effective.

Improved Wall Performance

This measure would decrease the area-weighted average mandatory U-factor of wall insulation to 0.148 for metal framed, 0.095 for wood-framed 2×4 construction, and 0.069 for wood-framed 2×6 construction, establishing a new backstop for buildings using the performance compliance pathway across all climate zones in California. Lower wall insulation U-factors reduce heat transfer in the solid portions of the walls in a building. Because wall insulation also has a prescriptive aspect that exceeds this maximum level (but is tradeable with other building systems in the performance compliance pathway), there would be no energy savings associated with this change to the mandatory portion of the code. This measure proposal is coordinated with proposed changes to the residential wall insulation R-value requirements.

High Performance Windows

This measure revisits the U-factor and SHGC prescriptive requirements for all multifamily window categories including curtainwall/storefront, NAFS 2017 Performance Class AW, and the All Other fenestration category. This includes both new construction and alterations prescriptive requirements.

• Lower U-factor from 0.30 to 0.28 in All Other window category. This measure proposes a slightly improved U-factor of 0.28 in climate zones where it is shown to be cost effective. For New Construction this includes Climate Zones 01, 03-05, 11, and 13-16. For Alterations this includes Climate Zones 01, 03-05, 11, 13, 14, and 16.
• Remove Relative Solar Heat Gain Coefficient (RSHGC) requirement in Climate Zones 1, 3, 5, and 16 for four habitable stories or more. This change would remove current maximum RSHGC requirement for curtainwalls, NAFS Class AW, and All Other window types in these heating dominated climate zones. This measure would also unify multifamily prescriptive fenestration requirements for buildings with three or fewer habitable stories with requirements for buildings with four or more habitable stories across all window categories, so the separate rows in the Table 170.2-A can be combined. This change is applicable to New Construction and Alterations.