Single Family High-Performance Windows and Walls

Proposal Description

The Energy Commission is moving forward with a window prescriptive U-factor proposal that is slightly revised from what is presented in the Final CASE Report. The updated proposal is for new homes to have a prescriptive maximum U-factor requirement of 0.27 in Climate Zones 1 through 5, 11 through 14, and 16 and 0.30 elsewhere. Homes with 500 square feet of conditioned floor area or less would be subject to the same requirements except a 0.30 U-factor in Climate Zone 5. Window alterations in existing homes would have a prescriptive maximum U-factor requirement of 0.27 in all climate zones.

This CASE Report presents code change proposals for prescriptive window U-factor requirements, mandatory window U-factor requirements, and mandatory wall insulation requirements, along with pertinent information supporting the proposed code change.

The proposed window measure would:

• Reduce statewide peak demand energy use by 0.51 MW in the first year.
• Provide a return on the investment in as few as five years.
• Save 1.48 GWh of electricity and 0.40 Million Therms of natural gas in the first year statewide, in newly constructed homes.

The code change proposals in this report would:

• Reduce the prescriptive maximum U-factor requirement of window assemblies in most homes to 0.27 in California Climate Zones 1 through 6 and 8 through 16 and to 0.28 in Climate Zone 7. Reduce the prescriptive U-factor requirement in homes 500 square feet or less to 0.27 in Climate Zones 1 through 4, 11 through 14, and 16.
• Require that installed windows match modeled solar heat gain coefficient (SHGC) values with a margin of ±0.01 in all climate zones.
• Reduce the mandatory window U-factor requirement to 0.40 for all climate zones.
• Reduce the mandatory U-factor for framed walls to the equivalent of R-15 cavity insulation for 2 x 4 wood framed wall assemblies, and R-21 for 2 x 6 assemblies.

Windows and walls can be a significant source of heat gain or loss within a home, which can lead to wasted Heating, Ventilation, and Air Conditioning (HVAC) system energy use. Therefore, these envelope components have been the subject of building energy code revision through many code cycles, including this current proposal. The measures in this proposal call for a change in assembly thermal performance characteristics whose relationship to energy use is understood and modeled.