Single Family Buried Ducts and Cathedral Ceilings – Buried Ducts is no longer being considered for the 2025 Code Cycle

Proposal Description

The buried duct measure as proposed is no longer considered for the 2025 code cycle. While the Energy Commission is not proposing any prescriptive changes, they plan on updating the buried duct compliance option in the CBECC-Res software to improve value of the credit and better represent the performance of buried ducts. The CASE Report aims to present a cost-effective code change proposal for buried ducts and an alternative prescriptive path for cathedral ceilings. The report contains pertinent information supporting the code change.

Buried Ducts

The proposed measure would create a simplified compliance method for buried ducts (compared to the current detailed process). It would modify Table 150.1-A, making buried ducts a prescriptive requirement under the Option B path for Climate Zones 1-3, 5-7, and 16 for new single-family homes. A proposed exception to Section 150.0(a)1 would also exempt new homes in Climate Zones 4 and 8 through 15 from the mandatory requirement to provide U-0.184 roof deck insulation for fully buried ducts. This measure would modify compliance software to accommodate a new set of effective R-values for fully buried ducts and remove barriers by simplifying modeling and verification procedures currently required for the existing optional buried duct compliance path.

Homes 500 square feet or smaller would be exempt from the prescriptive buried duct requirement due to lack of cost-effectiveness, but ceiling insulation requirements would increase from R-30 to R-49 in Climate Zone 3 and from R-30 to R-38 in Climate Zones 5 through 7. Current prescriptive insulation requirements for small homes in Climate Zones 1 and 16 would not change but would require a radiant barrier.

Additions of 700 square feet or larger would require compliance with Standards Section 150.1(c), prescriptive standards/component packages, so proposed changes to Option B and C would apply. The new requirements would not apply to alterations, but they could use the simplified compliance path under the performance method.

As defined, a fully buried duct is surrounded by loose fill (blown-in) insulation, with none of the duct’s exterior surface visibly exposed to the attic air. In Climate Zones 1-3, 4-7, R-49 is the proposed minimum prescriptively allowed ceiling insulation for vented attics and would fully bury a nominal 12-inch duct. Climate Zone 16 would require R-60 and would fully bury a 16-inch duct.

Cathedral Ceilings

This proposed code change outlines an alternative prescriptive compliance pathway under Option C of Table 150.1-A, Chapter 8 of Title 24, Part 6 for constructing cathedral ceilings in single-family new construction and additions. Cathedral ceilings are exterior assemblies where the interior ceiling surface is parallel to the roof surface and separated by framing. These ceilings may be flat or sloped and vented or unvented. Builders must use the performance pathway with no prescriptive path for cathedral ceilings in the current code.

Cathedral ceilings would require a maximum U-factor of 0.026 (or a minimum R-value of 38) across all California climate zones. Revisions to Option C of Table 150.1-A would introduce insulation requirements for roof constructions with either above or below roof deck insulation and increase the prescriptive requirement for ceiling insulation to R-38 for Climate Zone 8 through 10. The R-38 insulation requirement definition was based on equivalency with Option B of Table 150.1-A.

In alignment with current Option C, the proposal also requires compliance with Section 150.1(c)9B regarding Verified Low Leakage Ducts in Conditioned Space, per Residential Reference Appendix Section RA3.1.4.3.8.

Relevant Documents

CASE Report

• Final CASE Report – Buried Ducts and Cathedral Ceilings
• Draft CASE Report – Buried Ducts and Cathedral Ceilings

Round Two Utility-Sponsored Stakeholder Meeting Materials

• Proposal Summary – Buried Ducts and Cathedral Ceilings (May 10, 2023)
• Presentation – Buried Ducts and Cathedral Ceilings (May 17, 2023)
• May 17, 2023 Meeting Notes

Round One Utility-Sponsored Stakeholder Meeting Materials

• Stakeholder Meetings Introduction
• Presentation – Buried Ducts and Cathedral Ceilings (Feb. 14, 2023)
• Proposal Summary – Buried Ducts and Cathedral Ceilings (Feb. 8, 2023)
• February 14, 2023 Meeting Notes

Related Measures

• Single Family High-Performance Envelope (Windows and Walls)