hybrid insulation envelope for fire codes - GreenBuildingAdvisor

hybrid insulation envelope for fire codes albertlarsen | Posted in Building Code Questions on March 29, 2026 09:26am Climate zone 8, I would like to use 2 inch of ROXUL inside 4″ closed cell spf and then 1″x6″ tongue and groove board lumber in my cabin. Can anyone relay comments? Specifically fire code compliance questions. Answer GBA Detail Library A collection of one thousand construction details organized by climate and house part Search and download construction details Join the leading community of building science experts Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field. Start free trial Replies Expert Member DCcontrarian | Mar 29, 2026 10:58am | #1 Roxul is stiff. Closed cell spray foam is very stiff and tends to be uneven. It's going to be difficult to get both to fit in a cavity unless it's quite a bit bigger than the thickness of the two layers, I'd expect at least 2x8 framing. Unless this is for a cathedral ceiling, high density fiberglass will give about the same R-value in a 2x8 at a much lower cost. If it is a ceiling it's pretty skimpy insulation for zone 8. Expert Member MALCOLM TAYLOR | Mar 29, 2026 12:48pm | #2 Albert, You may find this useful: https://www.greenbuildingadvisor.com/article/thermal-barriers-and-ignition-barriers-for-spray-foam Log in or create an account to post an answer. Sign up Log in