Fastening a Bonfiglioli / Thermally Broken Wall
1. Thank you universe for Green Building Advisor. My current energy retrofit plans are forever indebted to the wealth of knowledge on this site.
2. My question: What’s the right type of fastener to attach 1″ of rigid insulation strips + wood strapping (probably a 1×3) parallel to wall and cathedral roof framing, a la Bonfiglioli. In the Fine Homebuilding article (https://www.finehomebuilding.com/project-guides/insulation/breaking-the-thermal-bridge) on the topic, 3.5″ ring shank framing nails are recommended, but for the life of me the only 3.5″ nails I can find to fit a framing nail gun are smooth shank. Will they work?
3. The context: I’m air sealing and insulating the 3rd floor/attic of my 1912 house on the far north edge of climate zone 4a. Due to complicated roof geometry (hip roof with 3 dormers), it will be an unvented assembly. The main roof rafters are 2×6 (100+ years old, so a true 2″x6″); dormer walls and rafters are just 2×4. To get more depth for insulation and lessen the impact of thermal bridging my current plan is to add 1″ polyiso cut into 2″-wide strips and then 1×3 strapping parallel to the rafters and dormer wall studs (thank you Bonfiglioli). Install 2–3″ of closed cell at the underside of the roof deck, then fill the remainder of the cavity with dense pack cellulose under the drywall finish. I’d love to do a bit more rigid insulation for a thermal break (1.5″ instead of 1″), but I’m afraid of being able to securely attach the 1x3s to the rafters and then having issues with hanging the drywall. Any thoughts or critiques on the big-picture approach are more than welcome.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part