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.
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Replies
R_Omalley,
Smooth shank are fine. Deck screws would also work well.
You'll have much better luck with 3 1/4 as it is much more common. 1" foam plus 3/4" strapping still gets you 1.5" of embed. My local supply places stock these.
If you have a coil nailer, search for 16CRD131G, 3 1/2 spiral.
Having done this here are a few things I learned the hard way:
Definitely go with a ring shank nail or screw. 3 1/4 ring shank are available and easier to find than a true 3 1/2".
Consider ripping plywood to strips as the 1x3 is prone to splitting.
Make sure you get the densest foam you can. The box store EPS sheets will compress at the ends and give you a wavy wall.
1" of foam is plenty to break the thermal bridging.
You don't have to over do the attaching of the foam to the strips before install. I just shot a few staples through the foam to tack it to the strips while assembling them.
Another consideration here is to forego the Bonfig altogether and just attach strapping or 2x4s perpendicular to the existing framing. This will pack out the space for more celulose insulation and the 1 1/2 x 2" intersections where the framing meets the strapping will have little thermal bridging. This would also be a much faster and easier assembly and if you used 2x4 you would get 3.5" of additional insulation space vs the 1.75" with the Bonfig.
Thanks very much for the suggestions. I'm planning on using polyiso for insulation, both for the higher R-value and density. I was hoping to save ourselves a little labor by using 1x3s instead of ripping plywood, but I'll do some test strips before going all in on materials.
I had considered doing perpendicular strapping instead of the Bonfig— I hear that helps with evening things out for drywall as well—but was concerned that might complicated getting a good dense pack fill since the cavities would bleed into each other. Guess that would be a good conversation to have with the cellulose installer. Would have to be 1x3 strapping or 2x4s on their flat side for us... losing 2" of ceiling height & 4" of dormer width was already a hard sell to my husband.