1″ Rigid Foam on exterior for thermal bridging
justfount
| Posted in Energy Efficiency and Durability on
New construction, 2×6 walls in climate zone 7 ( Upper MI.) Can I put 1″-2″ of foam on the exterior with fiberglass bats in the cavities to stop thermal bridging, while not worrying about the condensation on the back side of the sheathing? Thinking about putting T-11 or similar on the exterior.
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Replies
https://www.greenbuildingadvisor.com/article/rethinking-the-rules-on-minimum-foam-thickness
https://www.continuousinsulation.org/content/2021-ibc-and-irc-adopt-improved-vapor-retarder-requirements
2" (R-value ≥ 7.5) can work. Outward drying improves performance - so unfaced EPS or GPS would be best. Vented cladding also improves performance.
There are rules as to the minimum R value of rigid foam you need based on your climate zone. John in post #1 already linked to an article discussing that. You can put as much MORE than the rule says as you want to though. 1" isn't going to be enough in your climate zone, especially with a 2x6 wall (assuming it's fully insulated).
If you use a vapor barrier type of rigid foam (foil faced polyiso or poly faced EPS), you don't have any drying through the foam, so the exterior siding makes no difference. I personally would use polyiso here, EPS or GPS would be my second choice. XPS would be my last choice and is not preferred here.
I would put a smart vapor retarder on the interior side of the studs for some extra protection. The interior side smart vapor retarder will help to limit how much moisture gets into the wall in the winter, and that's a Good Thing.
Bill