Thermal bridging solutions when closed-cell foam is used?
What is the best way to handle the thermal bridging of the wood framing of my house — recognizing that some closed cell foam is specified for the wall cavities? We are constructing a house in climate zone 5 with 2×6 studs using the “hybrid wall assembly” as defined by Joe Lstiburek in his article “Hybrid Attics and Hybrid Walls.” I have seen no comments about managing thermal bridging using this type of wall system.
The specifics of our walls, from the inside out, are: drywall; R-11 unfaced batt insulation; 2″ closed cell foam insulation; 1/2″ plywood sheathing; WRB/Air Barrier (such as Henry Blueskin); vinyl siding.
We will have closed cell foam inside the wall cavities which will NOT allow the sheathing to dry inward. As a result, we cannot heed the advise (from “Can Exterior Foam Insulation Cause Mold and Moisture Problems?” and other articles) to use an adequate amount of external rigid foam insulation (approx. a minimum of 30% of the wall’s R value for zone 5) to keep the sheathing warm, because houses with excellent external insulation also must allow the sheathing to dry to the inside of the house.
Is there a product (presumably with high permeance) that could be used on the exterior that breaks the thermal bridging for both the studs and rim joists? If not, should we apply strips of foam to the interior sides of just the studs to minimize whatever thermal bridging we can?
Or should we just live with the thermal bridging — because too little exterior foam is worse than no exterior foam, and anything else we might do may just cause unintended consequences elsewhere?
Framing likely starts next week. As I have read in many questions, “I probably should have asked months ago …”
Your thoughts, comments and recommendations are greatly appreciated.
Thanks! ~ Judy
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