Interior thermal break?
I’m renovating a barn. The existing wall assembly is beautiful rough-sawn 3/4″ board & batt cedar siding that cannot be removed; [unknown air barrier?]; 1/2″ plywood; 2×6@16″; R-21 batt insulation; interior finish is plywood. One thought for increasing the R-value of the wall is to add a 1/2″ layer of rigid insulation as a thermal l break, furr over that with horizontal 2x, which can also be used as a chase for new wiring, etc.
Is it worth establishing an internal thermal break? Would the 1/2″ rigid over the 1/2″ interior plywood impede moisture movement, for example drying to the interior? This raises many questions, but I’ll start there.
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Replies
Lydia,
Adding rigid foam insulation on the interior should be fine. The thicker, the better, however; 1/2 inch isn't much. Two inches would be better.
But what's your climate?
Ditto: 1/2" of foam does offer much of a thermal break. If it's foil-faced isocyanurate, that would eliminate drying to the interior, while up to one inch XPS would only limit but not eliminate inward vapor migration.
And concealed horizontal cavities (between furring) are required to be firestopped at maximum 10' intervals. You should check with your building inspector about whether a furred fire-resistant wall surface is acceptable over combustible foam insulation.
Martin & Robert -
Thank you. The project is in the San Juan Islands in Washington State. I haven't been there in bad weather so I don't know if there's wind-driven rain, etc. The point about the foam being combustible is one I hadn't thought of, and I will ask the inspector before I start drawing it.
Thanks again!