Vapor Retarder for Exterior Renovation
Hello GBA,
I live in Michigan (Zone 5 but near Zone 6) and have started an exterior renovation to my home. I plan to replace siding, exterior sheathing, and the cavity insulation as part of the renovation. The interior drywall will not be removed. I will be using R15 Rockwool Mineral Wool in the stud bays, Zip sheathing, 2″ polyiso rigid foam exterior insulation, 3/4″ rainscreen, and fiber cement siding. My question pertains to a vapor retarder, is the interior wall paint sufficient for a vapor retarder? My home was built in 1965 so there are many layers of interior paint. Would a better approach be to use a product like CertainTeed Membrain or would that add any risk? If this type of product is recommended, should I cut strips and stable between stud bays on interior side of wall? I’m assuming installing it over the studs from the exterior is not an option. Thanks
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Replies
According to the below, you have sufficient continuous insulation for zone 6, even derating polyiso for temperature:
https://www.greenbuildingadvisor.com/article/calculating-the-minimum-thickness-of-rigid-foam-sheathing
Ordinary latex paint should provide enough vapor control. If you are still uneasy, vapor retardant paint is available, and would be much less costly and labor intensive to retrofit.
The use of exterior insulation can complicate flashing details, so pay close attention during installation. There is a far greater moisture risk from flashing defects than from inward vapor diffusion.
https://www.greenbuildingadvisor.com/article/how-to-install-rigid-foam-sheathing
https://www.greenbuildingadvisor.com/article/installing-windows-in-a-foam-sheathed-wall
I opted for the belt and suspenders approach and applied the retardant paint. Thanks so much. with respect to flashing, I plan to flash everything back to the ZIP sheathing.
Don't neglect the testing that will verify your air sealing.