cold climate + interior poly vapor barrier = ?exterior insulation
We live in Northern Vermont (5A) We have a midcentury prefab home we are re-siding. We’ve peeled off two layers of siding (shingle and the original clapboard siding underneath). Unfortunately when a previous homeowner installed new windows they nailed the flange over the clapboard siding and then shingled over that. Now that we’ve taken down both layers of siding we have a window that sticks out 3/4 further than the sheathing because of the clapboard. We need to pad out the wall by 3/4 before we can reshingle. Considering the current cost of plywood we were wondering if we can use a vapor impermeable product like rigid foam instead? I realize 3/4 wouldn’t provide much insulation but it’s better than plywood and about the same cost right now.
The biggest consideration for us here is the interior side has poly barrier between the drywall and the fiberglass insulation. Also we would be installing this rigid foam on exterior walls of the kitchen and bathroom (so very humid indoor rooms)
-we are leaving old tar paper on house and we are adding new tyvek rain screen over that and then new cedar shake siding. The builder we are using to help feels like the tar paper was vapor impermeable and he doesn’t see an issue adding rigid foam on top of it before he puts up the rain screen.
-house is on slab. lots of poor craftsmanship and detailing through the years = many breaks in interior vapor barrier and external sheathing.
-this is a heated home but not cooled. so internal humidity in summer is similar to outdoors.
I saw this post which makes me thing EPS would be the way to go if adding insulation board https://www.greenbuildingadvisor.com/question/renovation-with-interior-poly-vapor-barrier
So external insulation:
Good idea? EPS?
Terrible idea? Just suck it up and use plywood?
Any other suggested products or detailing would be GREATLY appreciated.
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