Insulation assembly for exterior walls on 1950’s house
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During a recent interior renovation project, which included taking off a few sheets of drywall on the exterior wall, I discovered that there was NO insulation in the exterior walls! I thought this couldn’t be right, so I cut a small hole through the old fiberboard sheathing and found that there is 1″ or polyiso that the siding was installed directly over. So our existing exterior assembly is:
– old/worn out looking vinyl siding
– plastic barrier (looks like 4-6mil poly)
– 1″ polyiso (I can’t imagine this old stuff has much of an R-value left)
– fiber board
– 2×4 studs
– drywall
I was planning on replacing the sad looking vinyl siding this summer, but now knowing this, if i’m opening up the exterior walls, I might as well remedy this situation. This is my initial thought on a solution assembly:
– Hardie board siding
– Zip R sheathing (the R6 version)
– dense pack cellulose insulation in the 2×4 wall cavity (R-13)
I’ve thought about filing the 2×4 cavity with closed cell spray foam, but I’m not a fan of it’s environmental impact and the cost of it would put me over budget. My questions would be, is there something else I should consider other than cellulose in the walls? (last summer I put about 12-18″ of it in the poorly insulated attic). And is the R6 zip sheathing sufficient without causing condensation issues within the wall?
Also, our house is in climate zone 6 and we’re about 1/4 mile from the ocean.
Thanks!
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Replies
Sturg,
I'll give you a bump. In CZ6, you want at least R7.5 of exterior insulation on a 2x4 wall, at least according to this article: https://www.greenbuildingadvisor.com/article/combining-exterior-rigid-foam-with-fluffy-insulation. It's possible you are close enough, but it might be better to attach the foam to the outside of the existing sheathing (if the fiberboard is in decent shape). I think using cellulose is the way to go since it can be blown into the ways with less mess and expense.
Putting additional foam on the exterior is certainly doable since you want to reside, however you're already at the limit of foam thickness that siding should be installed over. You would then add vertical strapping and choose a siding that can attach directly to the strapping. With the extra depth you'll have to figure out the best way to trim out windows and doors. With the right thickness of foam, you don't need anything in the stud cavities to meet code. In zone 5 you can do it with 3" of XPS