I want to know if I am detailing my thermal envelope correctly
I am building a 3300 sq ft house just north of Phila PA so I considered it as zone 5 just to be safe. From inside to out: 2×6 walls with dense packed cellulose, air sealed with tremco sealant according to GBA strategies. Then plywood sheathing. I’m adding one layer of 1 1/2 polyiso taped and sealed with z flashing all horizontal jts with 1×4 furring strips and vinyl siding. The vented roof will have a raised heel truss of 18 inches for 16 inches of cellulose. I’m following the air tight drywall strategies throughout the house. There will be a full basement with 2 inches of xps foam board then 10 mm poly over top, then 4 inch slab. Also on walls in basement 2 inch of xps on walls taped and sealed. I have two questions: how do I detail (seal) the bottom and top of the polyiso on the outside and the bug screen? I have not seen any good details describing that! Also when I fold up the poly sheet under the slab, should it attach to the concrete wall behind the foam board or to the foam board? And what is the best way to attach it to the surface?
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Replies
John,
Q. "How do I detail (seal) the bottom and top of the polyiso on the outside and the bug screen?"
A. Check out this video -- it should answer all of your questions:
https://www.greenbuildingadvisor.com/video-how-install-rigid-foam-insulation-outside-house
Q. "When I fold up the poly sheet under the slab, should it attach to the concrete wall behind the foam board or to the foam board? And what is the best way to attach it to the surface?"
A. You may be overthinking this detail -- it isn't that critical. You can attach the poly to the bottom of your concrete walls if you want, using Tremco acoutical sealant, duct mastic, or housewrap tape. Or you can just cut the poly at the edge of the floor, and seal the crack between the floor slab and the wall with caulk after the concrete slab has cured.
Martin,
Thanks, I wasn't sure if I had to seal the bottom or top of the exposed edges of the polyiso. I also see that some people use a 2x4 or a metal drip edge for the bottom. Any preference? Also am I alright with just one layer of polyiso instead of two? I was talked into it do to costs by the builder.
John,
Q. "I also see that some people use a 2x4 or a metal drip edge for the bottom. Any preference?"
A. I guess I would vote for the metal flashing -- but I think either detail will work fine.
Q. "Am I alright with just one layer of polyiso instead of two?"
A. Yes. Pay attention to air sealing at the polyiso seams; I suggest that you seal the seams with a high-quality tape. Two layers with staggered seams are better, of course, than one layer -- but you have definitely improved the thermal performance of your wall, even with one layer of polyiso, so I wouldn't lose any sleep if I were you.