Outsulation hits a road bump: 1/2-inch polyiso sheathing
So I had my entire plan laid out, for an exterior crawl space insulation. Excavate to footer, waterproof, insulate with 4″ EPS, French drain, stone fill, copper termite flashing, then 4″ foam from sill plate up 4′ onto the house. Main reason for foam above silk was to make insulating rim joist easier than doing foam from inside. I’m replacing all the exterior doors. I have excavator coming next Friday, so I was pulling siding off up to 4′ aboveboard sill. When pulling a siding nail, the prybar went through the exterior wall. I thought, oh great, rotten osb. I now have siding pulled half way up all along the front of the house, and I only have OSB on the corners. The rest of the sheathing is 1/2″ polyiso. I am concerned with the vapor barrier that poly ISO would be. I didn’t even consider this would be on the house. Eventually 4″ of foam on whole house.
1. Should I leave the poly ISO or replace with plywood? Since I intend to use 4″ EPS foam.
2. If I keep polyiso can I airseal it at the seams like I was going to do to the osb I thought was there? Or will this cause an issue with dewpoint?
3. I already have tyvek drainwrap, I can still use this over the polyiso but under the EPS?
4. Will the case of 3m 8067 tape I have fix/repair any holes/breaks in the face layer I created?
5. Should I give up and just insulate the crawl from the inside?
My main concern is with dewpoint/moisture issues, cavity fiberglass behind poly ISO looks fine.
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Replies
Joshua,
First of all, it's hard to answer your question unless you tell us your climate zone or geographical location.
It's not too unusual for a house to have rigid foam sheathing, with a few sheets of OSB at the corners as bracing.
If you are planning to install 4 inches of EPS on the exterior side of the existing sheathing, then you aren't expecting any outward drying. The vapor permeance of the rigid foam layer is irrelevant, because this type of wall isn't designed to dry outward. The point of the rigid foam is to keep the stud bays above the dew point during the winter.
Q. "Should I leave the polyiso or replace with plywood? Since I intend to use 4" EPS foam."
A. I can't think of any reason to remove the polyiso if everything looks sound.
Q. "If I keep polyiso can I airseal it at the seams like I was going to do to the OSB I thought was there?"
A. Yes.
Q. "Or will this cause an issue with dewpoint?"
A. As I said, you aren't getting any outward drying in any case. So there won't be any problems due to leaving the polyiso.
Q. "I already have Tyvek Drainwrap. I can still use this over the polyiso but under the EPS?"
A. I suppose you could, but it isn't really necessary. Most builders would probably locate the housewrap on the exterior side of the EPS.
Q. "Will the case of 3m 8067 tape I have fix/repair any holes/breaks in the face layer I created?"
A. The answer to that question depends on the size of the holes you made with your prybar. If the tape isn't sufficient to repair the damage, some canned spray foam might help.
Q. "Should I give up and just insulate the crawl from the inside?"
A. I'm confused. You wrote, "Eventually 4 inches of foam on whole house." So this project isn't just about the crawl space, is it? The project involves the whole house. I see no reason to give up.
Sorry, I'm in central Indiana, so zone 5. Thank you for the response.
Four inches of EPS is plenty for Zone 5. You shouldn't have any problems.
Thank you. I have read the post about where the house wrap goes, and even asked a question there about under or over the EPS. I had decided to go under the foam when I thought the house was OSB sheathed. I went with the crinkly Tyvek, but discovering the 1/2" polyiso just confused me.
1. I am still unsure whether to use the tyvek under or over the EPS. I was leaning towards under because of the OSB corners. I am just not sure the tyvek between the EPS and polyiso makes sense.
I am also doing the "wainscoting" with EPS and a covering 3' above the sill; mainly because I am not doing new siding or windows for a year or two but I still want the rim joist insulated. So I was concerned about only the 1/2" polyiso above the wainscoting and dew point issues, but the house has been like this since 1991, so I'm guessing another 2 years will not cause any further issues than have been realized (if any) in the past 26 years.
Joshua,
Installing housewrap between the polyiso and the EPS shouldn't cause any problems. You could save money by using ordinary Tyvek instead of crinkly Tyvek, though.
The Tyvek DrainWrap is serving as a Weather Resistant Barrier, so putting it over the old polyiso should be fine.