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EPS/OSB layers over existing sheetrock sheathing (housewrap, etc.)

Sum_Guy | Posted in General Questions on

I’m in the process of removing a second floor patio door and replacing it with a window and will be adding EPS to the outside wall.  House was built approx 1976 in Southwestern Ontario.  Exterior of the second floor was sheathed with half-inch sheet rock (I’m assuming exterior grade if there is or was such a thing).  No idea why sheetrock was used instead of plywood or osb.  Maybe a fire-code thing?   Ground floor has brick exterior but walls were sheathed with (I think they call it) ten-test.   Aluminum siding was nailed to second-floor sheetrock.  There is some sort of black paper (oily feeling, smooth, not tar paper) between the sheetrock and the siding.

My plan is to take the black paper off, fix any exterior sheetrock problems with new material (what-ever I can find at the local big-box, again not sure if an exterior-grade sheetrock is actually available).  Calk or use some sort of sealant or appropriate tape between the panels to make the sheetrock sheathing layer as airtight as possible, then cover it with what-ever housewrap I can buy locally (home depot canada doesn’t seem to sell housewrap, but lowes does).

Next I’ve bought some R8 EPS panels (almost 2″ thick) that I think were intended for below-grade (ie exterior basement wall) insulation.  I’m going to use that because it seemed to have a higher R and was quite a bit cheaper than above-grade versions.  These do have half-inch channel every 2 inches so I’m not sure which side I’ll put that on (house side or side facing out).  Over the EPS my plan is to put 7/16 OSB so I can squash the EPS tight to the house and have something to nail the siding to (the OSB will add a little more R to the picture I suppose).  So I’d like some feedback on that plan, and also if I should add a second layer of house wrap over the OSB.  I want a membrane between the house and the EPS to hopefully prevent EPS off-gassing from getting in the house.  Housewrap over the OSB might trap the offgas but I’m thinking would help the OSB from absorbing ambient outdoor humidity.  Or instead of a second house-wrap layer, maybe a paper layer over the OSB?

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  1. Expert Member


    If the exterior gypsum board was there because of fire code restrictions, those won't have gone away. Is your house close to the lot line? That's typically what drives rated exterior wall assemblies. If so you need to continue using non-combustable materials.

    Rather than adding a layer of OSB that needs to be fastened through all that foam, what people usually do is attach 1"x furring to fasten the siding to. This also can be used to create a rain-screen cavity, making the wall perform better.

    The place for the house-wrap is on top of the foam. That way the flashing at windows and door doesn't need to go back behind the foam. Think through how you want to handle the windows and doors with that much exterior foam. That's the tricky bit.

  2. Sum_Guy | | #2

    There's probably at least 10 feet between houses in my neighborhood. Houses today in this city are being built with exterior foam board. Is EPS flamable / combustible? Does OSB covered by aluminum siding qualify as fire-resistant? I don't know. I still like the idea of a solid, continuous, full-coverage layer like OSB over the EPS rather than furring strips. I'd put the window flashing over the OSB, but the window would be sitting on full 2x6 (or 2x7) plate that would be sitting on the 2x4 wall studs.

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