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