Retrofitting slab on grade workshop to MIL Zone 5a
Long time reader, first time poster. Recently purchased a home with a 720 sq foot 2×6 workshop that I am converting to a mother-in-law, for my mom. I’m in Zone 5a, in the mountains of Western NC at 3000 ft altitude. so significant winter heating and high winds. Current construction is uninsulated slab, 2×6 framing with t1-11 exterior sheathing and fiberglass batt insulation.
My plan is to excavate down 2’+ and install 3″ XPS (have a source for reclaimed) to the exterior slab. (need to cover about 12″ above grade https://www.greenbuildingadvisor.com/article/how-to-finish-exterior-foundation-insulation)
On the exterior above grade 2-2.5″ Poly-iso (again, reclaimed). I have the option of fiber faced or foil faced, which is more appropriate? I’m thinking of doing the WRB to the outside of the foam boards, and windows to the exterior (hoping for simplest installation). (https://www.nrel.gov/docs/fy15osti/63337.pdf)
Would it be inappropriate to leave the T1-11 siding on, and apply the insulation and WRB on top of that? T1-11 being pressure treated, this seems like a bad idea. If I remove it, what would be the simplest product to replace for structural rigidity, and should I install an air barrier between the sheathing and the exterior insulation?
In the attic, I plan to blow-in cellulose. Is there any reason to leave the attic vented, or should I seal it up?
I’ll save my HVAC questions for another post, but planning to use a mini-split and possibly an HRV.
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Replies
2" of reclaimed polyiso provides enough R-value in CZ5 that either facing would be fine. If you are installing a separate WRB, then I'd go for the fiber-faced to allow drying to the exterior, a small extra margin of safety in case water ever leaks in somewhere. But, you could use foil-faced if you wanted to tape it to use as your WRB.
I'd install it right over the T1-11. I'm not aware of any real health concern with pressure treated wood. However, you could install a plastic vapor barrier on the inside and detail it as an air barrier, sealing it at all the edges and penetrations. With foil facings on the exterior being a class I vapor retarder, you would have to use a variable vapor retarder inside, such as CertainTeed MemBrain. With fiber facings, you could use either a variable vapor retarder or regular polyethylene sheet. You could also set the ventilation system to slightly pressurize the house.
Normally, my preference would be to tape the sheathing to use as the primary air barrier, but I'm not sure how feasible that would be with the rough texture of T1-11. My next choice would either be using a sealed inside vapor barrier as above, or sealing the WRB (whether the taped foam or a separate sheet product). A fully-adhered membrane is another option.
With cellulose on the attic floor, you need to leave it vented. Do everything you can to air seal the wall top plates and any ceiling penetrations before blowing in the cellulose.
alembadelic,
+1 on all Frank's advice.