Daylight basement insulation – exposed XPS and other questions
I’m about to insulate the floor and 2 walls of my walkout/daylight basement, and I was hoping to get a little advice on a couple of points I’m not clear on. I’ve researched quite a lot, and learned a lot on this site, but I’m still not sure on a few things. Any help is much appreciated.
It’s a poured concrete foundation with no existing exterior or interior insulation. East wall is concrete up to 36” and the rest stud framing. South wall is concrete up to 9” and the rest stud framing. So it’s a sloping grade with part of that east wall below grade and the south wall pretty much above grade. West and north walls of this room are existing interior walls. No signs of major leaks or water damage in the year and a half we’ve been in the house (knock on wood!), but a lot of moisture in the air and a terrible musty basement smell.
My plan is to put 1” XPS on the floor (I already have it and since found out that EPS is more environmentally friendly. I’ll source and use EPS or PolyIso on the walls though) seams caulked and taped and leaving ¼” expansion around the edges which I will fill with caulk or foam.
1” EPS or PolyIso glued/caulked to the east and south walls will meet the XPS on the floor and be caulked at that corner seam
2×4 stud walls tight to the EPS on those walls. PT bottom plate with Tapcons through the XPS into concrete. Fill cavity with Roxul?
Then ¾” T&G plywood on top of the XPS on the floor, and Tapcons into the concrete using caulk in the holes before the Tapcons go into concrete. ¼” expansion gap between plywood and walls
Finish walls with drywall.
1) If I leave that expansion gap between the ¾” ply and my framed stud walls, that will leave ¼” of the XPS on the floor exposed. I think that code says that the XPS cannot be left exposed. How can I get around that? Will the baseboard covering that small gap suffice, and shouldn’t I be leaving a small air gap under baseboard anyway to allow moisture to escape? Or is it better do the subfloor first and frame the walls on top of the plywood?
2) Should I put the rigid foam and the framing all the way up to the ceiling on those two walls, being that the height of the concrete walls are only 36” and 9”, or should I just cover the concrete part with the foam insulation, and leave the existing walls as they are? They already have existing drwall. I could frame up to 36” on the east wall and make a ledge/shelf and up to 9” on the south wall which would make a sort of weird step I guess. The R value is minimal, but I guess it all helps, and going up to the ceiling will look a lot cleaner. Can I glue the EPS to the existing drywall with no worries?
3) Should I fill the2 ½”cavity in the new framing between the EPS and the drywall?
Is this air gap a fire hazard? Would Roxul in that cavity be a good call, giving me extra R value and solving the fire/code problem?
Sorry for the long post. Hope it all makes sense. Thanks for taking the time to read it.
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