Hi - homeowner trying to create an efficiency apartment in half of the basement. Cold climate (northern MN). Poured concrete walls and floor. 3 questions. 1. I've read that you want the moisture to dry "inward" so I will use unfaced rockwool batts. I have a bunch of 1" foil faced polyiso foam board. Can I use that against the wall before the studs? -- What if I tore off all (or most of the foil on one (or both?) sides?) 2. I was planning to insulate the ceiling in that half of the basement - for both sound and to keep the heat "down" there - but I'm not sure how effective it'll be for either of those desires? 3. The basement has always been "dry" except for condensation on the cold floor during the summer primarily. I plan on using either "dri-core" or "dry barrier" panels from menards for most of it. Is there anything I can (or should) put beneath a ceramic tiled floor? -- The other half of the basement is laundry storage and workroom and a treadmill. It will be "unfinished for the foreseeable future and stays "usably" warm due to the mechanicals being on that side. (I will insulate the wall between the two sides.) Thoughts?? -- Thanks very much!! Chuck

I would put the foam board over the studs and then the dry wall.

It seems like that would put the vapor barrier where they say there shouldn't be one? - not allowing wall to dry to inside... ?

Actually your using the foam board as a vapor barrier where it should be in the conditioned space. It's pretty simple. If you keep the vapor out of the insulation, no problemo. If you let the vapor enter the wall assembly then you have to worry about drying it.

Posted By smartwall on 30 Jan 2021 04:48 PM
Actually your using the foam board as a vapor barrier where it should be in the conditioned space. It's pretty simple. If you keep the vapor out of the insulation, no problemo. If you let the vapor enter the wall assembly then you have to worry about drying it.

--- BUt it's a concrete basement wall. I believe the moisture comes from the outside through the wall in that case.

I know it's a basement wall. Moisture condenses with heat. Keep the heat separate from the moisture and no problem. I would build a 2x4x24" wall away from the concrete wall. Maybe 1/2" and you shouldn't have a problem. Everyone thinks that concrete is this big sponge but that's actually not true. Unless you have actual water coming in thru micro fissures in the wall, concrete does not wick water like everyone thinks. Did my own experiment with a concrete paver and a bucket of water. The furthest the water migrated was 1 inch above the water level. Even put the whole thing in a plastic bag to make sure that evaporation wasn't at work. Still 1 inch. Got this idea from my own sidewalk. Noticed that my sidewalk dried before the asphalt road in front of my house.

My gut tells me the same thing - but this all gives me a headache. I don't want to get it wrong.