Internal concrete basement wall
I am finishing a section of my basement, Zone 5, 1989 construction. This area is an addition added in 1999. On the exterior walls I’m using 2″ XPS, 2×4 stud walls and mineral wool insulation. Because this area was an addition, the interior wall is also concrete; it was the old exterior wall, but has been interior for 20 years. It is insulated and framed on the other side, but I don’t know exactly how as it was finished by the previous owners.
I’d initially intended to simply build the stud wall directly on the interior walls without any XPS, as there’s no cold air coming in. But then I got thinking about moisture. I can’t seem to find any results for a situation like this.
Do I need to add a moisture barrier of some sort to this wall, to keep the framing dry? Should I use more XPS (maybe 1/2″?)? I was hoping to avoid taking up additional space because there are some inconveniently located pipes hanging from the joists only 4″ off the wall. Or maybe poly sheeting, and then the frame wall directly on top of it? The other idea I had was using pressure treated lumber as furring strips, and then building the wall off of that.
Thanks for any advice here!
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You will get some thermal bridging to the ground. Depending on how humid it gets in the summer that may cause condensation. Is the rest of the floor insulated? If not, keep in mind that you won't lose any more heat through the wall than through an area of floor equal to the footprint.
Thanks! I don't think the floor is insulated below the slab, but I don't actually know. I don't plan to add any additional insulation on the floor, as this is intended to be a workshop space.
I'm less concerned about heat loss than about moisture/mold concerns. A poly barrier was my first idea but I don't know if putting a barrier like that would cause problems on the other side or if it's just a bad idea in general.
To prevent condensation you need an air barrier, plus an insulation layer to keep the air barrier warmer than the dew point. Poly would work for the air barrier but if the wall is cold enough to cause condensation you'd just get condensation on the finished side of the poly without insulation.
There shouldn't be any concern about liquid water with an interior wall. If there is you've got bigger problems that a barrier isn't going to solve.
This is a pretty common situation in older homes here, usually an uninsulated stud wall is built against them.
You do get thermal bridging near the perimeter as DC suggests, it wouldn't hurt to put up 1" of foam on the first 2' or so (frame here with 2x3) and leave the rest uninsulated. I would skip the poly, use foil faced insulation and spray foam the edge to seal it where it ends.