Insulating Basement Walls, Modular Inside Out Design

[DanCostello]

I’m hoping to start framing out my basement studio build next week and I have a couple questions about insulation.

The existing foundation walls are a combination of fieldstone and brick, covered in parging (mortar, essentially) and Drylock (which is a quasi-waterproofing paint). I’m planning on doing inside-out inner leafs, partly for space, partly for aesthetics. And I’m planning on installing modules between the studs vs building the whole wall with sheathing on the floor and standing it up - I’m planning on this modular method because I have to do the work completely by myself and I can’t safely manage the weight of a whole wall section at once.

Anyways, because of nuances in the contours of the floor and the walls and assorted other things that have to be worked around in a 90yo house, there will be some gap between the studs and the foundation walls. That gap will usually be about 1” but will be upwards of 6” or more in some places.

1.) The gap between the foundation and the stud walls - fill it with insulation or leave the gap? I’ve seen some conflicting ideas on this - some see the gap as enabling air circulation, which allows things to dry out. Some want insulation for thermal insulation purposes. Fireblocking would put a point in the pro-insulation column, too. If it matters, I live in Maryland and the mid-Atlantic climate never gets too cold in the winter or blazing hot in the summers, though it does get humid AF in the summers. I don’t have any leakage through the foundation walls, and this is a rowhouse, so two of the walls are shared with adjacent units, not against the earth.

1A.) If I insulate, is rockwool and/or pink fiberglass fine or do I need to use the rigid foam stuff? Or something else?

2.) Vapor barriers: Does the 1”+ gap and/or the existing DryLock negate the need for a vapor barrier altogether?

2A.) If I need the vapor barrier, then as I’ve read on here, the vapor barrier goes on the cold side of the inner leaf, which would mean installing it on the back side of the inter-stud modules, which is fine. Covering the back side (i.e. the cold side) of the studs themselves is a little more tricky, however. Not impossible, but annoying. Is it necessary to cover the studs, too, or would covering the modules be sufficient?

Thanks,
-Dan.

[DanCostello]

I think I’ve answered some of my own questions:

I live in climate Zone 4A, whixh is warm enough that “condensation” build up in the wood isn’t much of an issue, so vapor barriers aren’t really even necessary.

Having a vapor barrier on the modules but not the framing is pointless.

Thermally, the air gap is a legit option, but I probably want to fill the gap for sound transmission reasons.

[Gregwor]

The way basements are done in my area is that insulation is put in the studs of the "frost wall" and there is a small gap left at the concrete. Vapour barrier goes directly under the sheathing. So in the case of an inside out wall, the barrier would be on the cold side of the inside out sheathing (the back side if you were to look at the wall).

You DO want as much insulation in the cavity as possible. It helps with isolation a lot.

Greg