Electrical – How to put electrical wires and boxes in insulated (but not drywalled) walls

I finished a basement in MN and spent a whole lot of time researching it. I eventually settled upon a method using XPS, steel studs, and paperless sheetrock as recommended by Building Sciences corporation and the US Dept. of Energy. I detailed the specifics on the answer to this question here:

https://diy.stackexchange.com/a/8644/1209

As for your questions:

1. I strongly recommend not ever using a vapor barrier in a basement. Read through my answer above for some more details. But, in summary: XPS acts a moisture retarder, but will still allow trapped moisture to dry to one side or the other. And since a basement can have moisture on either side, it's good to let it breath. When I finished my basement in MN, this was the biggest challenge I had with the St. Paul building inspectors. They insisted on vapor barriers, though invariably, in every basement I visited in MN with a VB, it smelled musty. I eventually did enough research to show them that the times have changed and VBs are not recommended that much anymore in basements by modern builders.

2. adding the fiberglass likely wouldn't hurt anything...it's more insulation. But it may not add a whole lot, either, if the XPS is already sufficiently thick enough. So, the question is how thick is the current XPS? If it's providing a good r-value already, you might be good to go there. Save the time/effort and just stud out from there (using Metal studs, IMHO).

Good luck!

I think based on the comments I better understand your scenario now.

Building Science has a lot of great information on basement insulation and the suggested method is to use XPS foam against the walls, optionally bats between the studs and no vapor barrier (since the XPS foam is the vapor barrier).

I would suggest that you use 1 1/2 - 2" XPS foam against the walls, and then space your studs another inch or so away from the wall (to allow your water to drain). If you need higher R values then the XPS provides you can use bats between the studs and I would suggest something like Roxul insulation which is stone wool instead of fiberglass since it is not affected by moisture like fiberglass is.

It would also be advisable to use steel studs so that they don't rot should they come in contact with moisture. You might also opt to put the bottom plate on top of a piece of XPS foam so that there is a less likelihood of the metal studs rusting.