I'm in the process of building an office and family room in my basement and need some help with the walls and insulation.

There is a perimeter drain in the slab around the exterior walls that collects a small amount of water that weeps through the walls during the wet season. This perimeter drain will require me to place the studs about 4" from the wall.

In this scenario, with an airgap between the interior/exterior wall, should I use rigid foam insulation attached to the block wall, or traditional fiberglass insulation within the studs? Also, if fiberglass is used, should I include a vapor-barrier and on which side of the wall should it be placed?

  • 4
    You need to fix the moisture problem before you build anything.
    – Steven
    Apr 19, 2013 at 12:46
  • I probably should have provided more detail - the moisture is being addressed according to approach #4 here: extension.umn.edu/distribution/housingandclothing/components/…
    – user12622
    Apr 19, 2013 at 14:23
  • Even with an interior drainage system, you should not have any water on the inside of the wall. Moisture in walls = mold.
    – Steven
    Apr 19, 2013 at 14:28
  • The only place the walls are wet, is at the very bottom where the holes drain into the channel. The rest of the walls are dry to the touch and are coated with a very old drylock-type substance which is flaking off in some areas. Do you think this is an acceptable situation to move forward with, or should I take additional steps first?
    – user12622
    Apr 19, 2013 at 14:44
  • I don't think anyone can answer this right without knowing the climate. If he is in Texas you don't insulate a basement (I know there are few basements in Texas) Also metal framing is a bad mix with water. I would use PT lumber to frame everything if water is that big of an issue.
    – DMoore
    Apr 19, 2013 at 15:10

2 Answers 2


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.

  • This is all great info and I appreciate everyone's input. I was referencing an old Building Science pdf, but the link you sent is more up to date and detailed - thanks.
    – user12622
    Apr 19, 2013 at 16:01
  • Based on the info, it looks like unfaced XPS is probably the way to go. I was thinking of using spray foam on the top of the open blocks, sill and band boards, but now I am wondering if I might use the XPS there as well?
    – user12622
    Apr 19, 2013 at 16:08
  • You can cut the XPS and use it to insulate joists and other areas - typically you seal it with caulking or great stuff. all of the XPS seams should be sealed with tape
    – Steven
    Apr 19, 2013 at 17:34
  • Great, that sounds like a good idea. Can the XPS be attached with construction adhesive, or should furring strips be used? Also, as for the base of the stud wall, I'm going between a couple of options: - treated for the base, with a sill gasket and regular wood for the rest, - Or, 1" XPS on the floor with 1/2 plywood then regular wood for wall. The question here is can the wall sit on top of the XPS & plywood, or should it be anchored direction to the slab? Laminate flooring will be going on top if that makes a difference. Thanks for all the info!
    – user12622
    Apr 19, 2013 at 19:16
  • There is a special adhesive needed to attach XPS - "PL 300". Furring strips can also be used provided that you use XPS that has channels for them already built in. You can use PTE with sill gasket if you wish - you could even use a piece of composite decking. But the plate should be anchored through it all to the slab
    – Steven
    Apr 19, 2013 at 20:08

Rigid foam is going to handle the moisture issue way better than fiberglass. Just the humidity of the area will stick to fiberglass and eventually the fiberglass will sag. If I were you I would get some rolled plastic sheating and put it on the interior part of the wall. I would then either spray foam, rock wool the walls, or put nothing in them. The need for insulation in a basement system is dependent on your climate. In a warmer climate insulation in a basement could bring negative energy returns.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.