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Insulating a basement wall – how to fix a bad situation?

Allister Tobias | Posted in Green Building Techniques on

A general contractor framed and insulated the exterior walls of my basement, however, the vapor barrier installed was poorly sealed, and now I have extensive condensation in the wall cavity. The GC installed a 6ml vapor barrier against the poured-concrete foundation wall from grade, down to the basement floor, wrapped it under the 2×6 framing, and then up to the main floor ceiling joists. The vb has many gaps throughout. The 2×6 framing is one inch away from the foundation wall. There are two layers of R14 Roxul insulation between the studs (24″ oc), measuring 7″ in total, so the insulation is currently snug to the poly-foundation wall. The condensation is on the warm-side of the poly and the insulation is wet as a result. Actually, this winter the Roxul was frozen to the poly.

For a bunch of reasons, I want to try and solve the problem as a DIY. To make it easier, I’d like to leave the framing in place. Because there are lots of staple holes, and some cuts, in the vapor barrier, I was thinking of removing it all, and starting with a new vp. I was also thinking of removing all the Roxul and recycling it as noise insulation around my furnace room etc. In its place, I would use R21 Roxul to fit the 2×6 framing and leave a one inch air gap between the framing and the foundation wall. This way any moisture on the foundation wall would not touch the insulation. Also, there would be no bridging. On the warm side of the framed wall, I would install a well sealed 6ml vapor barrier from the bottom plate to the rim joist, which is insulated with about 2″ of spray foam insulation.

There have been no moisture problems in the basement other than the condensation. I live in Zone C where we typically have very cold winters and not a lot of precipitation. My house is located in climate zone “C” – different system in Canada.: http://oee.nrcan.gc.ca/equipment/windows-doors/1371

Also, I tend to avoid the spray foam insulation except in awkward places like the rim joist (where it’s been excellent). The reason I avoid it is because it’s difficult to remove later on down the road when making changes (eg plumbing/electrical) and/or if I need to investigate the condition of the basement walls etc.

I would really appreciate advice on how to fix this problem in the best way possible given the constraints (i.e., would prefer to work around the framing). Thanks very much.

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Allister,
    For years, I have been telling homeowners (1) Never install any polyethylene when you insulate a basement wall on the interior, and (2) Never use fiberglass batts or Roxul when you insulate a basement wall on the interior -- because these two types of insulation are air-permeable.

    Some people doubt my advice, thinking they can make these materials work. Thanks for providing another example of a failure, proving my point. It's no surprise that your insulation is wet.

    Sorry -- that probably sounds unkind. You have a mess on your hands, and you have my sympathy.

    Remove the polyethylene and the Roxul. You can stack the Roxul in your attic if you want, but the polyethylene belongs in a dumpster.

    Once everything has dried out, you can insulate properly. The easiest way is with closed-cell spray polyurethane foam. Using rigid foam is an alternative, although your studs are in the way, complicating the work.

    More information here: How to Insulate a Basement Wall.

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