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Insulating interior basement walls if exterior has waterproof membrane and rigid board

user-7114034 | Posted in General Questions on

The exterior basement walls have waterproof membrane and two inch foam board installed. To insulate the interior basement walls, can i use the old school method of studs, fibreglass and poly?

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  1. GBA Editor
    Martin Holladay | | #1

    User 7114034,
    First of all, can you tell us your name? (I'm Martin.)

    Q. "The exterior basement walls have waterproof membrane and two-inch foam board installed. To insulate the interior basement walls, can I use the old school method of studs, fibreglass and poly?"

    A. Briefly, no. I advise you to stick with rigid foam or closed-cell spray foam, as explained in this article: "How to Insulate a Basement Wall."

    Admittedly, you could probably get away with using fiberglass batts -- assuming that the exterior details are perfect -- but the interior polyethylene would always be a mistake.

    The reasons that I don't advise installing interior fiberglass batts against the concrete are the following: (a) It's extremely difficult to inspect the exterior waterproofing details and insulation details, so you can't determine if there are any flaws in the installation. (b) Small flaws in the exterior details could result in wet concrete or cold concrete, and it's not worth the risk of letting fiberglass batts touch wet concrete or cold concrete.

  2. Expert Member
    Dana Dorsett | | #2

    Moisture wicking up from the footing into the foundation wall can wick toward the conditioned space side of the assembly. If there isn't a good capillary break between the top of the foundation and the foundation sill, putting any interior side vapor barrier (or even thick or foil faced foam board) can increase the moisture content of the foundation sill.

    A thin layer of foam board with a vapor permeance greater than 1 perm trapped to the foundation wall would provide a capillary break for the fiberglass, and allow any ground moisture wicking up the foundation wall to dry toward the interior, offering some protection for the foundation sill.

    Local climate matters too. When there is no interior side vapor barrier there has to be sufficient R value exterior to the fiberglass to prevent moisture accumulation at the above-grade portion of the wall. It's legitimate to add the R-value of both the 2" exterior foam and whatever semi-permeable interior side foam gets installed when making that calculation. Using the IRC R702.7.1 prescriptive minimums for the fully above grade walls would work (and would have a hint more margin for a foundation wall):

  3. user-7114034 | | #3

    I'm really not sure what to say. I live in climate zone 7/8. Recommendations from our local power heat company still recommend using poly in the basement. I checked my basement and it looks like the three walls that were waterproofed from the outside have about a half inch gap between the inside of the concrete basement wall and the 2x4 framing wall. There's fibreglass insulation in between the studs, followed by poly and drywall. Parts of the wall have been left uninsulated - like the area directly beneath the electrical box. I checked the insulation in the areas on either side of the uninsulated area and the insulation is dry and still sitting upright within the studs and therefore not touching the concrete wall. Unfortunately the fourth wall was not waterproofed from the outside. Don't ask. The basement wall on the non-waterproofed wall is done like this: 1/2 inch gap, poly, 2x4 wood frame with fibreglass in between the studs, poly and drywall. Yes two polys.
    I have no idea what to do. Nor do I know if the current setup is an issue in my climate. Yep we've got it all here. Extreme hot and cold, humidity, wind, and expanding contracting clay soils. Good news is the the drywall has not been mudded and sanded. I could undo the screws, remove the drywall and have the poly and fibreglass exposed...
    On another note, I know i need to put insulation in the rim joists. I am wondering about how to stop air from blowing through he insulation and making the basement cold.

  4. GBA Editor
    Martin Holladay | | #4

    User 7114034,
    It would still be helpful if you could tell us your name.

    If your house is already built, and it is built the wrong way, you don't necessarily have to tear everything apart and re-do it. If you know that your wall assembly is dry, you can leave the polyethylene in place. Keep an eye on things and don't worry too much.

    If any GBA readers are wondering what to do in a new-construction situation, my advice is unchanged: You never want to include polyethylene in a wall assembly if you are insulating a basement wall on the interior. If you live somewhere (Canada?) where local building inspectors insist on interior polyethylene, try to convince the inspector to accept a "smart" retarder like MemBrain instead of poly.

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