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Community and Q&A

Insulation on hollow cinder block foundation wall

obg8 | Posted in General Questions on

Hello, the lower level of my house is slab with about 4ft of block foundation wall and then regular wood framing above. I’m remodeling it and decided to frame out the block wall with 2×4 to add in electrical and insulation to the block wall on the inside. (it was originally framed with furring strips and drywall.)

I’m using R-15 faced fiberglass insulation, my question is related to possible moisture, etc. I have cleaned and painted the inside with loctite and eventually plan to seal the outside with a waterproofer for the portion below ground and I’m not sure about the portion above ground.

Would this setup lead to any kind of moisture/mold, etc? Should the insulated wall also be covered with some type of plastic barrier on the inside?

Thank you for your help.

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  1. Jon_R | | #1
  2. GBA Editor
    Martin Holladay | | #2

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

    You never want to install fiberglass batts against a below-grade wall made of concrete or concrete blocks (CMUs). If you are insulating this type of wall on the interior, you need to insulate with rigid foam or closed-cell spray foam.

    For more information, see "How to Insulate a Basement Wall."

  3. obg8 | | #3

    Thank you for the information. I went and installed 2" XPS against the concrete and then framed up a 2x4 wall to run electrical. It says in the article I can possibly insulate the stud bays with fiberglass now that I have R-10 against the concrete? I'm located in Connecticut, would this be acceptable? I used Dow weathermate tape to seal the seams and Greatstuff spray foam to seal the corners, I already purchased the fiberglass insulation and would love to use it if moisture isn't going to be a problem.

    One day I also plan to dig up around the foundation to re-do the drainage (old clay weeping tile) and was considering sealing the block with a waterproofer and applying more XPS against the outside of the block. Is there any chance I could do that as well? If I'm sealed on the inside and sealed on the outside wouldn't that lead to issues?

    Thank you for the help, my name is Chris by the way.

  4. Expert Member
    Dana Dorsett | | #4

    With 2" of polystyrene you have plenty of dew point control for R15 fiberglass or rock wool between 2x4 studs (or even for an R20 2x6 studwall) in your climate. Kraft facers are fine for this application, foil facers are not. Foil facers create a potential moisture trap.

    It's fine to insulate on the exterior with polystyrene as well. But with 2" of XPS (R8.4, when the HFC blowing agents are fully depleted) and a 2x4/R15 studwall (R10 "whole wall") you're already beyond code minimum.

    XPS is the least-green option here, since it is blown with climate-damaging HFCs (usually a mix of HFC134a and others, the exact mix & proportions being proprietary information held the vendors). Using reclaimed XPS is fine, or use EPS, which is blown with hydrocarbons, usually a variant of pentane (only ~7x CO2 compared to 1000-1400x CO2 for the HFCs), most of which escapes the foam and is recaptured at the manufacturing plant. The R-value of EPS is not dependent upon the blowing agent, and is stable over time. At the same density XPS and EPS will eventually have the same R-value.

    In southern New England there are several vendors of reclaimed and factory seconds rigid foam. The largest two are in MA (Green Insulation Group in Worcester, Nationwide Foam in Framingham), but there are others. At less than a third the price of virgin stock goods reclaimed cheap enough to be worth the drive if you need very much of it. Just don't use polyiso on the exterior of the foundation between the foundation & soil. (It would have been fine on the interior, between the foundation & studwall though.)

  5. GBA Editor
    Martin Holladay | | #5

    I agree with Dana's advice. Your fiberglass batts will be fine, in light of the fact that you have installed 2 inches of XPS against the concrete block walls.

    Adding rigid foam on the exterior side of the foundation at a later time won't cause any problems.

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