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Community and Q&A

Insulating interior basement walls with rigid foam board

user-7114034 | Posted in General Questions on

My house was built in the 40s. The basement walls are not smooth. There’s approx. 6 horizontal lines from the basement floor to floor joists that stick out about 1/4inch. The rigid foam board will not be sitting perfectly flush on the wall, is this a problem?

The joists were set in the concrete. How do i insulate the space between the bottom of the joist and the top of the joist? With scraps of rigid foam or with spray foam?

I understand that to make the wall ready for electrical and drywall, i have to fasten wood to the foam. The thought of putting that many holes in my concrete foundation scares me.

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Replies

  1. User avatar GBA Editor
    Martin Holladay | | #1

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

    Q. "The basement walls are not smooth. There are approximately 6 horizontal lines from the basement floor to floor joists that stick out about 1/4 inch. The rigid foam board will not be sitting perfectly flush on the wall. Is this a problem?"

    A. There are several ways to approach this issue. One way is to insulate the wall with closed-cell spray foam. For more information, see "How to Insulate a Basement Wall."

    Another way is to press the rigid foam against the wall, so that the concrete ridges leave marks in the foam. These marks can then be enlarged -- for example, with an old-fashioned can piercer or a more sophisticated blade from your tool box -- so that the foam sits flat against most of the wall.

    Q. "The joists were set in the concrete. How do I insulate the space between the bottom of the joist and the top of the joist? With scraps of rigid foam or with spray foam?"

    A. Either of these methods will work, as long as you have assessed the risks (and accepted the risks) of possible joist end rot. Insulating a concrete wall with embedded joists makes the concrete colder and potentially wetter, raising the risk of joist end rot. The risk is increased if the following factors are present: (a) the basement is damp, (b) the exterior grade is relatively high -- with 7 inches or less distance between grade and the lowest wooden components of the building, and (c) if there are bushes shading the exterior of the basement wall.

    The risk is decreased if the following factors are present: (a) the basement is relatively dry, (b) the distance between the exterior grade and the lowest wooden components is 8 inches or higher, and (c) there are no bushes blocking sunlight from reaching the above-grade portions of the foundation.

    If you have a risk of joist end rot, the traditional solution is to cut off the ends of the joists (the embedded portions) with a Sawzall, and then to support the joist ends on a new beam supported by new posts on new footings.

    Q. "I understand that to make the wall ready for electrical and drywall, I have to fasten wood to the foam. The thought of putting that many holes in my concrete foundation scares me."

    A. Once you have finished installing your rigid foam, you can erect a 2x4 wall on the interior side of the rigid foam. This wall sits on a pressure-treated bottom plate, and has a top plate that is secured to the ceiling joists or blocking between the joists. There is no need to fasten the 2x4 studs to the rigid foam or the concrete wall.

    If you need to fasten anything to the concrete wall, you should read this article: "Fasteners for Concrete and Brick."

  2. User avatar
    Dana Dorsett | | #2

    There is no need for the foam to sit perfectly flat with the wall- a 1/4" gap is also a good capillary break, and would manage bulk moisture incursions a bit better. With any foam board against the wall it would be important to air seal the top &.or bottom to prevent convective air transfer of ground moisture toward the conditioned space side of the assembly, but a heavy bead of foam board construction adhesive or polyurethane caulk formulated for concrete on the top & bottom horizontal step could even help in that regard. (You'd still want to seal the 1/4" gap at the top with can-foam or caulk after the foam board is up.

    Installing an inch of foam board under the bottom plate of the stud wall as a capillary and thermal break from the slab would avoid the need for pressure-treated bottom plate.

  3. user-7114034 | | #3

    Hi Martin, My name is Ellie. Thanks for answering my question. Regarding #3 - are you saying it's ok to fasten the foam to the wall with adhesive foam only, no concrete screws?

  4. user-7114034 | | #4

    Hi Dana, I'm not sure I understand how to create the 1/4 gap you're suggesting.

  5. User avatar GBA Editor
    Martin Holladay | | #5

    Ellie,
    Dana was talking about the 1/4 inch gap created by the concrete ridges. He's basically saying, "You don't have to worry about the presence of an air space between the rigid foam and the concrete as long as the perimeter of the rigid foam is air-sealed with a heavy bead of adhesive or caulk."

  6. User avatar
    Dana Dorsett | | #6

    Martin's interpretation of my statements is correct.

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