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Community and Q&A

Basement insulation with exterior wall waterproofing

ChrisJS | Posted in General Questions on

I am a new home owner trying to answer a question that evokes different responses depending on which site I visit!! Or perhaps, I am just dumb!! Anyway here goes.

The newly built home has an unfinished, uninsulated, poured concrete foundation. Water is channeled (piped) away from the house and there are drainage channels under the concrete floor. Drainage precautions were taken because the home sits in a granite hill top. In addition, exterior basement walls were waterproofed with Mar-Flex 5000 (15 year warranty). The basement appears dry, and there is minimal heat loss from heating system. It is significantly cooler than the ground floor.

I do not intend converting the basement into finished living space, but would like to use it as an exercise area. With that in mind, I began to look seriously at how best to insulate the walls.

The solution appears to be a layer of XPS glued directly to the concrete wall and covered with a layer of Dow Coring Thermax sheathing for finish and fire protection, to achieve R15 overall.

So now to the question I am trying to resolve.

Even though the basement appears dry and is “sealed” externally, is it OK to flush mount the XPS/Thermax insulation that will also act as an additional low perm interior vapor barrier.. My concern is moisture “wicking” up inside the concrete from below, or possibly still coming in through the exterior “seal”.

What is the risk of moisture accumulating inside the “sealed” wall and potentially causing damage to the concrete. We are in zone 5 (Massachusetts) and do get pretty cold Winters. Given the exterior “seal” should the wall be allowed to “breathe” to the inside? And if so, what other options might best solve the insulation need?

I am not concerned about mold since the XPS and Thermax should not support growth. I do not intend to put up a stud wall with batts and gypsum wall board.

Sorry for such a long first post. I am hoping a member may have encountered and resolved the same issue. Thanks for any help here.

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  1. GBA Editor
    Martin Holladay | | #1

    Your basement walls can stay damp for 200 years without any problems. In fact, damp concrete cures more slowly than dry concrete, which makes it stronger. Your concrete walls can continue to gain strength for many months after the concrete was placed.

    In short, your concrete walls don't have to "breathe" or dry out. Cover them with rigid foam and stop worrying.

    For more information, see:

    How to Insulate a Basement Wall

    Joe Lstiburek Discusses Basement Insulation and Vapor Retarders

  2. ChrisJS | | #2

    Thanks for the help, Martin.

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