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Please Help!!! Dimple membrane a necessity?

user-1021391 | Posted in General Questions on

I am doing a finished basement that has a perimeter drain (see photo). To clarify, I want to put a stud wall along the edge of the perimeter drain. I purchased some 2.5-inch felt backed poly-iso and had intended to stud against it. Do I put the 2.5-inch rigid foam on the cinder block wall (it would be over the drain), and then add the stud wall to meet the rigid foam? Would dimple membrane be needed or would any condensation run down the foam board into the french drain to the sump? Walls are painted with drylock; no water problems in basement with sump working in heavy rain. I need advice on best method. Thanks for any suggestions.


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  1. user-2310254 | | #1


    Where are you located? Advice is often climate-specific.

  2. user-1021391 | | #2

    Sorry. I'm located in Cleveland, Ohio area (Climate Zone 5A)


  3. user-2310254 | | #3

    Hi Rasheed,

    You would not want to apply the polyisocyanurate directly against the wall since it can absorb moisture and the perimeter drain suggests you have an ongoing water management issue.

    There are a number of threads on GBA that discuss using dimple matt to create an air space between the foam and the wall. Search on terms such as wet basement and dimple matt. Here is a link to get you started:

  4. user-1021391 | | #4


    Thanks you so much for your help. Your recommendation makes sense. Walls being dry today don't guarantee tomorrow during a 100 yr storm. With a basement finishing project to follow, it is certainly better to be safe than sorry and for a 200 sf basement the extra cost is worth the peace of mind.


  5. GBA Editor
    Martin Holladay | | #5

    I agree with Steve. If you have an interior drain, the dimple mat is a good idea.

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