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Community and Q&A

Foam over Platon dimple mat on foundation

Jessie Pratt | Posted in Green Building Techniques on

We are stumped at a stage in our building process. Winter is here and we need to deal with our foundation. Our site is dry dry dry, but we are not ready to just attach foam to the cement and backfill without any effort to damp/water proof the foundation.

We are essentially forced to use a mechanical system as the weather and temperatures (-10 degrees celcius and wet) are making any other sort of damp/water proofing impossible (as far as we know).

The main concerns with applying the Platon dimple mat is the difficulty of application and the question of what you do once you’ve reached the top of the dimple mat and you want to continue up the foundation/onto the rim joist with the foam. Should we just add a shim under the foam so that it remains in the same plane as the foam below (which is over the dimple mat)? Any other suggestions on damp/water proofing?

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

    The simplest detail, it seems to me, would be to continue the Platon dimple mat right up to the top of the concrete foundation wall. That eliminates the need for shims.

    Just be sure to complete the air barrier at the top of the dimple mat; canned spray foam might make sense at this location. If your wall plates aren't cantilevered over the concrete -- or if you aren't installing rigid foam on your above-grade walls equal in thickness to the rigid foam on your foundation -- you'll need Z-flashing at the top of the Platon dimple mat. The Z-flashing should also protect the top of the foundation foam.

  2. Jessie Pratt | | #2

    Thanks Martin!

  3. Joshua Terry | | #3

    I know this is old, but: as long as the dimple mat is air sealed, it won't reduce or get rid of the insulation value of the foam correct?

    I am struggling to come up with an acceptable waterproof membrane for my exterior crawl space encapsulation. I am skeptical of liquids (staying attached over time and proper coverage) and peel and stick is pricey, so if I could use dimple mat and air seal to avoid loosing the insulating benefit of 4" of EPS, that would be great.

  4. User avatar GBA Editor
    Martin Holladay | | #4

    Q. "As long as the dimple mat is air sealed, it won't reduce or get rid of the insulation value of the foam -- correct?"

    A. Correct.

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

    I think that your approach makes sense, as long as you have a plan to protect the above-grade rigid foam from sunlight and physical abuse.

    To learn more about materials to protect your rigid foam, see this article: "How to Insulate a Basement Wall." (Scroll down until you see the heading in bold font that reads, "If I insulate on the outside, how should I protect the above-grade foam?")

    To learn more about integrating exterior rigid foam and dimple mat, see this article: "Using a Dimple Mat to Keep a Basement Wall Dry."

  6. Jay Lista | | #6

    So I've already got dimple mat installed about half way up my walls (to grade level) that feeds into a french drain.

    The dimple mat sits 5/16" off the wall and was installed with ramset nails with washers so they protrude another 1/2" or so off the dimple mat. Complicating matters is the bottom 2' of the wall had an extra 1/4" layer of concrete installed many years ago, prior to the dimple mat.

    My plan was to install 2" polyiso directly on the concrete above the dimple mat with foamboard adhesive. Then, I would put 1 1/2" polyiso on the upper portion of the bottom half, and 1 1/4" (1/2" + .3/4") polyiso on the very bottom of the wall, and get it all as level as possible on the wall, with all seams taped and as many gaps that I can access filled with can foam. These boards over the dimple mat would be installed with mechanical fasteners.

    Then, I would put a second layer of 3/4" inch polyiso over the entire wall, using foam board adhesive.

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