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Community and Q&A

Rigid foam and dimple mats

Nina In CNY | Posted in Green Building Techniques on

My concrete contractor will be applying bituminous waterproofing tar on the outside of the basement walls up to grade. We are considering putting 4″ of rigid to the exterior of the concrete after that. Someone advised us to also put up a dimpled waterproofing membrane, but is his necessary if we are applying the 4″ of rigid foam?

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  1. Dick Russell | | #1

    A dimpled membrane will help provide free drainage of water (rain) only if there is something over the membrane to allow the dimples to provide a drainage gap. A good waterproofing application directly onto the concrete (not thin "damp-proofing") will keep water out, although it would be good practice to backfill with something porous, like sand. A layer of rigid foam against the dimpled membrane will provide that drainage gap. However, for insulation purposes, the foam ought to go up to the sill, as the foundation above grade sees the coldest winter temperatures. Bringing the foam up to the sill introduces the need to protect it against UV and physical damage. Then, too, is the matter of the foam extending beyond the plane of the siding and thus the need to flash it. On my house, I put 2" outside my foundation, and later another 2" inside. Thus I had to protect the outside foam (I used surface bonding cement, which has provided durable), but also I had to protect the inner layer for fire protection. If I had to do the house again, I'd just put all the insulation on the inside, with the fire protection over it, which I had to do anyway.

  2. Malcolm Taylor | | #2

    The dimples on membranes like those made by Delta, don't need an additional cover. The bumps provide a gap between the solid plastic surface and the concrete behind.

  3. Nina In CNY | | #3


    Thank you for your insight. I'm in zone 5, and this is exactly what I am struggling with now. I know in the future we will finish the basement, which will require us to potentially insulate on the inside based on if/or how we insulate on the exterior. I fear damage to the exterior foam, or my husband getting frustrated with it and just cutting it to grade and flashing it, which which will impact interior insulation requirements.

    I was thinking of just putting 2" on the exterior with the assumption that we will still need to insulate on the interior, and the worse thing is that the basement is warmer than it would have been if I did not put anything on the exterior.

    Malcolm, I am learning that there are 'dimple' membranes and 'dimpled' membranes. Based on the type used, does the foam go to the interior or exterior of the membrane?

  4. User avatar
    Michael Maines | | #4

    Nina, the issues stated above are exactly why I prefer, when possible, to insulate foundations on the interior. An elastomeric waterproofing compound on the exterior is important if you want a dry basement, and a dimple mat is good insurance. Thermax-brand foam insulation is usually allowed to remain exposed on the interior, but it's not inexpensive and can't be used to cover another, cheaper insulation. Another option is to spray foam the interior of the foundation, preferably with a product that has low global warming potenial, such as Lapolla's Foam Lok 2000 4G or Demillac's Heatlok XT HFO.

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

    Some installers sandwich the dimple mat between the concrete wall and the rigid foam. Other installers put the dimple mat on the exterior side of the rigid foam (between the rigid foam and the backfill).

    The detail below shows the latter approach. It comes from the Building America Solution Center.


  6. Nina In CNY | | #6

    Michael, Thanks, I was considering the spray foam option, particularly since it will not need to happen right away.

    Martin, Thank you for the diagram, I have been looking for one all weekend. Do you think the decision to put the membrane on the inside or outside typically depends on the the type of membrane used (rigid/exterior to foam or the peel -n- stick/interior to foam)?

  7. User avatar GBA Editor
    Martin Holladay | | #7

    Peel-and-stick is not a dimple mat. If you are using a peel-and-stick product, it goes on the concrete.

    If you are using a dimple mat, it can either go against the concrete (assuming there is no peel-and-stick), or between the peel-and-stick and the rigid foam (assuming you use peel-and-stick), or on the exterior side of the rigid foam, as shown in the illustration I posted.

  8. Jonathan Lawrence CZ 4A New Jersey | | #8


    My vote is to insulate from the inside too. I just went through a foundation pour a few months ago and I avoided a whole bunch of issues by choosing to insulate from the inside. I plan on placing 4" of EPS sub slab and 2 1/2" on the walls. I have been eyeing a product by Insofast and while I see them at all the trade shows, I finally decided to buy some to test it out - see pics. What I like about this product is that it is easy to install, it has electrical raceways built in, and it has plastic studs every 16" OC that you can screw the gypsum directly to so no need to create an interior wall for the gypsum. It is not cheap (pricing is on their website), but it is a DIY friendly product.

  9. Nina In CNY | | #9

    Thanks for the clarification Martin.

    Jonathen, That is an interesting product, and definitely worth my time to research; thank you. We were intending to insulate the sides of the slab with 2" of foam also. Do you see any problems with using that product should we do that?

  10. Bryanw511 | | #10

    Martin, I'm interested in doing exactly what's shown in the picture you shared (comment #5). How do you suppose the dimple mat is fastened? What would the detail look like beneath an attached garage floor slab?

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