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Foundation insulation & moisture protection

bryanw511 | Posted in Green Building Techniques on

I would like to insulate and protect my basement foundation walls using a water based spray on waterproofing, XPS insulation board, and a dimple mat. In what order would they be fastened to the wall? I feel like the insulation should go on after the waterproofing for it to be effective as an insulator.

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  1. Expert Member
    Michael Maines | | #1

    @Bryanw511, for insulating foundation exteriors, I recommend type 2, 15 psi EPS treated with borates for insect resistance. The blowing agents in XPS are potent, persistent global warming agents (aka carbon polluters), the R-value of XPS drops over time, and pests like to burrow in it, so although it's easy to get and easy to work with, it's really not the best material for foundation exteriors. The R-value of EPS remains constant, and about the same (R-4.2/in) as aged XPS, and its blowing agents are much more environmentally friendly.

    The dimple mat can go on either the interior or exterior of the insulation, since water won't affect the foam, the air gap is small enough that it won't affect the insulating value significantly, and the reason for the dimple mat is to keep water away from the concrete. The small gap between insulation and concrete does not significantly affect the insulating value, in this case, but either location will work.

    The waterproofing should go directly against the concrete, of course. You also need to cover the assembly on the exterior, to a minimum of 6" below grade. You can parge a stucco-like coating directly over foam, use a sheet of fiberglass mesh for reinforcing, or attach expanded metal lathe. Or use sheet metal, pressure treated plywood, or another covering. A lot of builders use fiber-cement panels for this but I'm not aware of any that warranty their product in this application.

    For more info, read this and the related articles listed:

  2. bryanw511 | | #2

    Thanks Michael. Doesn't EPS absorb more moisture compared to XPS? Also, I'm not sure how these materials would be fastened since typically they'd be applied to the concrete and not each other. How would the dimple mat be attached if the foam board were installed first? How would the foam be attached (if even necessary) if the dimple mat were applied first?

  3. Expert Member
    Michael Maines | | #3

    Bryan, the lightest weight EPS has pores that will take on water but it drains readily, and the type that I recommend is about as dense as standard XPS and takes on little if any water. I recently wrote an article for Fine Homebuilding about rigid foam ( but did not include pest resistance information; we have had a fair amount of feedback including photos of just how much damage can be done to XPS. Then again, some people seem to have no problems with it.

    As for fastening, in the article I linked to, Martin writes, "Above-grade insulation may or may not need to be attached to the concrete — fastening methods include foam-compatible adhesive, TapCons with washers, and specialty fasteners like Hilti IDP fasteners or Rodenhouse Plasti-Grip PMF fasteners — depending on the height of the exposed foam and the method used to protect it." On my projects I do my best to avoid exterior foam, but I spec Hilti IDP fasteners for interior foam. They (or most other appropriate fasteners) can go right through the dimple mat as well.

  4. bryanw511 | | #4

    Here's a question, is all 3 of these layers overkill?

  5. bryanw511 | | #5

    How are dimple mats attached to ICF? I'm also wondering what the detail looks like where a dimple mat is fastened to a home basement foundation wall underneath an attached garage slab, since the top normally terminates above grade with some sort of termination bar.

  6. Expert Member
    Michael Maines | | #6

    Bryan, when I design my own foundations, I typically spec all three layers (although I prefer the simplicity of interior insulation) with a note that if the soil drains freely, is not silty and foundation drains lead to open air, that the contractor may eliminate the dimple membrane and/or substitute damp-proofing for water-proofing, at their discretion. Most existing foundations and even most new foundations have only damp-proofing, not water-proofing. But I feel safest with a belt-and-suspenders approach, as a leaky foundation is difficult to fix.

    Brian, I have not placed dimple mats over ICFs so I'm not sure, but I imagine you can find the answer in manufacturers' literature or by contacting them for support. I imagine the dimple mat could hang from the top plate, unless you need a termite inspection strip or flashing.

  7. Expert Member
    Michael Maines | | #7

    Brian, I wanted to find an answer for you, and ironically the first good explanation came from a project I designed! I did not specify the installation details, and had forgotten what Mike Guertin used here:

    Below a garage slab I would just have the term bar stop below the slab, unless I'm missing something from your question.

  8. gotakeawalk | | #8

    When I built my workshop twelve years ago the foundation waterproofing company applied 1" fiberglass drain board which they cut off at grade. I installed 1 foam above their drain board to the top of the foundation to make a flat surface for attaching another layer of foam the entire wall height. It worked quite well. At the time I had a problem with glues holding the foam tight so I used screws and washers. Before screwing I added metal lath to the outside of the foam to hold the cement parging. As far as I can tell its held up well with no problems.

  9. bryanw511 | | #9

    Yes Michael, the article 'A Foundation Like a Cooler' is very similar to what I'd like to do. Possibly with exterior foam board instead of ICF, but that doesn't change the detailing much, if at all. I'm curious to know how the dimple mat would be attached. Also, how would this detail continue around the basement wall beneath an attached garage floor slab? Typically the insulation and dimple mat would extend to above grade. Thanks for all your help so far!

  10. Expert Member
    Michael Maines | | #10

    Bryan, on the ProHome, they just fastened the dimple mat to the attachment strips built into the ICFs. You really shouldn't need much for fastening once the foundation is backfilled. I'm not sure how to fasten through foam, but I would think attachment at the top would be enough. Have you read manufacturer's installation instructions for the product you are planning to use? I answered on another thread where you asked about the garage--I would just stop the dimple mat at a convenient height; there is virtually no hydrostatic pressure near grade.

  11. bryanw511 | | #11

    Thanks Michael.

    I really like the idea of a dimple mat, but layering all these materials makes me feel like I'm, as we say at the office, building a piano. It's pricey, a lot of materials, and possibly complex detailing. I'm all for keeping things simple yet functional. That's why I'm considering waterproofing and then using an insulated drainage board. There are many on the market such as Owens Corning Insul-Drain or Dow Perimate. Does anyone have any experience or opinions on how well these boards actually drain, or are they a gimmick?

    I know some will say why don't I just do waterproofing on the exterior and insulate the interior. Personally I'd be paranoid covering my foundation wall. I'd like to be able to see any possible cracks, leaks, etc.

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