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Using dimple mats and rigid foam in Philadelphia basement

smoke_teff | Posted in General Questions on

Hello all,

I am finishing the basement of my Philadelphia row house and have a few questions about insulating it. In typical row house fashion, the basement is very long (about 50′) and not very wide (about 11′).

We recently had a French drain installed on one of the two long walls (the lower of the two). The contractors installed a dimple mat above the French drain, but it only rises about six inches above the ground–the rest of the wall is uncovered.

1) Should I extend the dimple mat up to grade (about six feet above the slab) on the wall with the French drain? Or on all four walls? Or is the six inches of dimple mat I have currently sufficient?

1b) If you think I should extend the dimple mat up to grade, then when I install the rigid foam insulation, would I simply ‘glue’ it to the back of the dimple mats? 

2) I was originally planning to simply tile over the concrete slab for simplicity and mold-proofing, but after reading around here I am considering a system consisting of a dimple mat (for added drying) underneath a layer of rigid foam, underneath some kind of subfloor. Would it make sense to use cement board above the rigid foam as opposed to plywood/OSB? Would cement board be any more durable than the ‘sacrificial layer’ of plywood/OSB in the event of flooding? 

2b) Assuming I install the flooring system described above, would I have to resurface/repair the concrete slab before installing the dimple mats and rigid foam, etc., on top? The slab is cracked and chipped in certain places, though it’s not in overall terrible shape. 

Thanks very much for your help!

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  1. Expert Member
    Dana Dorsett | | #1

    Has the basement ever flooded above the 6" level of the dimple mat?

    Unless it's an end unit the long walls probably have unheated basements of other units on the opposite sides (?) and only the 11' wide walls are exterior(?). Only the exterior walls need to be insulated to the code-prescribed R10 continuous insulation (2.5" of EPS, or 2" of polyiso). If polyiso is used the bottom edge has to stop a few inches above the high-tide mark of any flooding history, since it can take on moisture which would take forever to dry should there be another event. EPS can go all the way to the slab, even if you have to dado-out to accomodate any dimple mat, but the bottom 6" doesn't add up to a big heat loss even if uninsulated.

    The common walls to adjacent basements can and probably should be insulated to at least R4 (an inch of EPS) if the neighbors aren't heating the basement.

    Unless you're putting down rugs (which could potentially get moldy in summer) it's probably not "worth it" to go full-out on insulating the slab from either a comfort or energy use point of view. Cementicious tile-backers used for walls don't really cut it as a subfloor under tiles.

    1. smoke_teff | | #6

      Hi Dana,

      Do you have any additional thoughts on whether extending the dimple mats is necessary?


  2. smoke_teff | | #2

    Thanks for the response!

    The basement has never flooded above the 6" level. We moved in 18 mo. ago. It flooded after a heavy rain once--maybe 1-2" of water in just one corner of the basement. We had the French drain installed a year ago and no flooding since.

    The 11' wide walls are the exterior walls, yes.

    In terms of the flooring, then, you think we should skip the dimple mat and foam entirely, just repair the cracks in the slab and tile over it?

    And it sounds like extending the dimple mats on the walls is unnecessary and we can just place the foam against the concrete?

    Thanks again for sharing your knowledge.

    1. Deleted | | #5


  3. Dadams | | #3

    Hi, Smoke: CMU block, or poured wall? In NJ, I had damp walls and ended up with dimple mat about 12" up and closed cell spray foam over top, slab up to floor joists. Behind the dimple mat, the contractor drilled into the void spaces in the CMU blocks for drainage, and the dimple mat contained liquid water and gave it a path to the newly installed French drain to sump. Maybe you have some similar drain holes behind the dimple mat?

    1. smoke_teff | | #4

      Hi Darren,

      I believe the foundation wall was originally stone (the house is roughly 75 years old), but has been parged over with concrete. There are some pores in the concrete behind the dimple wall, though it's hard to tell whether the bottom of the parging deteriorated a bit over time or whether it was deliberately broken as a path for the water by the dudes who installed the French drain. My guess is the former.

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