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One or two dimple mat/basement questions

smoke_teff | Posted in General Questions on

Hi All,

I posted last week but didn’t get much of a response and could still use some guidance on a few questions.

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 (two common walls, each about 50′) and not very wide (two exterior walls, each about 11′).

We recently had a French drain installed on one of the two 50′ common 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. Note: The basement has never flooded above the 6″ mat. The most flooding we ever had was about 2″ in one corner of the basement, but that was before installing the French drain

1) Should I extend the dimple mat up to grade (about six feet above the slab) on the wall with the French drain? Or is the six inches of dimple mat I have currently sufficient? Should I add dimple mats to the walls that don’t have any?

2) I have seen some basement flooring systems that make use of dimple mats, rigid foam, and plywood as subflooring. I was thinking it might be easiest (and most mold-resistant) simply to tile over the existing concrete slab. Is there any problem with this idea? Is there any need for dimple mats or other subflooring underneath the tile?

Thanks very much for your help!

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  1. GBA Editor
    Brian Pontolilo | | #1

    Hi Smoke Teff.

    If you have water intrusion issues on your basement walls and you plan to finish them with insulation, a stud wall, drywall, etc. then you will want to continue the dimple mat up to the top of the wall. If you plan to leave them unfinished, I'm not sure that it is necessary. You'll find some more discussion in these two articles:
    Using a Dimple Mat to Keep a Basement Dry
    How to Fix a Wet Basement

    As far as the slab goes, it is possible to use a dimple matt or a basement subflooring product if the floor has the potential to become wet. Even if your slab stays dry, it likely doesn't have a vapor retarder beneath it. Before installing flooring, you may want to consider some vapor control and a layer of rigid foam insulation is a good way to do this.

    It is also a good idea to decouple tile from the slab to prevent cracking. Many builders use Schluter's Ditra membrane or similar products.

    I know that there are a bunch of GBA members with more experience installing basement floors than I have, perhaps they will chime in.

    1. smoke_teff | | #2

      Hi Brian,

      I wouldn't say we have 'water intrusion issues.' Before installing the French drain, there was a bit of flooding after especially heavy rains due to the rising water table. No flooding or seepage issues since installing the drain, and in particular no water coming in through any walls so far (though of course this could change?).

      In the last few months I have covered the walls in an additional layer or two of parged concrete, plus waterproof paint. When it comes time to add the insulation, I plan to use EPS for its water-resistant properties relative to polyiso.

      Is a vapor retarder necessary even with mold-proof flooring such as tile? Would the benefit just be keeping vapor from coming into the living space? We plan to have a dehumidifier running and draining into the sump pump, for what it's worth.

      In terms of the tile cracking issue, we have been planning to do a sort of broken tile mosaic, partly to prevent future cracking and partly because it will enable us to use leftover tiles from many sources (which we expect to be cheaper than buying 500 sq. ft. of the exact same kind of tile).

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