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Foam instead of drain board behind foundation wall

blamus20 | Posted in Energy Efficiency and Durability on

Very typical scenario: sloping lot with daylight basement. The uphill concrete foundation wall is 10′ tall x 70′ long and will be below grade (its a very steep slope – house is 22′ wide). Peel n stick membrane as waterproofing (fluid applied is 5x more costly @ 26sqft/gal, $300/5gal)

Normally you would install drain board/dimple mat on top of the waterproofing membrane, the idea is to direct hydro-static pressure away from the concrete wall, and more importantly, protect the waterproof membrane from the backfill.

This is where I’ve been thinking of an alternative. Instead of spending the money on the drainboard – about $500 before adhesive, why not spend say $700 on 2×2″ XPS? The XPS will protect the membrane, and even though it will allow water to get to the membrane there wont be enough space to build up hydrostatic pressure on the concrete. And then do the french drain as usual, or in my case, use a product like Miradrain HC + outlet channel instead of pipe+rock. Miradrain HC is basically a 2′ wide strip of extra capacity drain board that connects to a 4″ pipe, it is designed to be used without drain rock and has a higher flow rate than 4″ perforated pipe. Think of it as a “strip drain”. This saves me a lot of headache as it saves me from importing more rock to behind the wall which is already extremely difficult to access. The extra volume taken by the 4″ thick foam also reduces the amount of backfill I’ll need – again, saving me labor on dumping dirt behind the wall which is now very difficult to access. 

The manufacturer probably won’t approve the use of miradrain HC without installing the normal miradrain 2000 (dimple mat) above for the entire wall, as it was designed as a complete system. But with XPS foam whats the point of using dimple mat? Even if the soil is saturated (because no drain rock), there should be little hydrostatic pressure on the concrete itself? Regardless, the soil should drain naturally to daylight because its a very steep sloping lot. 

If this is my only insulation there will be some thermal bridging, but this foam can be thought of as additional to whatever insulation I was going to frame onto the interior side of the concrete wall. If it end up being necessary.

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  1. Expert Member
    Akos | | #1

    I've used grooved EPS around foundation. These have channels in the back side and work the same as dimple mat. My local box store carries these but much cheaper from a commercial supplier (typically used for EIFS).

    For the extra bit of cost is definitely worth it as now you can insulate your basement interior with standard batts.

    The only challenge is finish it, simplest I've found is covering it with metal flashing. There are some stucco products sold by ICF manufacturers that could also be applied that will last longer then the metal.

  2. blamus20 | | #2

    I've been looking at products just as you decribed - insulfoam from CCW and insudrain boards. But at $130 for 4x8 that seams really cost prohibitive.

    I guess at this point Im more interested in the question of the need for drainage channels if the damp soil is only going to be bearing on the foam, and together with a good waterproofing membrane behind the foam, and for a steep sloped site where water can drain out of either sides of the foundation wall to daylight, is it too risky to just put normal EPS or XPS infront of the waterproof membrane?

    1. Expert Member
      Akos | | #3

      Regular foam should work just as well provided your water proofing is solid. Memrane type waterproofing is pretty robust.

      Grooved EPS should not be that much, I would try to find a better source:

      1. blamus20 | | #6

        Thank you, I will go look at my local lowes tomorrow but online I dont see any EPS boards with drainage channels. But if I cant find any, it sounds like you agree that just regular foam boards will work just fine. The $130 price I qouted from this website:

        but i agree thats ludicrous, I hope I can find a more reasonable supplier.

  3. onslow | | #4


    I am curious about two things based on the photo you posted of the foundation. How will anyone fit in that gap to apply the membrane waterproofing? Also, if the uphill wall is 70' long, did you have an engineer certify that the midpoint of the wall can take the potential slumping pressure of the soil? It does look quite rocky, so maybe it won't move, but if there is much ground water coming down slope has anyone discussed the possibility of the hill moving?

    I remain puzzled by your description of what your plan is with the Miradrain products. The literature would appear to present a very aggressive dimple mat with attached geofabric to be applied to the wall. The convenient dimple mat plus pipe connector boots would be applied at corners of foundations. where the dimple mat is no longer placed.

    It would seem perfect for simultaneously promoting very fast drainage and protecting the membrane on the wall. It might even eliminate the need for the membrane by virtue of virtually eliminating any hydrostatic pressure on the wall. Perhaps damp proofing would be sufficient. Only question is how clean the back fill would need to be. The fabric might not like backfill as rocky as the soil in your photo.

    1. blamus20 | | #5

      we actually have enough space to walk through behind the wall so clearance isnt a problem. see attached photos. should be enough space to apply peel n stick membrane and a few inches of foam. The entire house and foundation is engineered and the engineer has approved us to backfill without floor framing. The site is extremely rocky and took us a year to excavate (main slow down because of snow) This is CO about 30 miles from rocky mountain national park, so everything has to be engineer to rather high wind (175mph) and snow loads.

      The miradrain HC is basically a strip of very large dimple mat with fabric as the filter, and is said to drain faster than schedule 40 perforated pipe. Its just that it was designed to be installed with the normal size miradrain dimple mat on top (e.g. miradrain 6000), but since I am now thinking about adding foam board, I'm hoping to skip the upper dimple mat installation. I'd like to install the miradrain HC as a standalone strip at the bottom of the wall, either directly ontop of the peel n stick membrane and therefore under the foam, or directly on the foam - in effect replacing the pipe. And then at both ends, connect to a pipe and drain to daylight. You can see in the photo that water should drain out the sides of the wall rather quickly due to the steep slope. (the opening you see if the garage door)

      They do mention that with the dimple mat you can do damp proofing instead of waterproofing, but since I do have living space there, I might as well do the waterproofing properly with a membrane, but yet I don't see the need for a full height dimple mat if I'm installing foam boards as well, which was my original question.

  4. onslow | | #7


    Wow, did I ever misread the photo. That is quite a tall wall. And very tough wall if the backfill doesn't require decking out first. When you get done, be sure to post some of the views, they are sure to be spectacular.

  5. JayMart | | #8

    Hi Bernard - I'm facing this same exact situation at the moment. I'm going with a sprayed on membrane for waterproofing (TUFF-n-DRI) and plan on using 2" of EPS (no grooves). What I'm facing now is do I need the drainage mat or not? What did you end up going with?

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