Foam instead of drain board behind foundation wall
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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