Foundation plan details: Block wall, XPS, and drain tile
Hi everyone,
Hoping I could get a bit of advice on my current exterior wall plan for my split level. I have 2 figures in my drawing. Fig. 1 is for section of the house that has no daylight basement, fig. 2 has a daylight basement. Daylight basement is a little less then 4ft below grade. This is a retro fit job. Inside of basement is already complete. House is running radiant heat.
House foundation is cement block, then it was parged, water dampening proofed and then a 3 mill plastic was added then backfilled (1960’s). The plastic is almost non-existent after digging up some of the wall. I had some questions on my plan.
Fig.1 – Above my water dampening I was thinking of adding a 6 mill of plastic then my xps, thoughts? Is it necessary to add miradrain on the outside of the XPS for this section of the house since there is no basement? Would the XPS if taped at seams be able to stop hydrostatic pressure and water? Perhaps I should add a 6 mill plastic on the outside of the XPS.
Fig.2 – The bottom of the foundation is the weak point (Circled). I can see hyrdrostatic pressure pushing water in at the bottom. XPS isn’t going to fit perfect down there since its sloped. Should I be worried here? Should I apply the miradrain flush to XPS or give it a bit of a curve as in the drawing? Should I maybe add some tar paper at the bottom below the xps and miradrain to help keep water headed toward drain tile?
I thought about going with a elastomeric coating like hlm5000 but not sure it will work on top of the asphalt based stuff on the wall already there.
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Replies
Dan,
Q. "Above my water dampening [dampproofing] I was thinking of adding a 6 mil of plastic, then my XPS, thoughts?"
A. Using 6 mil polyethylene in this location is not typical. It won't be durable and it won't be waterproof (because it is so easily damaged.) I would skip the 6 mil poly and trust the other elements of your assembly to handle any water.
Q. "Is it necessary to add Miradrain on the outside of the XPS for this section of the house since there is no basement?"
A. Strictly speaking, no. But if you have everything opened up, and you're doing all this work, you might consider including the Miradrain product all around, just to keep your foundation area as dry as possible. The cost of the Miradrain is much less than the cost of the excavation and all of your labor costs.
Q. "Would the XPS if taped at seams be able to stop hydrostatic pressure and water?"
A. No. The way you handle the hydrostatic pressure and water is with the Miradrain, along with (ideally) free-draining backfill material (gravel) and a good footing drain.
Q. "Perhaps I should add a 6 mil plastic on the outside of the XPS?"
A. Again, no one uses 6-mil polyethylene for this purpose. Trust the usual materials -- dampproofing, the Miradrain product, good backfill, and the footing drain -- to do their work.
Q. "Fig.2 - The bottom of the foundation is the weak point (Circled). I can see hyrdrostatic pressure pushing water in at the bottom. XPS isn't going to fit perfect down there since its sloped. Should I be worried here?"
A. Probably not. If your footing drain is working, everything will be fine.
Q. "Should I apply the Miradrain flush to XPS or give it a bit of a curve as in the drawing?"
A. I don't think it matters.
Q. "Should I maybe add some tar paper at the bottom below the XPS and Miradrain to help keep water headed toward drain tile?"
A. Tar paper (asphalt felt) will do nothing here. It will be consumed by microorganisms in one year.