A Dimple Mat To Nowhere?
Hello!
I am hand excavating a small section (40 linear feet) of my home’s old fieldstone foundation in a couple of weeks to have a stone mason friend repoint the exterior wall above grade and down below grade a few feet.
Depending on how tough the digging is, I hope to get down about 3 feet. The whole foundation wall below grade there is just over 5 feet.
I plan on adding below-grade insulation while I’m at it.
Question is: should I add a dimple mat up against the foundation while I am at it too, even if there is likely no footing drain? This section of the house was built in 1830, and I hear the building inspectors often looked the other way about foundation drainage systems back then.
I’ve read the dimple mat article and all the basement insulation articles, but they all discuss how the dimple mat is part of a system with a foundation drain. But I haven’t seen anything about a foundation without a drain.
I do have a french drain pitched to a sump pump in the interior of the basement, and that has dried our basement out really well.
Silt-loam New England soils, seasonally high water table, Zone 5.
Thanks in advance!
Andrew
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Replies
I say no. You want to be directing water away from your foundation. A dimple mat gives it a conduit down to the basement. I can't think of any scenario where the bottom of the dimple mat is a better place for that water than the surface.
Without perimeter drains you want to encourage water to flow away from the foundation. Obviously grading and downspouts are important. Some people with fieldstone foundations will even put a horizontal layer of EPDM a few inches below ground around the foundation. Packed soil can be pretty water resistant, and sometimes it helps to pack the soil around the foundation. So when you fill that trench in fill it a few inches at a time and then tamp it well. You'll probably have to bring in more soil. See if you can get clayey fill.
If you haven't had any water issues in the past, there isn't much need for anything fancy in terms of additional water control like a dimple mat.
Dimple mats provide a means to drain water away to prevent it from building up pressure against the wall and eventually getting through into the basement or crawl space. To be able to work, the dimple mat really needs to be able to drain somewhere, typically into a footing drain. If you don't want to add a footing drain, then there isn't much point to adding a dimple mat. Without a way to drain, all that will happen is the dimple mat will fill up with water like a tiny tank, then you're right back where you started with water pressing against the foundation again.
Bill
Thanks DC and Bill.
Now for the digging fun to begin...