Protect new CMU wall from backfill?
Hi all,
I just had a new CMU wall, ~20″ high put up between the finished portion of my basement (slab poured by previous owner) and the portion that is still a crawlspace (home is on a hill, walk out basement, crawl starts where basement walls are approximately 2-3 ft below grade and extend “uphill” to the front of the house from there, where the crawl is several feet below grade.)
The previous owners had just poured a slab and put the 2×4 plate on it, no vapor barrier. The dirt in the crawl was on the back of the wood framing and Sheetrock. Real Mickey mouse stuff.
The new 20″ high wall comes a few inches higher than where the crawlspace dirt resides at that point in the basement, giving me a clean demarcation between the finished and unfinished parts of the basement and separating any wood framing from the dirt.
I want to backfill the wall with gravel so there’s not any risk of someone breaking an ankle/knee working back there on the mechanicals and to arrest any movement of the dirt to a degree. How should I prep the wall for this to keep any moisture from the dirt from crossing into the finished space? It’s “interior” dirt and generally quite dry, but I’d rather be safe than sorry.
Cheers,
Nathan
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Replies
I hope others more knowledgeable than myself answer this. I'm following this discussion because it's a very interesting problem.
My gut feeling is that the poly vapor barrier should continue all the way to the wall and down the crawlspace side to below the finish floor level. I'd be inclined to take a poly sheet all the way across, backfill with a well draining gravel, and the lap the sheet over the crawlspace sheet and tape the joint. I'm also thinking to treat it like a french drain and put a small perforated pipe down there, daylighting somewhere outside the house. Maybe I'm overly cautious but I'm thinking about those rare occasions when things get really, really wet. Those are the times we need to plan for.
My tentative plans is to roll an asphaltic fluid applied water barrier onto the back face down to the slab, and then cover that with 2" of EPS before backfilling with #57 gravel, potentially with as you mentioned a French drain in the bottom. I was then going to terminate the poly to the top of the wall. Since the EPS can be totally under a slab and VB, enclosing it like that shouldn't cause an issue?
I worry about poly VB against the block trapping interior moisture and causing mold issues.
The existing slab was done 20+ years ago and has no signs it was installed with a vapor barrier, so sealing down to the slab is probably the best I can do. I'm doing work on the lot to improve drainage to give it the best shot to stay dry possible.