Crawlspace Vapor Barrier
In a recently flooded crawlspace,I have a concrete rat slab(approx 2 1/2″ thick).However the slab has no vapor barrier under it.The slab itself is below outside grade about 18″.I want to control the moisture level in this crawlspace.The crawlspace,doesn’t get standing water in it,nor does the groundwater seem to exert any problems to it.The flood was a freak occurrence.
The slab is rough finished and not very smooth.I wouldn’t trust poly over it,with any activity at all.The subfloor will be removed soon,completely exposing the crawlspace.I want to take care of things then.I suppose I could remove the slab,put poly down and pour a new slab.Or I could put a very heavy poly over the existing and pour a new slab.Or I could make an attempt to seal the existing slab some how.Any suggestions?
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Replies
Dennis,
If you want to avoid the expense of a second slab, you could cover the existing slab with a layer of sand, followed by a plastic vapor barrier (as thick as you want) held in place with bricks or rocks.
If you want a more durable solution, you can pour a second slab on top of the new vapor barrier.
There are epoxy vapor barrier paint-on products that will work over rat-slabs, but I have little direct experience with how to apply them. It's a lot more expensive than poly, but a lot cheaper than a new rat-slab, eh?
I assume you'll be sealing the walls and closing off the vents too(?).
The coating idea sounds good,but how effective?Joe Listiburek talks about it a bit http://www.buildingscience.com/documents/insights/bsi-003-concrete-floor-problems
He mentions calcium silicate in there.However all of the crawlspace recommendations I see show poly under the slab and running up the foundation wall a bit.
Dana,good question.Not sure if insulating the bottom of the joists with foam,or sealing up,insulating the foundation walls and making it conditioned space is the way to go.There are issues either way.
There is another consideration.After Hurricane Irene in 2011,there was about 2 inches of water in the crawlspace and a perimeter french drain and sump pump was installed.A 12' wide trench was created and filled with gravel and perforated drain pipe.After monitoring this crawlspace for a few months,I would say that the pump probably has never come on since it was installed.I think,if anything it just creates more dampness in the crawlspace.I want to fill it back in with concrete and place the pump at the low point and plumb it to daylight.
Thank you for that! From Lstiburek's comment in the link:
"On a positive note, calcium silicate-based penetrants do clog up the capillary pores—i.e. cause “arterial sclerosis” for concrete and do reduce vapor emissions—but they should be limited to concrete slabs that do not get covered with impermeable floor finishes. So they are pretty good for exposed concrete floors that will remain exposed."
I presume you're not going to be putting finish-floor on your rat-slab, which means the penetrating silicate concrete waterproofing solutions will work as well or better in this application than a more expensive vapor retardent epoxy coat. They're not vapor-barriers, but work by dramatically reducing the capillary draw through the concrete, which is usually at least an order of magnitude larger path for moisture than vapor diffusion.
I've used acrylic sealers on my own antique basement slab with reasonable success (a measurable reduction in average basement relative humidity), but it has a shorter lifespan than silicate solutions that penetrate better. (On my "maybe someday" list is to demo the whole slab, dig in better drainage & capillary break, and install a vapor barrier & insulation, but it's pretty far down the list.)