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How do I seal a crawl space, with a below-grade basement with an existing slab?

Aaron Lubeck GC | Posted in Green Building Techniques on

I do a load of work retrofitting “new” green building practices into 80-100 year old homes. Sealed crawl spaces is a particular retrofit specialty, but even after installing a half dozen or so we still run into our head scratchers. See, here in the clay filled piedmont of North Carolina, no one builds basements on a flat lot. The drainage problems are too many. Of course, builders didn’t figure that out until mid-century, so there are plenty of historic homes stuck with below-grade basements.

I want to seal them. Here’s how I want to do it:

These basements are designed to let moisture in and get it out via sump pump, so moisture management is key. Sealing it without dealing with the water will trap water and create a pool under the vinyl. Adding to the problem, these old basement slabs have no vapor barrier underneath, so short of pouring another slab I have to waterproof from above. We have been using a roll on waterproofer on the floor, similar to what you’d use on exterior waterproofing. It doesn’t give me total confidence, so on the next project, I want to seal the ENTIRE room with our 6m reinforced vinyl. That includes the walls and floors. I’ll through scrap carpet on the floor, just to protect the seal. For water, I’ll cut out a channel around the edge of the slab (under the vinyl) to direct water to the sump pump, and the pump is engineered to take water from below and above the vinyl barrier.

Covered with our bright white poly, the basement room will look like a looney bin (I say this lovingly); it’s the perfect place for the client’s 3 year old to serve a few time-outs.

Question: For a retrofit, does the vinyl floor and wall seem like a good idea?

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  1. GBA Editor
    Martin Holladay | | #1

    I'm not familiar with North Carolina basements, but your description sounds unusual to someone from New England (like me). Some thoughts:

    1. Most homeowners who end up with a dry basement will want to use it. That means a different floor than carpeting over white vinyl.

    2. How much water does this basement get? Here in New England, we focus less on installing a tight plastic bag, and more on insulation. That means that the walls get either closed-cell spray polyurethane foam (if they are stone walls) or XPS foam (for concrete walls). In many cases, a retrofit job includes 2 inches of XPS on top of the slab — especially if a new finish floor is being installed.

    3. Unless this basement regularly gets an inch of standing water, I would trust the sump pump. If you're worried about the sump pump's dependability, include a battery backup system for the sump pump. Spend a little less money on vinyl and more on insulation.

    4. Anyway, those are the thoughts of a northerner. Maybe Michael Chandler will chime in with a North Carolina perspective.

  2. Michael Chandler | | #2

    Hi Aaron

    We've had to deal with sumps in NC basements a number of times. If you are worried about pump failure it's easy to set an float alarm on a separate circuit than the pump.

    You need to remember that in North Carolina in the summer the humidity outside is so intense that any air entering the basement will condense on any cool masonry surface. (long story there.)

    If you don't see any obvious seepage coming through the walls you are probably fine to seal them with a paint-on cementitious sealer like Thoroseal or Surewall. I'd seal the top of the wall to framing connection with spray foam, remembering to be careful about inadvertently creating termite access to the wood.

    The procedure I like for the floor is to break a 12" channel around the perimeter of the slab and run a perimeter drain with socked slotted drain tile and washed stone. This drain runs into a packaged sump tub with an ejector pump in it that drains out of the basement through 2" pvc with a check valve and union fitting to allow for easy removal of the pump if needed. Set the top of the sump three or four inches above the top of the existing slab. Lay regular 6 mil plastic over the entire floor, cut tightly to the plastic sump tub. Run this poly up the sealed wall then tack on 4" x 1/2" felt slab edge expansion joint material and pull the poly down over that to seal the top edge of the expansion joint. Pour a "rat slab" to the top of the sump to protect the poly and make the floor usable for storage. the "rat slab is not generally held to a very high standard, 2-3" of fiber mesh concrete w/ no re-bar is pretty common. It is often pumped out of a hose very wet on the contour of whatever the grade is in the basement.

    This is somewhat more of a Charlotte NC detail and not seen so much in Raleigh-Durham NC area. (They don't use metal drip edge on their roofs down there like we do, amazing how things change in such a short distance) It leaves you with very clean and dry storage and seals the basement with no white vinyl loony bin effect. If you like you can add a radon vent to the sump and kill two birds with one stone.

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