Sealing a Crawlspace in North Carolina

Close the vents and let a heavy-duty vapor barrier keep moisture, mold and radon out of the living space.

See and feel the dramatic transformation!

Proper detailing changed this crawlspace from a spelunker's nightmare into a bright, clean, dry storage space.

If it's in your crawlspace, it's in your house
As warm air rises inside the house, replacement air enters from the lowest part, often the crawlspace. Properly sealing the crawlspace and channeling rainwater away from the house can provide mold-free and radon-free living spaces.

Problem Roof runoff and improper grading allow water to collect, where it can enter the crawl space through cracks in the foundation walls. Humid summer air enters the crawlspace through foundation vents and condenses. Radon rises through the soil, into the crawlspace, and ultimately into the living space.
Crawl Space Solution Roof runoff is directed away from the house with gutters and proper grading. Sealing foundation vents keeps humid air out of the crawlspace. A sealed crawlspace keeps moisture from the ground out and passively mitigates radon. In tough cases, a PVC pipe and inline fan exhaust radon outside.

GOT WATER? PUMP IT OUT

Dirt crawlspaces are never-ending sources of moisture.
Even if the dirt's surface is dry, digging down a little bit reveals moist earth. Moisture ruins houses by providing a hospitable environment for the fungi, mold and insects that destroy wooden framing. Moisture in a crawlspace affects not only the floor system directly above it but also the entire house. Warm air in a heated building rises. As it rises, replacement air is sucked from the lowest part of a house. This natural air movement, called the stack effect, is how chimneys work. Consequently, whatever is in the air at the lowest point eventually flows through the upper sections of living space (drawing right). If mold spores and radon are present in the crawlspace, you can bet they're in the living space as well.

Separate the house from the earth and keep out the weather
With little headroom, light and habitability, the crawlspace may not seem to be an important part of the house. In fact, it's very important. Moist crawlspaces may be a bigger problem than wet basements (see "Keeping a Basement Dry," FHB #140, pp. 64-69) because they can produce an unseen moisture stream through the building envelope (drawing above).

To avoid moisture's negative effects, a crawlspace should be fully sealed and isolated from the ground and the outside.
Part of my technique involves placing a 20-mil, 7-ply sandwich of high-and low-density polyethylene with polyester-cord reinforcement on the dirt floor and up the walls. I have this pool-like liner-like sheeting made specially for my system. It is easily strong enough to crawl on and to store materials on. Its bright white color makes the crawlspace a bright, pleasant place to be.

A vapor barrier such as Tu-Tuf could also be used, but it offers less durability, UV-resistance and fire-resistance. Some contractors solve the problem of moist crawlspaces by pumping in concrete. But for this method to work, the concrete needs a vapor barrier. Even with a vapor barrier, this alternative doesn't address water-vapor diffusion through the walls. If the vapor barrier is doing all the work, then why use expensive concrete?

Despite what the building code says, many colleagues and I believe that venting crawlspaces is a bad idea. Code requires 1 sq. ft. of ventilation for every 150 sq. ft of dirt floor. Using a vapor barrier over the dirt floor reduces the ventilation requirement to 1 ft. per 1500 sq. ft. of floor space. The intent is to vent out the humidity that the exposed earth lets in.

But venting a crawl space creates its own problems. In winter, there's an energy penalty: cold floors and higher heating costs. In summer, vents actually admit moisture in the form of warm, humid air. Warm air can hold more moisture than cool air. Warm air entering a cool crawlspace can reach its dew point and give up its moisture as condensation. Relative humidity, dew point and the stack effect combine to make crawlspace vents more likely to compound a moisture problem than to alleviate it.

For these reasons, I close the vents outside and seal them from the inside with 2-in. thick foam insulation and polyurethane caulk. And yes, I sleep well at night because I am doing the right thing for my clients.

Heavy-Duty liner is cut to size in the daylight
Plastic liner is more easily handled in the driveway. After the plastic is cut to size and rolled up, it can be installed in sections in the crawlspace. I have liner material custom-made, but lighter plastic such as Tu-Tuf can also work.

Damp crawlspaces can make your living room moldy
Almost weekly, we see news stories about mold spores. Molds and individual tolerances to mold species vary. Some mold has little effect other than an unpleasant odor, but some can be toxic (most notably Stachybotrys chartarum; see "Cross Section", FHB #150, pp. 46, 48). Mold needs organic material, moderate temperatures and moisture to grow. Remove one factor, and mold can't grow. Of these factors, moisture is the easiest to control. Mold thrives in damp environments. Using a hygrometer, we routinely find humidity readings well above 50% in dirt crawlspaces. Some are as high as 80%. This is compared with readings around 27% in conditioned space. According to the Environmental Protection Agency (EPA), relative humidity should be kept below 60% (ideally between 30% and 50%) to control mold. Properly sealing a damp crawlspace lowers relative humidity to within these levels. For more information on mold prevention, contact the EPA Indoor Air Quality Information Clearinghouse. - L.J.

Wet crawlspaces need drainage

In addition to water vapor, many crawlspaces leak groundwater. Such cases require a drainage system appropriate to the details of the crawlspace. Outside, I make sure downspouts are directed away from the house. Inside, I grade to one corner and install a sump and a pump with a sealed lid. Groundwater that leaks in can make its way from the dirt floor into the sump and be pumped out.

If a lot of water is leaking in, I create a swale in the dirt at the perimeter to channel water into the sump more directly. As another option, I can trench in a perforated pipe around the perimeter pitched to the sump. To prevent the pipe from clogging with silt, I slip a filter-fabric sleeve over it. Except in cases where there's extreme flooding or where a concrete floor will be poured, I avoid using crushed stone on perimeter drains because it's heavy and it's hard to lug though small openings.

I do one more thing before installing the liner: clean the dirt. All sharp or large rocks are buried when I regrade, thrown around the sump liner or removed along with any wood or other organic material.

Installing and sealing the crawlspace liner

Once the crawlspace is cleaned out and drainage issues are solved, I install the liner. The liner material comes in a 24-ft. by 50-ft. accordion-folded roll. It's much easier to handle the 105-lb. roll outside than in the crawlspace, so I roll it out in the driveway to cut it to size (photo above). I then fold and roll up the liner like a carpet, black side out, and bring it into the crawlspace.

I start with a piece of liner wide enough to cover the floor from the center row of piers to the perimeter, and up to the top of the wall. Next, I cover the other three walls, making sure the liner is long enough to overlap the floor by about 1 ft. Then I cut the remaining floor piece. After I roll out the liner, I take off my boots for the rest of the job to keep the liner clean.

Now, I turn back to the walls. I cut the liner 2 in. from the top of the wall and fasten it with nylon expansion fasteners that press in 1/4-in. holes that I drill every 3 ft. or 4 ft. along the top edge. I install the fasteners 3-in. down from the top of the liner (photo right) so that I can pull down the liner to seal it to the wall with polyurethane caulk (photo below right). Polyurethane caulk is the only caulk I use on my projects; it sticks to anything and lasts for ages. I use Bostik 916 or Vulkem 116.

I never seal the liner to the sill plate because doing so gives water vapor a path to
the wood and can be a route for termites and other insects to get to the house framing. Leaving the liner 2. In. down from the top of the wall allows a routine termite inspection.

At wood posts, I lift the weight with a hydraulic jack, if possible, and slide the liner under. If I can lift the post only a bit, I slide a piece of aluminum flashing under the post and seal the liner to the edges of the flashing with caulk. At masonry piers or columns, I cut slits and wrap the liner up 6 in. or so, then caulk. At seams, I caulk under the overlap and use a 4-in. wide peel-and-stick tape made from a matching material, which makes the seams disappear. A high-quality builders' tape such as Tyvek tape also would work.

The finished system looks fantastic. My customers are pleasantly shocked when they see the end product. The white color reflects light, and it's clean and mold-free. Indoor air quality is improved, and the crawlspace can be used for storage.

Nylon Fasteners secure liner to foundation
To keep water and insects away from floor framing, the liner is cut in place to 2 in. below the top of the wall

Holes are drilled though the liner North Carolina crawl space repairinto the foundation (photo above), and nylon expansion fasteners are driven into the holes.

Seal the liner to the foundation with polyurethane caulk.

It seems like a single sheet, but it's not. Overlapping seams of the liner sections are sealed with caulk and matching tape for a seamless barrier.

Developed and distributed exclusively by Basement Systems Inc., manufacturer of a full line of basement waterproofing and crawl space products.

Larry Janesky is president of Basement Systems Inc. in Seymour, CT. Photos by Harold Shapiro, except where noted.