Sealing aCrawlspaceClose the vents and let aheavy-duty vapor barrierkeep moisture, mold andradon out of the living space

BY LARRY JANESKY

Dramatic transformation. Proper detailingchanged this crawlspace from a spelunker’s night-mare (inset photo) into a bright, clean, dry stor-age space.

FEBRUARY/MARCH 2003 95Drawings: Mark Hannon

Problems Solutions

A. Roof runoff andimproper grading allow water to collect, where it canenter the crawlspacethrough cracks in thefoundation walls.

A. Roof runoff is directedaway from the housewith gutters and proper grading.

B. Sealingfoundation ventskeeps humid air outof the crawlspace.

B. Humidsummer airenters thecrawlspacethroughfoundationvents andcondenses.

As warm air rises inside the house, replacement air enters from the lowest part, oftenthe crawlspace. Properly sealing the crawlspace and channeling rainwater away fromthe house can provide mold-free and radon-free living spaces.

irt crawlspaces are never-endingsources of moisture. Even if thedirt’s surface is dry, digging down

a little bit reveals moist earth. Moisture ruins houses by providing a hospitable envi-ronment for the fungi, mold and insects thatdestroy wood framing. Moisture in a crawl-space affects not only the floor system direct-ly above it but also the entire house. Warm airin a heated building rises. As it rises, replace-ment air is sucked from the lowest part of ahouse. This natural air movement, called the

stack effect, is how chimneys work. Conse-quently, whatever is in the air at the lowestpoint eventually flows through the upper sec-tions of living space (drawing above). If moldspores and radon are present in the crawl-space, you can bet they’re in the living spaceas well.

Separate the house from the earth,and keep out the weatherWith little headroom, light and habitability,the crawlspace may not seem to be an impor-

tant part of the house. In fact, it’s very im-portant. Moist crawlspaces may be a biggerproblem than wet basements (see “Keeping aBasement Dry,” FHB #140, pp. 64-69) be-cause they can produce an unseen moisturestream through the building envelope (draw-ing above).

To avoid moisture’s negative effects, acrawlspace should be fully sealed and isolatedfrom the ground and the outside. Part of mytechnique involves placing a 20-mil, 7-plysandwich of high- and low-density polyeth-

D

I F IT ’S IN YOUR CRAWLSPACE, IT ’S IN YOUR HOUSE

Crawlspace dampness canbreed mold in livingspaces.

Dry crawlspacediscourages moldgrowth in livingspaces.

C. Radon rises through the soil, into thecrawlspace and ultimately into the living space.

C. A sealed crawlspace keeps moisture from the groundout and passively mitigates radon. In tough cases, a PVCpipe and inline fan exhaust radon outside.

96 FINE HOMEBUILDING

ylene with polyester-cord reinforcement onthe dirt floor and up the walls. I have thispool-liner-like sheeting made specially for mysystem (www.basementsystems.com; 800-541-0487). It is easily strong enough to crawl onand to store materials on. Its bright white col-or makes the crawlspace a light, relativelypleasant place to be (photo p. 94). A vaporbarrier such as Tu-Tuf (Sto-Cote ProductsInc.; 800-435-2621) also could be used, but itoffers less durability, UV-resistance and fire-resistance. Some contractors solve the prob-lem of moist crawlspaces by pumping inconcrete. But for this method to work, theconcrete needs a vapor barrier. Even with avapor barrier, this alternative doesn’t addresswater-vapor diffusion through the walls. Ifthe vapor barrier is doing all the work, thenwhy use expensive concrete?

Despite what the building code says, manycolleagues and I believe that venting crawl-spaces is a bad idea. Code requires 1 sq. ft. ofventilation for every 150 sq. ft. of dirt floor.Using a vapor barrier over the dirt floor re-duces the ventilation requirement to 1 ft. per1500 sq. ft. of floor space. The intent is to ventout the humidity that the exposed earth lets in.

But venting creates its own problems. Inwinter, there’s an energy penalty: cold floorsand higher heating costs. In summer, ventsactually admit moisture in the form of warm,humid air. Warm air can hold more moisturethan cool air. Warm air entering a cool crawl-space can reach its dew point and give up itsmoisture as condensation. Relative humidity,dew point and the stack effect combine to makecrawlspace vents more likely to compound amoisture problem than to alleviate it.

For these reasons, I close the vents outsideand seal them from the inside with 2-in. thickfoam insulation and polyurethane caulk. Andyes, I sleep well at night because I am doingthe right thing for my clients.

Wet crawlspaces need drainageIn addition to water vapor, many crawlspacesleak groundwater. Such cases require adrainage system appropriate to the details ofthe crawlspace. Outside, I make sure down-spouts are directed away from the house. Inside, I grade to one corner and install asump and a pump with a sealed lid. Ground-water that leaks in can make its way from thedirt floor into the sump and be pumped out(photo left).

If a lot of water is leaking in, I create a swalein the dirt at the perimeter to channel water

Got water? Pump it out

If gutters and exterior grading won’t stop water from coming in, a trench,perforated drainpipe and a sump pump may be required to move it out. Awater-removal strategy could be one of the following scenarios.

If the ground slopesto one corner,

install a sump in thelowest corner.

If the ground slopesto one side, dig atrench to channel

water to the sump.

If the ground slopes more than

one way, trenchin a perforated

pipe around theperimeter to carry

water to the sump.

Plan views

FEBRUARY/MARCH 2003 97

Almost weekly, we seenews stories about mold

spores. Molds and individualtolerances to mold speciesvary. Some mold has little ef-fect other than an unpleasantodor, but some can be toxic(most notably Stachybotryschartarum; see “Cross Sec-tion,” FHB #150, pp. 46, 48).

Mold needs organic materi-al, moderate temperaturesand moisture to grow. Re-move one factor, and moldcan’t grow. Of these factors,moisture is the easiest tocontrol. Mold thrives in dampenvironments. Using a hy-grometer, we routinely findhumidity readings well above

50% in dirt crawlspaces.Some are as high as 80%.This is compared with read-ings around 27% in condi-tioned space. According tothe Environmental ProtectionAgency (EPA), relative humid-ity should be kept below60% (ideally between 30%and 50%) to control mold.

Properly sealing a damp crawl-space lowers relative humidi-ty to within these levels.

For more information onmold prevention, contact theEPA Indoor Air Quality Infor-mation Clearinghouse(www.epa.gov/iaq/molds/;800-438-4318).

—L. J.

Damp crawlspaces can make your living room moldy

Plastic liner is more easilyhandled in the driveway. Afterthe plastic is cut to size androlled up, it can be installed insections in the crawlspace(photo above). The author hasliner material custom-made,but lighter plastic such as Tu-Tuf (800-435-2621) also can work.

HEAVY-DUTY

LINER IS CUT

TO SIZE IN

THE DAYLIGHT

98 FINE HOMEBUILDING Fastener photo: Roe A. Osborn

Holes are drilled through the liner into the foundation (photo above),and nylon expansion fasteners aredriven into the holes (photo right).

To keep water and insects away from floorframing, the liner is cut in place 2 in. belowthe top of the wall.

into the sump more directly. As another op-tion, I can trench in a perforated pipe aroundthe perimeter pitched to the sump. To pre-vent the pipe from clogging with silt, I slip afilter-fabric sleeve over it (photo p. 96). Exceptin cases where there’s extreme flooding orwhere a concrete floor will be poured, I avoidusing crushed stone on perimeter drains be-cause it’s heavy and it’s hard to lug throughsmall openings.

I do one more thing before installing the lin-er: clean the dirt. All sharp or large rocks areburied when I regrade, thrown around the

sump liner or removed along with any woodor other organic material.

Installing and sealing the linerOnce the crawlspace is cleaned out anddrainage 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 han-dle the 105-lb. roll outside than in the crawl-space, so I roll it out on the driveway to cutit to size (photo p. 97). I then fold and roll upthe liner like a carpet, black side out, andbring it into the crawlspace.

I start with a piece of liner wide enough to cov-er the floor from the center row of piers to theperimeter, and up to the top of the wall. Next,I cover the other three walls, making sure theliner is long enough to overlap the floor by about1 ft. Then I cut the remaining floor piece. Af-ter I roll out the liner, I take off my boots forthe rest of the job to keep the liner clean.

Now, I turn back to the walls. I cut the liner2 in. from the top of the wall (photo top left)and fasten it with nylon expansion fasteners(Outwater Plastics; www.outwater.com) thatpress in 1⁄4-in. holes that I drill every 3 ft. or

NYLON FASTENERS

SECURE LINER TO

FOUNDATION

FEBRUARY/MARCH 2003 99

4 ft. along the top edge (photo bottom left, fac-ing page). I install the fasteners 3 in. downfrom the top of the liner (photo right, facingpage) so that I can pull down the liner to sealit to the wall with polyurethane caulk (top pho-to). Polyurethane caulk is the only caulk I useon my projects; it sticks to anything and lastsfor ages. I use Bostik 916 (800-472-9430;www.bostikfindley-us.com) or Vulkem 116(www.roancorp.com; 800-321-7906).

I never seal the liner to the sill plate becausedoing so gives water vapor a path to the woodand can be a route for termites and other in-

sects to get to the house framing. Leaving theliner 2 in. down from the top of the wall al-lows a routine termite inspection.

At wood posts, I lift the weight with a hy-draulic jack, if possible, and slide the liner un-der. If I can lift the post only a bit, I slide apiece of aluminum flashing under the postand seal the liner to the edges of the flashingwith caulk. At masonry piers or columns, Icut slits and wrap the liner up 6 in. or so, thencaulk. At seams, I caulk under the overlapand use a 4-in. wide peel-and-stick tape madefrom a matching material, which makes the

seams disappear. A high-quality builders’ tapesuch as Tyvek tape (www.tyvek.com; 800-448-9835) also would work.

The finished system looks fantastic. My cus-tomers are pleasantly shocked when they seethe end product. The white color reflectslight, and it’s clean and mold-free. Indoor-airquality is improved, and the crawlspace canbe used for storage. �

Larry Janesky is president of BasementSystems Inc. in Seymour, CT. Photos byHarold Shapiro, except where noted.

Seal the liner to the foundation with polyurethane caulk.

Seems like a single sheet, but it’s not. Overlapping seams of the liner sections are sealedwith caulk and matching tape for a seamless barrier.

Properly lappedliners ensure thatwater runningdown the wallhas no chance toget inside thecrawlspace.

Tape

Floor lineroverlaps wall liner.

Overlapping sequence is critical

POLYURETHANE CAULK

AND PLASTIC TAPE SEAL

THE DEAL

Polyurethane caulk