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Fixing a Wet Crawlspace

Fixing a Wet Crawlspace

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    Upon opening the crawlspace door, the author saw large, obvious patches of white wood rot and black mold.

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    Before the remediation

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    The OSB ceiling looks considerably better at the end of the job, after the crew has scrubbed it with a borate solution.

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    Every surface in the crawlspace was dripping wet, including electrical outlets.

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    Even the staples holding insulation batts in place were rusty and gleaming with moisture.

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    Droplets of water were also visible on the filaments of this white fungus.

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    Moisture condensing on the cold masonry columns on the perimeter of the crawlspace was wicking into the adjacent cripple wall framing and insulation batts.

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    There was fungus even on the surface of the treated wood wall plate.

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    In the worst parts of the crawlspace, wood and insulation facings were covered with slick black coatings of wet, slimy mold.

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    Mold on the face of insulation

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    The presence of mole crickets also indicated that the space was constantly damp.

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    The entire crawlspace was equipped with just two small vents. One is shown here as viewed from outside the crawlspace under the exterior deck.

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    OSB and Tyvek between the crawlspace and the floor protected the floor structure from damage, saving the floor (and perhaps the house) from total ruin. The OSB was heavily attacked by fungus, but could still be cleaned up with brushes and a borate solution.

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    The author's old battery-powered sprayer is small enough to drag into hard-to-access spaces.

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    The sprayer allows him to saturate the underfloor OSB with a solution of Tim-bor borate, killing mold, rot organisms, and insects without toxic risk to people.

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    To finish off the space, the author's crew seals the vent openings with duct tape and mastic.

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    The crew applies a poly ground cover, sealed to the base of the crawlspace wall.

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    Carpeting protects the 6-mil poly in the area near the access, creating the perception of a space that should be treated with care.

A Beach House in the Mountains

To cure this crawlspace's problems, we had to understand why they had gotten so bad. Generally, these wet conditions mean that there's a lot of moisture getting into the home, and no way to remove it. In this case, the source was the ground, and the crawlspace details were trapping the incoming ground moisture. The two small vents in the crawlspace walls were totally ineffective at taking moisture out; that's not surprising considering that the air outside the crawlspace was also nearly saturated most of the time.

This house was built by a builder for himself, and he put great effort into the foundation details. As I worked on the house, I was struck over and over by how much this builder cared, and how hard he had tried to do a good job. But each new thing he chose to do made things worse instead of better. The builder has since passed away, and so we can only guess at why he chose the methods he did. But he clearly didn't understand what he was dealing with. Given some guidance, I'm sure he would have built an outstanding house.

The difficulties started when he built a beach house in the mountains. The home sits on columns, with insulation between the floor joists and OSB nailed up underneath the whole house. But in the mountain climate, the floors were too cold in the wintertime, so he went back and closed up the under-floor space with framed wood walls between the columns. I guess he thought this was a good way to trap heat in wintertime — which it is, in principle. But the enclosure also trapped moisture, because he didn't put down a poly ground cover.

In a few places, we could see that the builder stuffed cracks in his cripple walls with fiberglass. It's another piece of thorough work, based on complete misunderstanding: Why would someone go to the trouble of air-sealing a space that they're venting?

What saved this house was the OSB under the floor joists, and the Tyvek under it (there is also insulation between the joists). Those details kept air infiltration from bringing the moisture right into the floor assembly and destroying the joists. The Tyvek didn't make sense at all as a vapor barrier in this application, of course — it's designed to let moisture through. But with the OSB on top of it, the assembly seems to have blocked both diffusion and air transport of moisture. The OSB was heavily attacked by mold and rot, but the joists don't seem to have been significantly damaged.

The Remedy

This job was as bad a case as you would ever want to see. But the solution is pretty much standard: Clean it up, dry it out, and seal it.

We first removed the insulation from the cripple walls, bagging it for disposal. We also had to excavate the crawlspace floor in places to gain access to back portions of the space. The builder had never scraped the organic soil from the site, he just dug trenches for a few center post footings.

I decided to leave the OSB in place, though. Structurally, it was garbage after being chewed on by rot for several years. But it was there as an air barrier, not for structural reinforcement. However, the mold and rot needed to be cleaned up. So we brought in a sprayer and saturated the OSB with a solution of borate. We use either Tim-bor or Bora-Care products (Nisus Corporation, 800/264-0870, www.nisuscorp.com); they are familiar to our clients (Grandma used borate), they don't harm people, and they are about the most effect thing available for killing mold and rot. Borates also kill insects, so we're getting rid of the bugs at the same time as we clean up the mold. In this case, we used Tim-bor, because it comes as a powder that is convenient to mix up and put in our small sprayer.

Any small sprayer or power-washer can handle this work. Mine is an old one that runs off a 12-volt car battery. Its portability helps when we need to access crawlspaces through rear or side doors in houses that are far back from the road.

We saturated the OSB thoroughly with the borate solution — I wanted it completely soaked — and scrubbed the surface with stiff bristle brushes. Then we laid down temporary construction poly and set up our dehumidifier. It took a week of constant operation for the dehumidifier to dry out the OSB.

On a less severe case — the typical crawlspace with just a few spots of mold and wood moisture content of 14 to 18% — I would just put down the permanent sealed poly, then dehumidify. But the 25 gallons of water I added to the OSB is my responsibility, and I needed to pull that out before I sealed anything for real. If I don't, it will come out by itself and end up causing mold.

In the week it took for the OSB to dry, all the bugs and flakes of fungus rained down onto the construction poly. When we pulled out the poly, all that debris came with it.

Next we installed our final poly. We used the same methods as in our new crawlspaces: Lay down 6-mil black poly, overlap it a foot, duct tape the seams, and apply non-toxic duct mastic over that. In this case, we used PS-1 mastic from RCD Corporation (800/854-7494; www.rcdcorp.com), our usual preferred product. We sealed up the vents with duct tape and mastic.

Unfortunately, I couldn't run my poly up the crawlspace walls like I usually do. That would be like putting the wood wall in contact with the moisture conditions under the poly, and it would have quickly rotted out the untreated framing. So we ran the poly up onto the side of the treated-wood plate, adhered it with mastic, and nailed a 1x2 batten over it for protection.

We install the final poly in our sock feet because we don't want to scuff up the poly even a little bit. When I give the crawlspace back to the homeowner, I want it to have that new-car shine and smell. In fact, some of what we do is just to influence the perception of anyone who looks in. That's part of the idea behind the carpet runners we install before we leave, for instance. We could use super-heavy-duty poly instead of 6-mil material, and not need to protect it with carpet. But I want the people who come down there to see the carpet and recognize that this is a conditioned space, not your average crawlspace.

When we left this job, we hadn't exactly created a formal living room, but we did accomplish our goal: We got rid of a mold and rot problem that could have destroyed the whole floor, we provided a boundary between the home and the soil that will improve energy efficiency and maintain good air quality in the home, and we created an underfloor space that people can enter and work in as needed without risking their health and safety.

Contractor and consultant Jeff Tooleyis owner of the Healthy Building Company, based in Bear Creek, N.C.