In North Carolina, where I live and work, enclosed crawlspace foundations with small vent openings are very common. Most of those crawlspaces experience high humidity, and many have problems with mold and rot.
My contracting firm, The Healthy Building Company, specializes in sealed and insulated crawlspaces, which the newest version of the International Residential Code now allows. Instead of vents, a sealed crawlspace has a continuous vapor barrier to keep ground moisture out of the space.
Part of my company's business is detailing crawlspaces for new homes, as I described in a JLC article last year (Building a Sealed Crawlspace, 10/03). The rest consists of fixing crawlspace problems in existing homes. In this article, I'll show how we recently cured severe moisture problems in the crawlspace under a house in the Blue Ridge Mountains.
This job came our way when we got a call from the retired lady who had recently purchased the home. She was happy with the house in general, but a tradesman working on her deck had looked under the house and advised her to have the crawlspace checked out. Once on site, it didn't take long to see that this crawlspace was in bad shape — about a nine on a scale of ten.
The house is just three or four years old, and when I went there to inspect it, I expected to find a typical mild case: small amounts of moist dirt, poly over most of the floor area, and a few fuzzy spots of early fungal growth on the wood. What I found was a disaster — a dank, dripping, moldy space, totally infested with bugs.
As bad as it was, we were able to fix this space in spite of the owner's limited budget, by cleaning up and and sanitizing the mold-covered materials, installing a vapor barrier, sealing up the two small vents, and dehumidifying the newly enclosed space until the structure was well dried out. I also advised the homeowner that some other changes should be made: Some wood infill walls at the crawlspace perimeter should be replaced with a better system, or they may eventually rot. But that's the nice thing about the way we seal up crawlspaces — when the owner is ready, all someone has to do is cut the poly back, do the work, then install new poly, sealing it to the old with the same kind of mastic and tape that we used.
Bugs, Mold, and Rot
The sources of moisture around this cabin were plentiful and obvious. A nearby creek and pond had saturated the air and soil. In a location like this, any crawlspace needs to have a fully effective vapor barrier or there will be problems.
Those problems were evident as soon as I looked under the house.
The mold and wood rot were everywhere. With water dripping from every surface, it was obvious that the crawlspace was at the dew-point temperature more or less continuously.
The concrete block columns supporting the house at the perimeter were exposed to the outside temperature, so their inside faces in the saturated crawlspace stayed constantly wet with condensation. Moisture from the block was wicking into the adjacent wood-frame, batt-insulated infill walls, built to close up the space and trap warmth during the winter. The pressure-treated wall plate was set directly on the damp earth, where it also wicked moisture into the air and into the wall framing above it.
When you assess a crawlspace, one thing to check out is what kind of bug life it supports. Certain critters are comfortable in a moist environment, and if they've set up shop there, you know that the conditions they prefer are present. In my experience, a sealed, dry crawlspace will only support spiders — they are the only ones that don't seem to care whether the moisture is there or not. But millipedes, ladybugs, and potato beetles, for example, aren't found in a correctly detailed sealed crawlspace.
The crawlspace under this house was packed with mole crickets, a species that thrives in damp conditions. That told me that this crawlspace had been wet all the time, for a long time. In a sealed crawlspace, you'll never see a mole cricket.
It was interesting to see that the spiderwebs under this house, as well as the electrical outlets and even the filaments of fungus, were beaded with moisture condensation. The spiders were nowhere to be seen — apparently they weren't happy with the conditions.
At the back of the crawlspace, things looked really bad. The OSB on the ceiling and the paper face of the insulation are both glistening with moisture and coated with black, slimy mold. If left unattended, these materials would have completely decomposed within a very short time.
