What causes crawl spaces to be moldy?

If the floor is dirty, there will be fungi in the dirt that may produce odors and even spore aerosols when disturbed. Fortunately, crawl space dirt is not generally disturbed, and if odors are not a problem in the occupied space, this source of mold can be ignored. If the crawl space is colder than the occupied space and air from the occupied space enters the crawl space, then condensation will occur on walls and floors regardless of their material. If this condensation is persistent, mold will grow on surfaces, and will produce odors. If the crawl space is consistently under negative pressure with respect to the occupied space, the mold can probably be ignored, although mold can damage wood and other cellulosic materials, and such a space can never be used for storage. If the water table is high and the crawl space becomes wet, then, again, mold will grow. In this case it is important to remediate the space so that standing water does not persist. This water will evaporate and may increase the humidity in the occupied space, possibly leading to condensation and mold on cold surfaces. Sampling and crawl spaces Dirt-floored damp crawl spaces are always moldy, so this is essentially a case of “if you see or smell mold it exists.” There are reasons for sampling, however. First, if the mold is growing on wood surfaces, you need to be sure that the mold is not a wood-rotting fungus (i.e., one that can degrade lignin and destroy the integrity of the wood). This would probably be done with a combination of tape and bulk sampling. For the latter, slivers of the moldy wood or other material should be collected and sent to the lab for analysis. Regardless of whether or not you can smell mold in the occupied space, you may want to sample to check whether or not spore aerosols are entering the occupied space. In this case, spore trap sampling in the crawl space and in the indoor air is appropriate. In addition, I would probably collect tape samples of any visible mold so that the spores can be compared with those in the air. Remediating mold in a crawl space There is no point in removing mold from a crawl space unless you have fixed the water problem, whether it is caused by leaks or condensation. Biocides are not generally useful. They may kill 99 percent of the mold, but the remaining 1 percent is very likely to learn to like the taste of the biocide and take over the environment. Virtually all of you know more about remediating water problems than I do, so I will leave the methods to you. However, I will say that negative pressure in a crawl space with the air vented to the outdoors will work in cases where the owners/occupants cannot afford extensive remediation. Much of what I have said here is based on personal experience of a dirt-floored wet crawl space in my house that has been resistant to remediation. Fortunately, our crawl space is under negative pressure and we don’t have moldy odors in the house. As always, I look forward to your comments. – See more here.