How to Keep your Crawl Space Dry

A crawl space has to be dry. There is no disputing it — there is too much misinformation out there. So here, for the record, is how to keep your crawl space dry and mold free.

Poor grading is usually the cause of water coming into the crawl space.  To keep you crawl space dry look at how the gutters and downspouts are working. Check during a rain downpour where the water runs as it leaves the downspout. The landscape should slope away from the foundation. Water should run over the surface and clearly away from your house. Don’t use mulch to make a slope because it simply doesn’t work. The soil beneath could well be angled back towards the wall. Make sure gutters are cleaned of debris and free flowing.

Keeping your crawl space dry with ventilation only works if the air being exhausted and the incoming air are humidity and temperature controlled. When you use a fan to push air from the crawl space air pressure demands that it is replaced. If the air coming in is the same humidity and temperature as the air it is replacing, the whole exercise is futile.

In summer months where we deal with temperatures over 90ºF and a relative humidity over 8o% the replacement air is not going to help keep your crawl space dry. We don’t want to be pulling in moisture-laden hot air into the crawl space when it is already considerably below the dew point of the imported air. Ground temperatures can be significantly lower than air temperatures. Typically once you are two feet below the surface the temperature stabilizes at around 55ºF. The cooler crawlspace drawing in hot or humid air will result in mildew. So the use of the fan has accomplished just the opposite of what you were trying to accomplish in the first place. If necessary refer to our mold removal guide.

Install a vapor barrier to keep your crawl space dry

keep your crawl space dryUse a black plastic vapor barrier over top of the dirt in the crawl space. It will keep your crawl space dry by preventing vapor pressure from the soil from moving up into the house. Black plastic is preferable to clear plastic because it prevents things from growing underneath it should it get any light. Seal overlapping seams with tape and use roofing tar to seal the edges of the plastic sheeting against walls. This is an effective vapor barrier that will allow air to flow over top and keep your crawl space dry.