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Q. Our home has a foundation crawl space that a person can stand in that narrows down to a shallow crawl area towards the front of the house. Our recent rains and poor drainage on one side of the house have caused the ground in this crawl space area to become partially damp and I’m worried about mold. The crawl space has vents along the sides that we have been told were the code when the house was built. We have been advised to use a fan or fans to pull out moist air. We are looking at a low amperage fan that is supposed to remove about 140 cubic feet of air per minute(CFM) which then is supposedly replaced by fresh air pulled in via normal vents. Is that enough?
A. Crawl spaces need to be dry. There is no argument about that. Where we get ourselves into trouble is wading through the myths and mysteries surrounding just how to get crawl spaces dry and how to keep them that way.
Normally most of the water that arrives in a crawl space starts life coming from the roof or the lawn due to poor grading. Getting water away from the building is not only step one but the most important step. Take a cold hard look at how the gutters and downspouts are working. Go outside during a rain storm and watch what’s going on and where the water wants to run once it’s exited the downspout. If the gutters are overflowing-- clean them. Make sure the grade slopes away from the building and water runs over the surface and away from the foundation. Remember mulch isn’t dirt so even if you’ve got mulch piled up so it looks like it’s sloping away from the house, the packed dirt underneath of it may be sloped back towards the house and not away and water will leak through the mulch, run against the house to find its way in. If you have exhausted any and all means to control water from the exterior, including adding an exterior drainage system if you have to, and you a still have wet crawl space soils then consider interior steps but only as a last resort. It’s like plugging leaks in the hull of a boat from the inside. It usually doesn’t work in the long run. Crawl space ventilation is only productive if the air being vented and the replacement air are controlled relative to its temperature and humidity. If you pull a cubic foot of air out of a crawl space, the decrease in air pressure caused by the removal of that cubic foot of air will pull replacement air from wherever it can until the air pressure inside of the crawl space is equal to the air pressure outside of the crawl. It’s Physics 101 and you can’t fight it. The fan you are referring to moving 140CFM is pulling about four times the air a common bath fan does. So where does make up air come from in June, July, August and early September when the outside air gets up to 96ºF with a 88% relative humidity? You don’t want to import hot, moist summer air into a crawl space whose ambient temperatures will be significantly lower than the dew point of the imported air. Ground temperature is 55ºF or so around here once you get about two feet below the surface. The cool crawl space environment will condense the water out of that humid air forcibly dragged into the crawl the results of which will be mold, mildew, rust, rot etc. Exactly the reverse of what you intended to have happen using the fan. If you could get a fan that had controls that sense outside temperature and humidity and turned the fan on only when the air is within a certain range then we’d be in business. Placing a black plastic vapor barrier over the earth is a good idea to keep vapor pressure from the soil from moving up into the house. I recommend black plastic as clear plastic allows things green to grow underneath of it should it get any light.