Using a fan or fans to extract crawl space air forcibly
21 May 2005
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 99.9% of the water that gets into a crawl space starts life at that house coming off the roof or from 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 while it’s raining 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 bui lding 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 sloping back towards the house and not away and water will percolate through the mulch and run against the house eventually finding 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 are still getting damp crawl space soils then consider interior steps but only as a last resort. It’s like pl ugging 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 and you can’t fight it. The fan you are referring to moving 140CFM i s pulling about four times the air a common bath fan does.
So where does the make up air come from in June, ,July, August and early September when the outside gets up to 96ºF with a 88% relative humidity? Do you want to forcibly import that 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 hot 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 with the fan. Winter air infusion just increases the heating loads and forcibly importing very cold air can threaten plumbing pipes. If you could get a fan that had controls that sense outside temperature and humidity and turned the fan on when the air is within a certain range then we’d be in business.
Modern building science is tending towards a program of getting crawl spaces dry then closing the exterior vents off and conditioning the air in the crawl somewhat with the home’s air system. It’s difficult to get done right and the whole time there is a battle with the code folks who enforce existing venting requirements. A long time ago while writing about crawl spaces, what leapt out of my head was the remark that anyone who builds a basement so shallow that they have to rename it “crawl” should be hit with a stick and not fed for a week. I still believe that.