Vented Crawl Spaces and the Psychrometric Chart Are Not Friends
Really, the argument about whether you should vent a crawl space in a humid climate is over. Advanced Energy's research project from 2002 proved that closed crawl spaces outperform vented crawl spaces. A quick look at the psychrometric chart shows that the argument should never have existed in the first place.
I know it's a bit hard to read, but if you click the image, you can see an enlarged version.
What's going on here is that we're starting at the blue dot to the right of the arrow. It's a summer day, the outdoor temperature is 90° F and the relative humidity (RH) is about 53%. I chose that RH because here in Atlanta, we have a good number of hours with the dew point at about 70° F. By looking all the way to the left of the chart, where the relative humidity is 100%, you can read the dew point.
The point of that blue dot is that when that air comes into the crawl space, it cools down. I chose 80° F as the temperature it reaches in the crawl space. By looking at the relative humidity curve it lands on, you can see that it went from 53% to about 70% RH.
That is NOT a good number to keep your RH at because it's where mold can start taking off. The more time your crawl space spends at 70% RH or higher, the more likely you are to have mold growing.
Vented crawl spaces can also get cooler than 80° F. I took the photo of the hygrometer above on a warm August day here in the Atlanta area, and you can see that the air was about 70° F with 92% RH.
And then there's the evidence, of course. If you've spent any time in vented crawl spaces, you know—and your lungs and nose know—that they have problems. The photo below is from one I was in last month. Duct leakage exacerbates the problem here.
You can't just seal up every crawl space you see, however. If it has atmospheric combustion appliances, you need to deal with that first. For the best guidance on how to do this right, go to crawlspaces.org, Advanced Energy's website on the subject, and download their 75 page pdf file.