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 below shows that the argument should never have existed in the first place. (Click the image to see an enlarged version.)
The psychrometric chart tells all
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 and don’t spend a lot of time above that number.
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 at which to keep your RH, because it’s where mold growth 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.
Of course, the final RH of the air in the crawl space is determined by the RH that the entering air gets to (once it cools) and the mixing of the two air masses. Still, the trend is clear. In a humid climate, when you bring outdoor air into a vented crawl space, its relative humidity goes up.
Vented crawl spaces have moisture problems
Vented crawl spaces can also get cooler than 80°F. I took the photo of the hygrometer below 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 the crawl space has atmospheric combustion appliances, for example, you need to deal with that issue 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.