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Vented Crawl Spaces and the Psychrometric Chart Are Not Friends

Replacing humid air with more humid air doesn't work

Posted on Apr 3 2013 by Allison A. Bailes III, PhD

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 hygrometerA device that measures relative humidity of air. Mechanical hygrometers that rely on a coil of thin metal are not terribly accurate; electronic hygrometers available at most electronic or hardware stores are usually accurate to about plus or minus 2 - 3%. 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, Advanced Energy's website on the subject, and download their 75-page pdf file.

Allison Bailes of Decatur, Georgia, is a speaker, writer, energy consultant, RESNET-certified trainer, and the author of the Energy Vanguard Blog. You can follow him on Twitter at @EnergyVanguard.

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  1. Energy Vanguard

Apr 3, 2013 10:56 AM ET

a question
by JoeW N GA Zone 3A

Allison, what about a crawl in a house the reads high for radon? If the preference is still for nonvented ..... ??

Apr 3, 2013 2:26 PM ET

Passive vents on the walls of
by Dana Dorsett

Passive vents on the walls of a crawlspace isn't really a radon solution, but a 6" layer of pea gravel under an EPDM or 10-mil poly vapor retarder (preferably with rigid insulation and a non-structural rat-slab over it) and either active or passive depressurization of the gravel layer can reduce radon exposures (as well as ground moisture drives) quite a bit.

Rather than looking at a Weather Channel snap-shots of current dew points, it's useful to look at graphs of historical data and averages when making design decisions. In most of the southeastern US (including most of Texas) the summertime average dew points are WAY above the healthy level for indoor air (mold-hazard, even at room temperature), and moldy-stinky crawlspaces are common. Even in the cooler northeast & much of the mid-west the summertime dew points are well above the deep subsoil temps, the net result being that vented crawlspaces under code-min insulated floors are cool enough to condense, adding FAR more moisture to the joist edges than it ever purges (save after flooding events), making a compelling case for sealed conditioned crawlspaces.

But in the Rocky Mountain states or further west (even the foggy-dew Pacific Northwest ) dew point averages are typically low enough that vented crawls offer a net moisture purge. So while there may be valid energy use reasons for building conditioned crawlspaces in these areas, the moisture accumulation aspect falls flat, even with (sealed & insulated) air conditioning ducts in the crawlspace below an insulated floor.

Apr 8, 2013 3:05 PM ET

Doesn't answer its own question!
by Jonas Robinson

I was printing this article to show, and realized it doesn't say anywhere "no, you shouldn't vent a crawlspace in humid climates," and neither the heading nor the subhead are helpful in 'splaining to a layman who just thinks you should.

Apr 8, 2013 3:25 PM ET

Response to Jonas Robinson
by Martin Holladay

If you want to print out an article to convince the unconvinced, maybe this one is the one you want: Building an Unvented Crawl Space.

Apr 9, 2013 9:30 PM ET

Re; Passive vents on the walls
by Tyler Dotten

Dana is 100% right about the fact that passive crawlspace vents are not the answer for radon mitigation (Although vented is slightly better than unvented if only looking at radon). A rat slab would certainly help your radon issues, but having a radon mitigation system installed is by far the best way to make sure radon does not get in to your house (and will cost less than a rat slab in most applications).

It's important to note how big of a role your specific climate plays in this decision, as Allison does in this article. It's easy for people to read an article like this and say vented crawls are bad. While that may be true for any humid climate (really a majority of the country), we here in the Pacific NW have the luxury of being able to choose whether we want vented or not. For me it almost always comes down to cost and b/c I don't like using closed cell spray foam, vented crawlspaces are generally the best bet.

Jul 20, 2013 1:19 PM ET

Edited Jul 20, 2013 1:21 PM ET.

psychrometric chart software
by Sibren Tadema

Recently I wrote an application which simplifies psychrometric chart calculations. Among the people that use it I found quite a lot of architects and building engineers. Therefore, I thought it might be useful for you guys as well. You can download it for free at

Mar 20, 2015 12:06 PM ET

There is a third way
by Shannon Holman

I'm late to this party, but thought it worth mentioning that many of us here in New Orleans live in small houses with open crawl spaces. My little house has soaring ceilings, lots of (leaky) windows, and great cross-ventilation. By keeping my window A/C unit set at about 78 and my ceiling fans humming, I can be quite comfortable in summer, while not creating the kind of high temperature differential between the inside and outside that is likely to cause moisture problems on the underside of my floors.

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