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Building Science

Four Sources of Crawlspace Moisture

To stop moisture problems in vented crawlspaces, you have to know where the moisture comes from

We know that vented crawl spaces are often wet, nasty parts of a house. But where does the moisture come from?
Image Credit: Energy Vanguard
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We know that vented crawl spaces are often wet, nasty parts of a house. But where does the moisture come from?
Image Credit: Energy Vanguard
This air conditioner condensate line is leaking in a crawl space, adding moisture to the already considerable amount of water in that space.
Image Credit: Energy Vanguard
Soil is another crawl space moisture source. The lighter soil appeared dry but the darker soil, which had been uncovered right before the photo, shows that there's a lot of moisture in the dirt.
Image Credit: Energy Vanguard
Water coming through foundation walls can add a lot of moisture to a crawl space, too. In this case, the backyard sloped down to the house, and the hydrostatic pressure in the soil drove water into the crawl space.
Image Credit: Energy Vanguard
Foundation vents are another moisture source in crawl spaces. On a hot summer day in the eastern U.S., the dew point of outdoor air is often higher than the dew point in a cool crawl space.
Image Credit: Energy Vanguard
An encapsulated crawl space, when done properly, controls all four of those sources of moisture.
Image Credit: Energy Vanguard

Here in the southeastern U.S., we have a lot of crawlspaces. Most are vented. Even most new ones are vented. It’s not because it’s the best way to keep them dry. That’s certainly not true. We have enough research on crawlspaces to know better. No, they’re vented because foundation vents got into the code decades ago and, once there, they’ve been difficult to dislodge.

So if you have a vented crawlspace, especially in a humid climate, it most likely has moisture problems. And where does that moisture come from? Let’s take a look.

1. Plumbing leaks

The first thing you might think of when we talk about water getting into a space is that there is a leak of some kind. A lot of plumbing pipes, both supply lines and drain lines, run through crawlspaces, and they do leak occasionally. Because crawlspaces are visited infrequently, those leaks can go on for a long time before being discovered. This is especially true when a pipe leaks onto dirt or gravel rather than plastic.

One time, we discovered a plumbing leak in a crawlspace after we encapsulated it. How many years was it there before we came along? Who knows?

The first photo below (Image #2) shows another source of leaks in a crawlspace: the air conditioner condensate line. Those pipes often aren’t installed to the same level of quality as regular plumbing pipes. They also pass lower through the space, making them more susceptible to damage.

2. Soil

Another big source of crawlspace moisture is uncovered soil. The second photo below (Image #3) shows our Georgia red clay in a crawlspace. The lighter area was uncovered and looked dry. You might think it’s not putting much moisture into the crawlspace air because it looks so dry. But you’d be wrong.

The darker patch of soil shown above had been covered by the plastic vapor barrier (6 mil polyethylene) before I pulled it back for the photo. It’s darker because it’s wet. The reason the uncovered soil seemed dry is because it was constantly evaporating water into the crawlspace air.

The good news is that most new homes do get vapor barriers put down on the ground, eliminating a lot of the moisture that comes from the soil.

3. Foundation walls

Moisture can also come from the ground outside the house by migrating through the foundation walls. Vented crawlspaces rarely get any kind of damp-proofing or perimeter drains on the exterior. As a result, wet soil outside the crawlspace can come right through, as you see below.

The crawlspace in the third photo below (Image #4) had a lot of this moisture. The backyard sloped down toward the house, putting a lot of hydrostatic pressure against the bare concrete block wall. You can see how well that worked. The whole crawlspace had an inch or two of standing water. And it hadn’t even rained in a while when I visited.

Another way water gets up against the foundation walls is from a roof without gutters, downspouts that don’t move the water away from the house, or rain water in a yard that slopes toward the house. I had an interesting water mystery to solve in one crawlspace I encapsulated, and the source turned out to be one of those three.

4. Foundation vents in crawlspace walls

Finally, we have the vents in the crawlspace walls as a source of moisture. Yes, it’s true. The idea behind those vents was to dry out the crawlspace, but they actually do the opposite, at least in the summer. Here in the Southeast, we have this stuff in the air called water vapor.

When outdoor air comes through those crawlspace vents, it actually raises the relative humidity in the crawlspace. It’s true! If you don’t believe my word, see what the psychrometric chart has to say about crawlspace vents.

How to have a dry crawlspace

If you have a crawlspace and want it to be nice and dry, encapsulation is the way to go. (I’m not a contractor anymore and Energy Vanguard doesn’t do this, but a lot of home performance contractors do.) You cover the ground, the foundation walls, and the crawlspace vents to stop those three sources from wetting your crawlspace. Then you may need to do something about the crawlspace air, too.

After getting it encapsulated, you have a crawlspace that’s beautiful, dry, and, if it’s done right, your indoor air quality should improve. Just ask my friend Perry.

Allison Bailes of Decatur, Georgia, is a speaker, writer, building science consultant, and the author of the Energy Vanguard Blog. You can follow him on Twitter at @EnergyVanguard.


  1. user-741168 | | #1

    make that 5
    5. When rainwater comes from outside through the vent. Most vents are in the top row of block, and it's rare to see 8" or more of exposed foundation in our neighborhood. Hey, that ain't a vent--that's a sluice.

    The very first US Code addressing venting of crawl spaces was the Minimum Property Standards from the National Bureau of Standards in1923. It required 7 square inches of vent area per square foot of footprint (!).. New Zealand adopted the US preposterous subfloor ventilation requirement in 1924,. Nigel Isaacs, on the faculty at Victoria University in Wellington came to the US this year, traveling on a Fulbright Scholarship, in search of the origin of this number. He found it, among one committeeman's papers, in Minneapolis. The guy had written ".7" rather than "0.7" in pencil on the draft and the person drafting the copy took it as "7". Nigel has a lovely photo of the document, and pleasant memories of travels throughout the US in search of the decimal point.

  2. GBA Editor
    Martin Holladay | | #2

    Response to Bill Rose
    Thanks to your research, I've grown accustomed to typing this sentence (which I type a lot): "These building code requirements are not based on science." The sentence is relevant to many discussions, including discussions of attic venting, vapor barriers, and crawl space ventilation.

    I admire your ability to scour old documents in search of "first instances" -- and I love this latest anecdote about the decimal point. Thanks.

  3. user-741168 | | #3

    Allison, Martin,

    Seven sq. in/ per sf is a typo, but it went for two decades before someone did an adjustment. It’s spooky to think that we in the industry may be genuflecting before a typo, and imposing penalties on typo noncompliance. Or worse—the adjusted value may be every bit as arbitrary as the typo, indistinguishable from a typo.

    But perhaps you’re more harsh toward code numbers than I am. For one thing, I’ve proposed cutoff values in ASHRAE standards that thankfully got changed to better values. The starting point is not the value but rather the belief that something must go there and what goes there is a simple coinflip call by the committee. The old guys certainly felt that soil-coupled cavities needed ventilation (not arbitrary), so a cutoff was imposed (arbitrary). That’s reasonable up until it’s time to rethink the legacy requirement. At which point, the first legacy defense is along the lines of—ok, smartypants, what’s the real number? A burden of proof falls on the challenger that the defender never faced. The more direct approach of challenging the assumption, in this case the assumption that the cavity is soil-coupled, encounters the problem of the pure authority that numbers exude over statements. Add to that the problem of confirmation bias in research (research requires funding, funding comes from the establishment, the establishment likes to be stroked not dissed).

    My approach is to swat the number out of the way, and go behind it to the underlying notion of ventilation. Venting has a serious energy penalty. Not venting requires a hard-to-specify level of soil decoupling. Where venting is required (code official) or desired by the owner, I’ve wondered about venting the space under the ground cover, leaving the space above unvented. (You can imagine the details.)

    That doesn’t make crawl spaces any less problematic. I’ve considered issuing a product recall on the crawl space. I appreciate everyone’s efforts here toward improvements.

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