4 Ways Moisture Enters a Vented Crawl Space

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Crawl space moisture sources

Here in the southeastern US, we have a lot of crawl spaces. 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 crawl spaces to know better. No, they're vented because foundation vents got into the code decades ago and, once there, something like that is difficult to dislodge.

So if you have a vented crawl space, 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's part of your house is a leak of some kind. A lot of plumbing pipes, both supply lines and drain lines, run through crawl spaces, and they do leak occasionally. Because crawl spaces 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 crawl space after we encapsulated it. How many years was it there before we came along? Who knows!

Water leaks are one source of crawl space moisture

The photo above shows another source of leaks in a crawl space: 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 crawl space moisture is uncovered soil. The photo below shows our Georgia red clay soil in a crawl space. The lighter area was uncovered and looked dry. You might think it's not putting much moisture into the crawl space air because it looks so dry. But you'd be wrong.

crawl space moisture source - uncovered soil

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 crawl space 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 crawl spaces rarely get any kind of damp-proofing or perimeter drains on the exterior. As a result, wet soil outside the crawl space can come right through, as you see below.

The crawl space in the photo below 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 crawl space had an inch or two of standing water. And it hadn't even rained in a while when I visited.

Crawl space moisture source - foundation walls

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 crawl space I encapsulated, and the source turned out to be one of those three.

4. Foundation vents in crawl space walls

Finally, we have the vents in the crawl space walls as a source of moisture. Yes, it's true. The idea behind those vents was to dry out the crawl space, 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.

crawl space moisture source - open foundation vent

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

How to have a dry crawl space

If you have a crawl space 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 crawl space vents to stop those three sources from wetting your crawl space. Then you may need to do something about the crawl space air, too.

A beautiful, dry, encapsulated crawl space

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

Related Articles

Solving a Crawl Space Water Mystery

What Is the Best Way to Deal with Crawl Space Air?

Vented Crawl Spaces and the Psychrometric Chart Are Not Friends

Beautiful Duct System in a Beautiful Crawl Space! Was I Dreaming?


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As John Tooley suggests:
Keep bulk water OUT!
Direct water that gets in to OUT!
Keep moisture out!
Be aware of & prevent diffusion!


John Tooley knows a thing or two about crawl spaces and moisture!

Volunteered to assist for several of his presentations at the HPC Conference in Greenville, SC last week ... got to spend a little time ... fascinating, personable, knowledgeable, enjoyable!
Love the "Tooleyisms"
"people don't make mistakes, processes do!"



Thanks, Michael.

Don't forget leaky ductwork can bring it in from the main house, especially in winter. Warm humid air leaking out & condensing in the coolness of the crawlspace. If you have leaky ducts you are literally pumping warm humid air in the return ducts into that space.

I'm surprised you haven't covered this topic before... or maybe I missed it? Vented crawls in humid climates are just dumb. Closed crawls are nice but are expensive when done properly.

For non-basement homes, slab-on-grade is more efficient and economical. For some reason, slabs are associated with low-end production homes in many parts of the country. OTOH, here in the Southwest, slab-on-grade dominates the market, including high-end. One benefit of slab construction in cooling climates: the slab provides passive cooling as long as there's no insulation beneath. Of course, edge insulation is critical, since the majority of heat loss is at the edges.

I prefer basement foundations because they offer the best opportunity to keep mechanicals inside without having to resort to an encapsulated attic, another pricey compromise. I always recommend suspended grid ceilings for finished basements. This provides easy access to all plumbing, electrical and mechanical systems for maintenance and future upgrades.


David, I've written about crawl spaces quite a bit. See, for example, Vented Crawl Spaces and the Psychrometric Chart Are Not Friends. It's been a while, though, so I took up the subject again, with a different way of presenting it.

Slab-on-grade foundations also dominate in Texas. I'm with you on basements, though. Unfortunately, for those not in a cold climate or building on a slope, the expense of digging that hole doesn't make sense.

Allison wrote: "for those not in a cold climate or building on a slope, the expense of digging that hole doesn't make sense."

Well, I'm in a warm climate, and preparing to dig a big hole for a basement home on a flat lot. Check back with me later to see how the cost compares, but it definitely will cost less than building the same size home on grade (slab and roof would be twice as large). OTOH, when compared with two floors above grade, yeah, a basement probably does increase the cost a bit.

Question. The inside vertical foundation walls are covered with 6 mil in an encapsulating best practice. This being sealed completely to the underside of the horizon ground spread 6 mil, Is this vapor,and bulk water against pressing the entire block wall letting moisture to wick up the wall around the termite barriers and into the home? In other words, if the block is wet under the 6 mil..how do we kept the wood dry on top where the deck plating is?

A termite shield would stop termites as would a roll of sill seal underneath the sill plates help stop wicking moisture from attacking the sill plates. The sill plates are also pressure treated so that in itself should alleviate all these issue.

A few years ago we encapsulated the crawl spaces under five 6-unit condominium buildings in central Vermont, built in the early 1970s. We used Delta Drain on the ground for our drainage plane and vapor barrier, and 3" SPF on the walls and band joist. These crawls are 4-6 ft. deep with poured concrete walls and dirt floors. We installed a 100 cfm Panasonic fan in each to provide some air exchange for the 2,000 sq.ft. basement crawls, in accordance with Building Science Corp.'s recommendations.
I have monitored the temperature and RH in each building since day one, using an on-site Acurite tower with remote, such that someone had to periodically go to the building to read the inside and outside temp. & RH. In the first 1 year, the temp. never went below about 50 degF and RH never went above 50%. Now I am finding the RH getting rather high in summer, as high as 80%RH at 60 degF (right now). In one building, someone apparently turned the fan off over a year ago, thinking that it was causing backdrafting of their wood stove (it wasn't), and that basement crawl has about the same temp and RH.
It appears that the only solution is turning off the fan in summer and running a dehumidifier in each basement?

Great articles.

BTW, at the end of the article, buried two levels deep in the 'encapsulation' link, is Advanced Energy's excellent webpage on closed crawl spaces: www.crawlspaces.org

@Brad, sounds like the fan is pulling most of its makeup air from outside rather than from the house. You can verify with two blower doors. The house with disabled exhaust fan has a moisture superhighway through the disabled fan housing.

Closed crawls don't work very well if they're not built tight. This can be difficult in retrofit projects. One option would be to add a small transfer grill between house and crawl, thus changing the makeup air source (it will follow the path of least resistance).

Also, keep in mind that 80% @ 60F corresponds to a dew point of 54F, which is not excessive. A small amount of heat would keep the crawl RH under control. Was the floor insulation removed? Mechanical dehumidification should only be considered as a last resort.

I have new construction clients with crawls that are sufficiently tight that code-required drying mechanisms are not needed. I advise my clients to stipulate in their contract that closed crawls be verified tight with a blower door (requires two rigs, one to null the house). I typically specify a small amount of supply air to satisfy code, then have homeowner close the vent(s) and monitor RH to see if stays below 70%.

Hi David. I did do some pressure diagnostics after the work was done and measured 10-15 Pa difference between the basement crawl and the units above, with the fans off. I did not do a guarded test of just the crawl spaces, since we had to use 6 blower doors for each building as it was. Between that result and visually verifying that all of the foundation and band joists were covered with at least 3" CC SPF, with no visible possible paths to outside from the crawl, I conclude that the basement crawls are well connected to the living spaces above them, and well sealed to the exterior. I can also visually ID leakage pathways between these spaces.
FYI, these are 3 over 3 units with every unit having a direct access to the exterior. The crawl space between each stack of 2 units is poured concrete with sufficient holes in each to consider them all pneumatically connected. The Association owns the crawl spaces, while the individual units are owned by private individuals (as well as the access to the crawls). Most units are second homes which are not occupied all of the time. None of them have air conditioning (not very common in Vermont). In one building, the owner of a ground floor end unit was adamant that we could not put any rigid or foam insulation in "her" crawl space. The Association had not pushed the issue in order to avoid a court battle. That one crawl is the only one that is not insulated and still has significant leaks to the exterior (we were able to completely seal it from the rest of the crawl space in that building). [I am now monitoring that crawl and the two adjoining crawl spaces for temp.&RH through a wi-fi connection. The differences are interesting.]
Some of the unit owners on the lower level did insulate the floor. In all but two cases, we removed that insulation (fiberglass) as part of our work. One unit had SPF applied to the underside of the floor and the other had fiberglass batts added, with netting. Since there was no mice activity in that insulation and since the unit owner wanted a "warm" floor, we did not argue and left it in place.
We are finding that in some crawls, the high RH is condensing on the plastic water lines passing through, creating puddles of water on the Delta Drain. In addition, it is my understanding that an RH of >~70% can allow mold to grow, even without condensation on surfaces. Owners are not to store any "stuff" in the crawl spaces, but some do. Then they complain when mold grows on their "stuff".
I would expect that since the basement crawls are well connected to the living units above, that moisture will diffuse from the warm humid air above to the cool crawls below, over the course of a summer.
Now what do you think?

Brad wrote: > it is my understanding that an RH of >~70% can allow mold to grow

I think that's a good guideline, although Lew Harriman opened my eyes to the complexities involved in mold growth.

I don't buy that the high crawl RH you observed is residual moisture that migrated down to the crawl over the summer. Partial vapor pressure differentials are highly cyclical. There can be significant lag if a building is reasonably sealed, as well as hygric buffering (storage), but I can't see that playing out over a period of weeks, let alone months.

Unless there's a significant interior moisture source (foundation moisture, inadequate spot ventilation, etc), I think you'd likely find that the crawlspace and exterior dew points swing roughly together. But again, 54F dp isn't that high. The only reason it manifested as 80% RH is because the temperature was allowed to drop to 60F. (BTW, here's a handy dew point calculator: www.dpcalc.org)

Looking at the temp./RH in crawl spaces vs. the living space above, for the past couple of weeks, shows me that the RH in the crawl is following the RH above, with just a bit of a lag and with swings not as wide as above. I am confident that there is no moisture entering the crawls other than what is in the air from living spaces above. I also do not see any potential for hygric buffering.
That is a handy dew point calculator. I have been using a psychrometric chart to determine dew points and changes in RH with temp.
There has been some condensation in the crawls from potable water pipes being at the dew point. That can be stopped by insulating all of the cold water piping.
My main concern for the crawls is possible mold, especially with the potential at >70% RH. I am also looking for guidance on ventilating the crawls during warmer months, so as to not introduce lots of moisture by ventilation. I could probably answer the question if I continue to monitor the crawl spaces remotely for another year, and see how quickly the dew point in the crawls responds to changes in the outside air, without the fan running.
Is there a particular article or book that you would recommend from Lew Harriman on this particular subject?

When I said crawl and exterior dew points would swing roughly together, I didn't mean the crawl is more connected to outside than units above. I was just saying that unless there's a significant interior moisture source, crawl moisture must be driven by infiltration, regardless of whether it's at the foundation or from units above. I was simply refuting the notion that crawl moisture was driven by an accumulation over the summer.

As you know, a significant internal moisture source can be diagnosed by monitoring outdoor and indoor dew points in the units, evidenced by dp spikes that don't correspond to outdoor conditions. That would be the best scenario in terms of remediation. But again, the high RH you noted in the crawl was associated with a not-so-high dew point.

BTW. since crawl perimeter is tight, I don't see how running exhaust fan in warmer months would be helpful. An exhaust fan can be useful if a crawl is more connected to the outside than the rooms above. Essentially, it tips the pressure balance to ensure crawl is dried by house air. In this case, it might make sense to run the exhaust during the cooler shoulder months when crawl temp drops too low. This would pull in warmer air from the units above, thus raising crawl RH.

Based on everything you've said (and excluding an internal moisture source, e.g., inadequate spot exhaust in the units), I think you need to focus on preventing crawl from getting too cool. One option would be to use a humidistat set to, say, 60% RH, to control the exhaust fan(s).

To those who would bypass diagnostics and go straight for a dehumidifier, I'd say that's an expensive band-aid rather than a solution. It's usually better to attack the problem rather than the symptom. In this case, the problem appears to be crawl temperature, not excess moisture.

Here's a link to Lew's excellent mold presentation at the 2015 Hot Dry Climate Forum: www.bit.ly/2frLBMo. Contrary to the title, the focus is not specific to dry climates!

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