moisture in crawl space - navigating my options

kimbermannySeptember 29, 2011

I have a home built in (very humid!) Atlanta in 1950, and expanded in 1970, so it's on two dirt crawl spaces. We're having issues with moisture in one specific area of the newer crawl that shows up after heavy rains - we notice because of a strong muddy smell (and sometimes a mildew-y smell follows that). We've gone under to look when the smell is there, and there is moisture that seems to be coming through the brick. We've never seen puddles of standing water, but there is mud splashed on the walls, so that suggests we may have had some puddles in the past. The ground on the outside of the house in that area is shady, and tends to hold moisture (there is moss growing there) but it slopes away from the house. We have gutters that we haven't always done a great job of keeping clear.

The older crawl does not have a vapor barrier at all, and is powder dry. The newer crawl has a 6 mil vapor barrier, but it doesn't cover all of the ground. There are vents for both crawls that are just open year round, covered with mesh - I don't think they can be closed. We have a new, expensive HVAC unit installed in the older crawl. There's no insulation under our house at all (sometimes I can see sunlight through the floorboards near the vents!), but I think that's a whole 'nother issue.

We've had 6 different companies come out to look, and each has a very different answer for us, at very different price points. I think we've got a consensus that this isn't a major foundation issue, and that while there is some mold and mildew in the crawl, that it's minor and isn't going to require any real remediation effort.

The options that are seeming the most sensible (er, cheapest) to us are to either:

option from contractor 1 - install a french drain to lead water away from the problem area, and then install a 6 mil vapor barrier in on the ground only in the crawl, and leave the brick uncovered. (cheapest quote)

option from contractor 2 - lay a thicker (12 mil) vapor barrier on the ground in the crawl, and run a thinner (10mil) vapor barrier up the brick, so that the entire crawl is covered up to the wood beams. (second cheapest quote)

But we've also had a contractor tell us we don't have a problem at all, one suggest we need a dehumidifier / air exchange system installed, one suggest we need total crawl space encapsulation, and one suggest we need $20k of foundation repair. No one seems to be on the same page about what to do! And I'm out of contractor referrals, and the internet is failing me.

Anyone dealt with this kind of thing, or know anything about it, or know where I could turn for advice?

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You can ventilate to allow water vapor from the bare earth to escape, or you can try to cover the bare earth to prevent water vapor from entering the crawl space in the first place.

If you place a vapor barrier on the bare earth and fasten it to the walls you can keep water vapor from the crawl space.

Wood battens on the walls with a butyl caulk seal to the wall and fastening the barrier to the wood battens will exclude most water vapor.

    Bookmark   October 1, 2011 at 6:18PM
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I tend to agree with brickyee and contractor 2. This will yield the best year round indoor air quality. You could probably seal your vents up with this approach.

It may not solve your bulk water problem in heavy rains though. It would be nice to get the french drain as well, if its done right and fixes the bulk water problem.

    Bookmark   October 2, 2011 at 8:42PM
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I just did this in my crawl space here in New England. This was a real DYI project since I am not handy. My contractor estimate was $5,200. I did it myself for $475.00. First remove all insulation and replace with new since it may be moist and not effective. I then ran construction foam board ( used on new exterior houses) horizontially along the walls of the crawl space. I attached them to the wall using "Good Stuff" the insulating foam in the can. I sprayed some between the foam board and wall and it's still attached. Then I applied 6 mil poly to the floor and up the sides of the walls ( now covered by foam board) for about six-eight inchs. THE KEY IS TO PREVENT ANY AND ALL MOISTURE FROM COMING UP THROUGH THE DIRT FLOOR. So you can leave / go over the exsisting poly. When done with laying the poly, where it met the walls I again sprayed "Good Stuff" where the floor met the wall still leaving the excess up the wall. When it dries to the poly, wall and floor you have a Tight seal. The worst part was working in the crawl space. Grab an old pillow or cushion for your knee's. Knee pads didn't work for me.
Also a dehumidifier works on a "closed area prinsible" If the vents are open, it will just take the moisture out of the exteriior air. GOOD LUCK

    Bookmark   October 14, 2011 at 10:55AM
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You have to get rid of the water that runs under there, no question about that. That said, in a hot-humid environment in a home with cooling, the air can be a much greater source of water than the ground below a house. If the floor temperature is below the dew point of the air, you will get condensation. In older raised homes without cooling, there was no problem. Add air conditioning to these old homes, and you get a very high potential for condensation. If the air conditioning is conventional ducted, condensation can happen in very strange places where air is sucked into the structure by the AC blower.

Builders and building code experts have been slow to come around to recognizing that a better solution might be to seal the crawl spaces. In an air conditioning environment, this also helps keep the house cool. That said, I don�t think that I want to do that with my house unless I air-seal the floor as well. Eventually, I will spray-foam the underside of the house with low density foam. After that is done, however, sealing the crawl will no longer be necessary.

More information can be found at

    Bookmark   October 14, 2011 at 2:10PM
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over the summer I had a big project similar to your situation. old home added on to over the years.
hot humid La.
after mastic sealing all the ductwork, return & supply plenums of 3 systems in the crawl space. this was hard pipe
with floor registers..(and you could see the ground around the cut in the floor for the supply boxes)
unwrapped hard pipe mastic sealed, reinsulated with R-8 ductwrap stapled taped and mastic sealed. sealed all the plenum & supplies. all supply boxes. big job.

then we came back with 3" closed cell foam on bottom of floors.

amazing the difference it made. we also foam sealed the attic @ the roofline. oh and home was balloon framed. lots of air movement.

the crawl space is semi enclosed, and vapor barrier is non existant in places. difficut access in some areas.

midways of the job I attended this class put on by LSUagcenter..heres the info they gave us:

best of luck

    Bookmark   October 16, 2011 at 10:31AM
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