Crawl space moisture barrier solutions

[Jack]

I have a little home repair business going after working as a construction carpenter for many years. One customer has a damp crawl space in which standing water accumulates during the rainy season, percolating up from the ground. The crawl space is actually quite high,

5 1/2 feet at the highest and maybe 3 1/2 to 4 feet at the lowest. The concrete foundation wall is about 6 to 8 inches above grade with 2x4 underpinning up to the joist level. One concrete foundation wall runs down the center dividing the crawl space into 2 sections of about 15 feet by 20 feet each.

I looked into crawl space moisture barrier systems in the net. The standard seems to be 20 mil plastic sheeting, doublestick tape and/or polyurethane caulk and plastic fasteners into holes drilled into the concrete foundation. This is all straightforward and common sense stuff. I am guessing that 6 mil plastic sheeting will do the trick just as well as the 20 mil stuff as long as no holes are poked in it. The only thing I am not sure of is whether the doublestick tape used in these systems, and regular polyurethane caulk, will stick to the 6 mil plastic available at any lumber yard or home improvement center as well as it sticks to the 20 mil plastic used in the various systems, or if the 20 mil stuff is made of plastic that is better in this regard.

Another thing that I am wondering about is whether it would not be just as well to apply 6 mil plastic to the bottom of the joists and seal the edges against the top plates of the underpinning. Once the moisture that gets sealed into the joist bays evaporates out through the floor and into the house, it seems to me that the sealed joist bays should remain stable as far as moisture content is concerned. The advantage to this method is that on days when relative humidity is very high, a crawl space will still be moist even with a vapor barrier on the ground.

Any thoughts?

[Jack]

One thing I neglected to address in my OP was the standing water. This accumulates at the low corner of the crawl space and will be dealt with either passively by punching a hole in the foundation or tunneling under it and filling the tunnel with gravel, or actively with a sump pump.

[hallerb]

yeah address the groundwater problem first, with a french drain preferably drained to daylight if possible, gravity tends to be highly reliable.......

then cover the ground with multiple layers of very heavy plastic sealed well.

[aemeijers]

Is this a high water table location or something? I'd first look at where the water is coming from. and try to alleviate that. All the usual suspects- gutters, yard grading, improperly directed downspouts or driveways sloped the wrong way, missing or failed exterior foundation drains at footer level, etc. I've been in hundreds of basements and crawlspaces over the years, and any water down there almost always is coming from ground level outside, not up from the bowels of the earth. Yes, sometimes people do build in a swamp where water does actually bubble up out of the ground, but it isn't common.

[aemeijers]

Forgot probably the most common one up north here: frost-heaved patios and porches causing water to pond right against the foundation.

[Jack]

The house is at the base of a hill. All the rain water that falls on the hill percolates down through the ground and saturates the ground of the area surrounding it. All the rainwater leaders are piped away from the house. The grade around the house is about 1 1/2 to 2 feet higher at the high end than at the low end. The foundation at the low end create a dam and the water pools up there. Other houses in the area have the same problem. The water that falls on the hill is traveling down the hill below the surface.

Dealing with water that collects under the house can done passively with a french drain or actively with a sump pump, or with both. The main question that I have is whether it would better, after a french drain and/or a sump pump is installed, to install a moisture barrier (plastic sheeting) against the dirt or against the bottom of the joists.

I don't see any reason not to put it against the bottom of the joists and at least one reason in favor: even if the dirt were sealed off completely, relative humidity of the air in the crawl space could still reach 100%. Sealing against the joists would prevent ANY moisture from penetrating into the living space above.

[Paul Franklin]

The current issue of the Journal of Light Construction has an article on this very topic. The author went through several unsuccessful iterations until he got the Building Science group to provide advice.

You may be able to get it online or at your library; it's worth the read. The author had to deal with standing water too.

I don't recall all the details, but know he ended using some kind of mastic along with fiberglass mesh tape to seal all the seams; it was the only thing that did the job and was long lasting.

HTH, Paul Franklin

[hrhofmann]

To answer your questions. Stapling the 6 - 20 mil plastic on the bottom of the joists seems reasonable, as long as the owner doesn't mind having a pool at one end of the crawl space. Any drainage to daylight using gravity would be helpful. You don't say if there is a frost situation for part of the year or not. If no frost, or enough to freeze the drain pipe, then as someone posted earlier, gravity is almost foolproof.

The trick will be to staple through some sort of material that will keep the staples from tearing the plastic. That is where the heavier plastic has an advantage. I am thinking of the round 1.5 inch plastic disks used to hold roofing paper down until the shingles are put on. There may also be some material used when Tyveking a house that would meet your needs.