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.