Wrote:
manhattan42 wrote:

4 mil plastic is "waterproof" but it is not "vaporproof"....4 mil
plastic has a relatively high perm rating which means that it readily
allows gaseous water to flow thru it...

Then again,

http://www.hornerflooring.com/techgu...watervapor.pdf

says 4 mil polyethylene film passes 0.17 gr/h-ft^2 per "Hg of water
vapor
pressure, vs 0.11 for 6 mil, vs about 20 (200X more) for 1/2" drywall.
A 1" hole in a sheet of drywall can pass a lot more water vapor.

A house with a 1000 ft^2 50 F wet basement floor with 70 F air at 50%
RH
would have Pw = e^(17.863-9621/(460+50)) = 0.367 "Hg at the floor and
Pa
= 0.5e^(17.863-9621/(460+70)) = 0.374 "Hg in the air, so moisture
would
flow FROM the air TO the floor through a 4 mil poly film layer at a
rate
of 1000(Pa-Pw)0.17 = 1.2 grains per hour, ie 1.2/7000 = 0.00017 pounds
per hour, ie about 1 drop of water every 2 weeks. Changing to 6 mil
poly
would reduce this to about 1 drop every 3 weeks :-)

Nick

The problem with all the scientific rhetoric is that is does not take
into account the reality that gaseous water (humidty) passing up
through the concrete can be well more than 100% and that the
tempartures can drop well below freezing.

This adds up to temperatures and conditions well below the dew point
for that portion of air even at 70F or above.

The calculations above also fail to take into account that gases flow
from areas of high concentration to areas of low concentration
regardless of the air pressure.

This means that the natural progession of moisture will be up from and
through the concrete basement floor and into the living space above
where it will condense under most circumstances.

One thing you got correct, however, is that 4 mil poly is 3 times less
able to retard water vapor than 6 mil poly, and why it is not permitted
as a true vapor barrier by building codes.


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manhattan42
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