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re: dehumidifier 70 better than 45 ?
2 jun 2004
ms  wrote:
>the basement is about 25' x 30' with painted walls...

maybe it needs some paint on the floor, eg ugl drylok, and
maybe some rugs to avoid foot traffic abrasion. 

>i figure my fedders 45-p dh is barely keeping up with the moisture
>supply... i've tried setting it lower - to 35, 40 and 45%, but that
>makes no difference since it can't seem to extract enough water
>to ever reach below 55% rh.

i have a similar situation. i think the slab was installed without
a vapor barrier underneath. the ugl ap engineer said drylok has
a low water vapor permeability, but it isn't used on floors because
it has a low abrasion resistance. he said the slab was probably
wicking water up from the ground and evaporating it near the surface.
i suggested putting poly film on the floor surface and rugs over that
to protect it from foot traffic, but he said that could cause mold
growth under the film and water might seep out at the edges. a much
more expensive epoxy paint would also work, but the drylok should
also work, even if some wears off in high traffic areas. 

the building science corp web site has an interesting basic 18 page
pdf on "insulations, sheathings, and vapor diffusion retarders"...

    ...a cubic foot of air containing 2 trillion molecules of water in
    the vapor state has a higher vapor pressure (or higher water vapor
    density) than a cubic foot of air containing 1 trillion molecules
    of water in the vapor state. moisture will migrate by diffusion from
    where there is more moisture to where there is less. hence, moisture
    in the vapor state migrates by diffusion from areas of higher vapor
    pressure to areas of lower vapor pressure... a hot-humid climate [exterior at 74 f dewpoint and interior at
    75 f and 50% rh] over a one-week period, 1.5 pints of water can be
    collected by diffusion through [a 4'x8' sheet of] painted gypsum board
    (about 5 perms); 14 pints of water can be collected through air leakage
    through a 1 in^2 hole under a 5 pascal air pressure differential...


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