Stretch wrote:

Storing mosture in a basement slab could encourage mold growth.

Nonono. Don't deliberately store mosture. Dry out a slab in wintertime
and let it absorb mosture in summer months to avoid energy-inefficient
compressive dehumidification, in a fairly airtight house, instead of
keeping the RH constant all year. Use the mosture storage capacity of
a house for good vs evil. But as you say, this won't work everywhere.

The device would probably work in some parts of the USA, but not here.

Which device? Wilmington NC has w = 0.0048 in January and 0.0168 in July.
If we dry out a 4"x2400ft^2 slab in a house at 65 F in January, how much
mosture can the slab absorb in July before the RH hits 55% at 80 F?

It weighs about 4/12x2400x150 = 120K pounds. In 55% air, its long-term
moisture content is about 5% of 55%, ie 2.75%, so it stores 0.0275x120K
= 3300 pints of water. Air with w = 0.0048 has Pa = 29.921/(0.62198/w+1)
= 0.229 "Hg, vs 65 F air at 100% RH with Pw = e^(17.63-9621/(460+65))
= 0.630 "Hg. RH = 100Pa/Pw = 36% makes the moisture content 0.05x36%
= 1.8%, so it stores 0.018x120K = 2160 pints. The difference is 1140.

If the house begins a humid spell with a 1.8% slab moisture content and
leaks 30 cfm of outdoor air with w = 0.0168, how long before it reaches
55% at 80 F indoors, after the slab stores 1140 pints of water?

At 36% RH, 80 F air has Pa = 0.36e^(17.863-9621/(460+80)) = 0.377 "Hg and
wid = 0.612198/(29.921/w-1) = 0.00794, and 55% 80 F air has wiw = 0.0122;
wiavg = 0.0101, and the slab might absorb 24hx60x30x0.075(0.0168-0.0101)
= 21.8 lb/day of water, filling up in 1140/21.8 = 52 days, approximately.

With a more accurate differential equation, if the slab stores
P = 60C0.075(0.0168-wi) net lb/h of water and wc = 0.018+Pt/120K
and 100% 80 F air has w80 = 0.02256 and RH = 100wi/w80 = 20x100wc,
wc = 0.4513wi, so wc = 0.018+dt-cwit, where c = 5.64x10^-4 and
d = 1.89x10^-5 and d/c = 0.0372 (the equilibrium moisture content
at 80 F with w = 0.0169 and 74% RH), so wc = 0.018 +(0.018-0.0372)e^-ct
= 0.0275 after 55 days, with an RC time constant with C proportional
to the slab mass in series with R = 1/cfm.

Summer sun might keep a concrete wall or a roof or a LiCl roofpond
under a greenhouse dry...

Nick