bob smith wrote:

Assuming the nighttime humidity is low and the daytime humidity is
high. If you use ventilation to bring in the lower humidity air only at
night, will that indoor humidity stay low during the day or will it
rise back up by noon?

That depends on how fast the moisture enters the house and what the
house is made of, ie how well it can store moisture. A house made
entirely with indoor tile walls can't store much. A more typical house
filled with wood and cloth and paper can store more. IIRC, the
equilibrium moisture content by weight of some woods is about 30% of
the RH of the surrounding air. Exposed or aerated concrete is also
good. See Kurt Kielsgard Hanson's 142 page catalog of sorption
isotherms as LBM technical report 162/86 under
http://www.byg.dtu.dk/publications/reports.htm

you might do it for 3.5 cents a month with a 90 W 2470 cfm
fan that automatically turns on when outdoor air is drier.

"Drier," as in less absolute vs relative humidity, ie a lower humidity
ratio w, in pounds of water per pound of dry air, which requires a
calculation, starting from RHs and temps. And the fan needn't run only
at night, or every night. The outdoor humidity ratio (vs RH) doesn't
vary much over a day. NREL says w = 0.005 on an average April day in
Phila with a 52.4 and 62.6 F daily min and max. If the house requires
heat, we might better ventilate during the day... 70 F air at 60% RH
has wi = 0.0095, so a 2470 cfm fan that moves 60x2470x0.075 = 11,115
pounds of air per hour can remove 11,115(wi-w) = 49.7 pints of water
per hour of operation, with an energy cost of 90/49.7 = 1.8 watt-hours
per pint, 330 times less than the 600 watt-hour/pint cost of a typical
dehumidifier.

Nick