Joseph Meehan wrote:

...most of the time the only fix is to keep the water away from
your foundation in the first place and second to provide additional
ventilation or dehumidifiers.

The problem may be too much uncontrolled ventilation, esp in an older home
with lots of air leaks or a slab with no vapor barrier.

Basement exhaust with a differential humidistat (a Smart Vent?) can use
100X less electrical energy than a dehumidifier, climate permitting. My
hunch is that most do. A Key West house with no AC might be one exception.

Smart Vent's 12/19/2000 US patent no. 6,161,763 "Module-controlled building
drying system and process" at http://www.freepatentsonline.com describes
a controller connected to indoor and outdoor absolute humidity sensors.
If outdoor air has a lower absolute humidity than indoor air, the controller
turns on a ventilation fan. If outdoor air has higher absolute humidity
than indoor air, it turns the fan off.

Oddly enough, this patent measures "absolute humidity" in grams/m^3 vs
a humidity ratio in g/g, so it looks like the controller would turn on
the fan with outdoor air at 95 F and 17.4 g/m^3 and indoor air at 80 F
and 17.5 g/m^3, even though cooling 1 m^3 of the 95 F air would lower its
volume to (460+80)/(460+95) = 0.973 m^3 and raise the absolute humidity
to 17.4/0.973 = 17.9 g/m^3, so this would add moisture to a house.

As an alternative to "absolute humidity sensors" or calculations,
we might mount an RH sensor on each side of a single pane window or
a steel plate and insulate the plate indoors, except near the sensor,
and turn on the fan when outdoor RH is less than indoor RH.

Nick