Pawlowski wrote:

How about a Crawlspace Smart Vent?

http://www.smartvent.net

They cost $365, but they only use 40 watts when moving 290 cfm of air
out of a basement when the absolute moisture content of basement air
is greater than the absolute moisture content of outdoor air.

The technical description on the crawlspace web page says they evacuate
crawlspace air when its RH is more than 35% and the absolute moisture
content of the outdoor air is lower than that of the crawlspace air.
They also evacuate crawlspace air when the crawlspace RH is less than
25% and outdoor air has 20% more absolute moisture. They say adding
humidity to a crawlspace is sometimes useful to keep it from drying out
to the point that hardwood floors buckle.

Does that mean when the absolute moisture content is the same (high or low)
it does noething?

Yes. This might work well in a climate with some humidity variability
and with a fairly airtight crawlspace and some building materials that
can store moisture. It would work better in Chattanooga (wmin = 0.0036
in January) than Key West (wmin = 0.0100 in January.)

Seems like it would have limited value in a naturally humid region
whee you want to make it lower.

Yes. It only works on dryish days. There's a nice graph of crawlspace
humidity over time on the web site.

Nick