Article 390463 of misc.consumers.frugal-living:
From: nick@acadia.ee.vill.edu (Nick Pine)
Subject: Re: Dehumidify Basement?
Date: 13 May 2002 10:33:41 -0400
Organization: Villanova University

SmegghedJ <smegghed@comcast.net> wrote:

>Any ideas on how to dehumidify an unfinished basement, short of going out
>and buying a dehumidifier? Thanks!

The yearly average air (and basement) temp is 54.3 F in Phila. At 50% RH,
this corresponds to a dew point of (460+54.3)/(1+(460+54.3)ln(0.5)/9621)
= 36 F. You might turn on a basement fan whenever the outdoor temp is less
than 36 F and turn it off when the basement RH drops to 60%, with Grainger's
$16 5E266 line voltage thermostat and Herbach and Rademan's ((800) 848-8001
http://www.herbach.com) $4.95 Navy surplus humidistat, item # TM89HVC5203,
which has a 20-80% range, a 3-6% differential, and a 7.5A 125V switch that
can be wired to open or close on humidity rise.

With another humidistat for outdoor air, other combinations are possible: 

To:   40   45   50   55   60   65   70   75   80 F...
Ro:   88   74   64   54   47   40   35   30   26 %...

Perhaps someone will make a smart basement fan control that figures this
out automatically (R = exp(-16.53(T-36)/(460+To))) and tries to keep the
basement dry, and warmer in winter and cooler in summer by ventilation.  

Nick