thelooch wrote:

... why can't this warm attic air be used to heat the basement?

It can, altho it may condense water in the basement if its dew point temp
Tdp = (T+460)/(1-(T+460)ln(RH/100)/9621)-460 is higher than the basement
temp. For instance, 70 F and 50% makes Tdp = 530/(1-530ln(0.5)/9621))-460
= 50.5 F, with condensation on 45 F basement walls. Then again, that warms
the walls. With enough airflow, they will warm to the dewpoint and the
condensation will re-evaporate. Without enough airflow, the basement will
gradually become an aquarium. We might avoid this with a "differential
humidistat" that only moves air through the basement when it contains less
absolute moisture than basement air, eg a Smart Vent (tm.)

My basement has a $5 humidistat and a $12 window box fan near the floor
that pushes air up through a duct to the first floor when the RH of the air
near the floor rises to 60%. Yesterday, I washed some clothes and hung them
to dry in the basement. The fan is running this morning, for the first time
this year. T = 65.5 F with RH = 69% on the first floor. The outdoor temp is
51.0, at 6:30 AM. This might be a good time to run a dehumidifier to warm
the house. Or wait till outdoor air warms up later today, and open a window.
If it were hot and humid outdoors, a window AC could help, with additional
net house cooling from the basement.

An attic with a transparent south roof can collect lots of heat,
as in our Soldier's Grove Solar Today story at
http://www.ece.villanova.edu/~nick

Nick