R-20 walls are unusual, R-11 is more common. (Mine are R-24, but I
have never seen anyone else do that). Probably figure average windows,
actual sizes would help, single or double pane?? Don't forget ceiling
and floor (Crawlspace) load. Also, don't use a propellor fan to push
air through a duct, not enough static available to be very effective.
A small squirrel cage forward curve blower would be much more efficient
and more effective.
An 8" duct with .1" static per 100 feet would deliver about 230 CFM
with metal duct, about 160 CFM with flex duct. To get 511 CFM ducted
would require 12" metal duct. 12" flex would provide 480 CFM. No
allowance was made for grille losses.
Otherwise I agree.
A window box fan in the hall would definately work the best for the