I live in the Baltimore suburbs. In the summertime, my 2nd floor can easily get 15 to 20 degrees hotter than my 1st floor -- especially with my 2nd floor thermostat programmed not to run during the day. For most of the summer, my 1st floor stays relatively comfortable with just the windows open.

I'm looking into installing a whole house fan to move hot air out of my 2nd floor in order to pull cooler air in through open windows. My soffits are very well ventilated, (though I'm not confident my ridge vent really allows for proper convection).

In any case, I'm wondering if it's really worthwhile to spend $1,000+ on a fan with heavily insulated automatic doors? I'm thinking that it might be relatively easy to fabricate a 3" or so foam-board box around the fan, just slightly taller than the fan itself, seal the corners and bottom w/ expanding foam, and then cap it with a cover made from the same foam board material during the winter in order to eliminate warm, moist air from entering the attic. (It would certainly be tighter than my current attic entry panel.)

Any thoughts, opinions, suggestions on this?

Thanks in advance.