On Tue, 24 Jun 2014 08:51:38 -0400, micky
wrote:
I'm thinking most of the air will enter through the open basement window
-- 200 times as big as the openings in the mail slot -- where it will
be cooled by the constant 64 degrees in the basement before it
eventually gets upstairs.

That may very well be correct, but don't forget about humidity. In
hot humid climates, the temperature often approaches the dewpoint at
night. When the outside air is very humid and you bring it into the
basement, it could easily cool down to the dewpoint temerature and
condense on walls, pipes, and anything else down there. You can
damage things stored in the basement and encourage mold and mildew
problems.

Pat