I was busy putting up that clear shrink-film window covering all around,
and decided to cover the opening to my fireplace. WOW! I just couldn't
believe how much air was flowing up / down that chimney, even though the
damper was shut tightly.

It's a typical main fireplace, about 4' x 5'. Once sealed, the film
acts almost like a large human lung sometimes! It's windy here in the
Chicago area today, and at times I was wondering if the double - stick
tape was strong enough to hold the film from pulling in and out.

I've read about inflatable chimney blocks. Has anyone used one, and if
so, does it do the job?

This is amazing. I don't think it's my imagination that the rest of the
house seems warmer, since there's so much less air infiltration, even
around windows that I haven't sealed with film. This is an otherwise
well-built home, but I guess that an air-tight fireplace seal wasn't
important in the early 1940's...