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.

For rarely used fireplaces, I stuff a wad of fiberglas batts cut the shape
of the flue just below the damper. I leave the paper moisture barrier on the
batts, then hang a little note from the batts so next time I build a fire I
remember to move the plug, and open the damper.