Sealing fireplace opening

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...
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Welcome to the chimney effect. We use plexiglass inserts with gaskets. TB
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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.
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Just curious, is your chimney on the main or upper level? I have a fireplace in a basement rec-room, and have assumed heatloss wasn't too bad, since it is in a basement. Maybe I should re-think that.
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Besides wind effects that may go right down the flue, much of the heat loss is due to a cold chimney connecting to a warm room, at any level of the house. The more chimney length is exposed, or greater height above the roof line, the greater the cold transfer into the flue. Cold air is denser than warmer room air, so the air sinks rapidly and spreads into the lighter density, warmer room air, forming a cold layer in the bottom third of the room - right where you don't want it!
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1st (and only) floor. I have a

You would if you saw this film moving around. Today is only mildly breezy, and the film just never stops moving.
Tape a sheet of newspaper from the top edge & let it hang down across the fireplace opening. I'd bet that newspaper never stops swaying around.
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Damn you. You just made me add yet ANOTHER thing to my list of annoyances to deal with! :-)
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