I mentioned this to a chimney sweep, and he suggested installing a top ceiling damper. Here is a description:
https://magicbroom.net/keep-cold-away-t ... ng-damper/A top sealing damper is located at the top of the chimney structure. It seals the entire flue off from the outside; this keeps warm or cold outside air from drafting into the chimney. It also prevents animals, debris, and moisture from getting into the chimney.
Like traditional dampers, top sealing dampers are operated by pulley or lever and should be opened when the fireplace is in use and closed while the fireplace is not being used.
There's a picture of one in action (up/down) on that site.
Is this worth installing? I have no idea to what extent it would mitigate the entry of cold into the fireplace. If it would be significant, I'll do it. But would it be?
Thanks in advance.
Update: I see there has been some discussion of chimney top dampers here: