m Ransley wrote:
You have heat going up and out as I did since my damper was broke , I
installed a cap that closes with a cable , then cold air started flowing
down and in from the cold chimney cooling air inside it. I screwed in
steel L channel recesed in the opening , cut foambord to fit and put
magnetic tape on the foamboard with weatherstripping for an airtight
seal. I painted it flat black and it just pops in or out for using the
fireplace. It made a noticable difference. No doors I saw actualy
sealed. A ballon sounds like alot of work to instal and remove and you
will still have the cold firebox convecting cold inside.

great idea! magnetic tape?? Home depot/lowes???