How to seal around chimney liner

[eric859]

Several years ago I had people that I thought were professionals install my fireplace insert. Apparently, I was duped. They installed the insert and liner only. There is no cap on the top of the chimney or a blockoff plate at the bottom of the chimney (things I learned about after the fact). Now I'm experiencing a lot of downdraft when the fireplace is not lit. The room is freezing. What options do I have to seal up the chimney with the fireplace insert and liner already installed? I can't disconnect the liner from the insert because they screwed up and cut the liner too short, so they had to add an extender between the insert and the liner to make the liner long enough to reach the top of the chimney. Then they used some sort of cement around the extender where it connects at the insert and liner to stop any smoke leaks (or so they said). Can I stuff some sort of fireproof insulation up around the liner to seal up the space between the liner and chimney? Do I need to fabricate a steel blockoff plate and cut it in half in order to install it around the liner? Any safe ideas to stop the downdraft would be greatly appreciated.

 

[begreen]

The first priority has to be getting a proper, sealed top plate on the chimney. The chimney should be stuffed at top with roxul insulation. At the bottom you can add a block off plate without disturbing the liner by making it in two overlapping pieces. Stuff the damper area with roxul before putting up the plate.
https://www.hearth.com/talk/wiki/why-damper-seal-is-needed/