Stagnant air is a pretty good insulation.
Chimney liner block off plate.
This will prevent any airflow through the area around the liner.
Heavy aluminum 26 or 24 ga sheet metal both work.
It is very common to have to ovalize a short section of the liner in order to pass the liner through the throat damper.
This can result in a significant cooling of the stove and large heat loss.
A block off plate is particularly helpful with inserts installed in fireplace on exterior walls with exterior chimneys.
If you have an internal chimney then insulation is better but you could get away with a good block off plate and stuffing some mineral wool down a couple of feet from the top.
To prevent this from happening install a block off plate.
Some are better than others.
The block off plate is a horizontal plate that blocks off the opening where normally a damper is in a fireplace.
The damper sealing kit is designed to seal off your damper around your stainless steel chimney liner kit when installing a fireplace insert or a freestanding stove.
Top plates and caps can be ordered through most liner companies.
This opening will vary in size but generally you will be installing a 6 or 8 chimney liner.
When you have such a large opening around the liner a large amount of the hot air generated from the wood stove will be rising up into the chimney cavity.