Fireplace insert insulation

Hi, I have recently had a wood insert replaced with a gas insert in my fireplace. The outer edge of the insert does not sit flush with the fireplace, nor did the company doing the work suggest, or recommend anything to go in between.
What can I put in between the insert outer-plate and the chimney that would be within code, but block the cold air?
Thank you, Dave
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Is it a direct vent fireplace (ie you don't need the chimney)? If so, block the damper. If not, check for clearances of the unit and go from there. Think most of the air coming through is from the damper. If they sent the exhaust up the chimney, they probably removed the damper door leaving a nice hole for cold air to rush down from. You'll need to do something with that to stop the drafting.

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