Cold air leaking in from fireplace insert

My wood burner fireplace insert is leaking cold air into the house (and sucking out the heated air I'm sure). I know the damper is closing completely and I think the outside air is coming down between the walls of the double-wall pipe. I've tried to find insulation to put in the void to stop the flow but haven't had any luck. Does anyone know if there is a specially designed insulation for use in such hot environments? Will regular insulation withstand the heat (minus the paper)? Your help would be appreciated.
James
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I think Rockwool is ok, check a fireplace store.
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If nothing else, a small piece of door gasket material for a woodstove might work in a pinch. If the B-vent pipe is leaking, it might be something you should think about replacing, since I don't think they're supposed to do that.
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down heatloss from the outside wall of the stove into the chimney space. This increased the heat output noticeably.
Bob
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