Gas Insert

Just had a gas insert installed (or partially installed.... a piece is on order). The insert has a fan for circulating heat. It also had a large hole for the gas pipe. On top it has a fresh air pipe and exhaust pipe which go up thru the masonry chimney. 2 pipes have caps on top and installer assures me that the chimney is well sealed except for the 2 ducts.
Seems to me that it would be good to put something in to seal off the gas access hole or insulate the top of the fire box even though there is a cap on top of the chimney othewise the room air will mix with the chimney air and there is probably some minor infiltration going thru the masonry. Any idea what would be the right thing to use? I will be asking the installer and store but they are not an encyclopedia of knowlege but we are stuck using them. Normally they put these in wood framed chimneys. Our builder screwed up a bunch of masonry fireplaces and after a house caught fire he is going back and putting in the gas insert units as a fix so he doesn't have to fix the wood clearance problem. Any suggestion for insulation would be appreciated.

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