I am just finishing up the install of my Quadra Fire 3100i insert into an existing ZC fireplace. Since the chimney flue meets the firebox almost at the back wall I had to use an adjustable offset box to connect to the outlet on top of the insert in order to meet installation requirements and maintain clearances. My question is would it be a good idea to seal the inside corners and seams of the SS offset box? I can see small air gaps where the sliding plate exits the back and I would imagine the base of the slider is not airtight either? I have a tube of the Rutland Furnace Cement I was thinking of using on the inside of the box to seal it up.