.....you are going to seal the top of the chimney? Probably going have a VC Montpelier insert and liner installed. I know it's important to stop the draft going around the insert and up the chimney somehow. My installer says they do this by sealing the top of the chimney where the liner makes its exit. Sounds good to me but thought I would ask the more knowledgable base of folks here. FYI, have an exterior brick chimney approx. 30 ft. length.