We bought a house that has a wood burning fireplace insert where a regular fireplace used to be. There is a strong cold draft that flows into the room around the insert. I believe most of the cold air is flowing down the chimney around the stainless steel exhaust tube running up the shaft. I want to try and seal the void around the exhaust tube. There is no live flame associated with the insert outside its chamber just the smoke and some heat. Can I use Roxul around the stainless exhaust pipe to pack and seal the void between the chimney and the tube?