Cold air leaking down around outside of flue and through walls
I have a basement fireplace that is on an outside north wall. Cold air leaks from the edges of the brick mantle and all along the basement wall adjacent to the fireplace. Directly above in the main floor cold air blows in through the baseboards where the chimney is run directly behind the wall and all along the entire length of the wall's baseboards. It blows enough to move a match flame when held in front of the baseboards directly where the chimney is running behind the wall and less further away so it's like there is air blowing around the flue and down and blowing into the house via baseboards and poorly insulated walls. Also the walls in front of a chimney normally get very warm when there is a fire blazing. On this wall, there is no increase in wall temperature whatsoever even though there is very hot air going up the chimney.
Our electric company suggested some sort of chimney kit that would seal the area around the outside of the flue and the chimney space. A chimney builder and repairman I consulted said to take the outside siding off of the chimney and insulate and recommended replacing the outside with brick instead of siding. He didn't have much to say about the "kit" suggested by the electric company.
Any other suggestions or comments on which of these 2 suggestions would be best?