My sister just moved into a house that the previous owners had installed a VC gas insert into an existing wood fireplace. While, I was relighting the pilot for her, I noticed a 10-20 mile an hour breeze going up the chimney. What is the best way to seal it? Also, is there anything that should be done to insulate the firebox of the old fireplace to keep it from sucking the heat out of the insert? Thanks much for the feedback.