Hi Jeff, I cut a section of fiberglass batt insulation and stuff it up against the flue and leave it there all summer. I know that flue is not the most air tight thing in the world even when they are working properly. I also cut a piece for the trap door in the floor of the fireplace you put the ashes through. I cover it with a piece of plywood cut to fit the bottom and with some little risers on it so as to not crush the insulation. I spray painted it black so it is not obivous.