Air intrusion is a common issue with traditional cast iron dampers. A top mounted damper is the best way to completely seal the chimney when it is not in use. These dampers feature a heavy silicone seal that will not allow any air to pass through. Is your fireplace a manufactured unit or a masonry fireplace? If you would like to provide the dimensions of the flue, I can suggest a damper to fit your needs.
Collin C. - NFI Master Hearth Professional
on December 3, 2013