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To many homeowners, central forced-air hvac systems spell the ultimate in comfort. However, buyers don't know, and contractors are just learning, that the comfort and efficiency of a central hvac system often come with an energy penalty. Recent research shows that ducts are responsible for 20% to 60% of the air leakage in houses. Some contractors have used duct tape to seal ducts — seemingly a logical move — but duct tape becomes brittle and loses its adhesion within a couple of years. So more residential contractors are turning to the treatment used in many larger commercial buildings — duct mastic. Applied properly, it can all but eliminate air leakage from ducts. Duct mastic doesn't hold ducts or their seams together. That's