Imagine opening a window in your house and leaving it that way 24 hours a day, all year long, like this attic hatch.
On balmy spring days, the breeze wouldn’t be so bad. But in the freezing cold of winter and the sticky heat of summer, with the furnace or air conditioner on, smart homeowners would recognize that they might as well be throwing buckets of quarters out that open window to pay for the escaping heated or cooled air.
InterNACHI’s Home Energy Book describes the common sources of air leakage in a home, including tips on how to finding air leaks.
http://book.energytool.nachi.org/ducts/air-escape