The ductwork in our house, at least in the places I could access, leaked a lot of conditioned air into non-conditioned spaces. For example, in the crawlspace and basement lots of air was escaping from the ductwork where two pieces joined. I figured I was loosing quite a bit of energy by heating parts of the house no one lived in and was probably making it comfortable only to the bugs and other critters that might like some warmth during the winter months. Conversely, if you live in a warm climate and have an air conditioner, sealing the ducts will stop the cold air from cooling unconditioned space as well.
