Q: Do you use duct tape to seal ducts?

A: If you mean the household kind of duct tape, no. Heat hardens the adhesive in most household duct tapes. Over time, the tape loses its stickiness and the duct leaks again.

It looks pretty ugly, but most energy-efficiency experts use duct mastic. It has the consistency of mashed potatoes and cures to form a durable seal. Mastic comes in one-gallon buckets or, for sealing certain joints, in caulk-gun tubes.

Seal gaps around ducts with spray foam where they penetrate the floor or ceiling. In addition, ducts in an attic or crawl space should be insulated. If your ducts are uninsulated or poorly insulated, seal them first, then add insulation to keep the air in your ducts at its desired temperature as it moves through the system. Use duct-insulation material rated at least R-6.

Why? At current fuel prices, heat is the last thing you want to waste.