Galvanized metal ductwork, which typically has air gaps at the joints, can be sealed with duct mastic. This is a gray substance, of about the same consistency as joint compound, which is troweled ...

Joint sealing can be done with aluminum-foil tape, mentioned above, or a special duct mastic and fiberglass tape. The special mastic and fiberglass tape are available from some heating-supply dealers.

Seal the joint by using metal foil duct tape -- not the fabric tape that is sold as "duct tape," which will not last -- and duct mastic. Clean off the duct before applying the tape or mastic.

This is typically a metal box where all duct runs originate. If you notice any leaks at the corners of the box, use a mastic adhesive to seal the openings.

However, if it is absolutely necessary to pan joists, then do it as sparingly as possible and, where it is done, smear the inner sides and top of the joist space with mastic before applying the sheet ...

Mastic is a fibrous, water-based, sticky sealant that dries to a pliable solid. When using mastic or metal tape to seal ductwork, the product should be UL 181 approved.

The foam, while expensive, will both insulate and air seal the ducts, taking the place of the tape or mastic. Dwight Barnett is a certified master inspector. Scripps Howard News Service.

Most homeowners are aware of the importance of insulation in attics, walls and floors, but they sometimes overlook other items, such as water heaters and metal furnace ducts, where insulation can c… ...