An HVAC system uses ducts to provide conditioned air to the vents in the home. Manufacturers insulate some ducts as they are constructed. Others are nothing more than metal boxes and benefit from insulation. The process involves working in the unheated basement or crawl space of the home. Insulating your ducts is time consuming, but the energy savings are significant.
Seal the ducts
Insulation doesn't stop leaking ducts. Use mastic from a can or tube to seal the duct. Duct tape doesn't provide an adequate seal by itself. Some mastics call for the use of a special tape to ensure a proper seal. Caulking is appropriate for small leaks.
Select the correct insulation
Choose insulation made specifically for ductwork. Don't use paper-backed standard insulation for ducts due to the fire hazard caused by the backing. Select an insulation that provides the proper R-value for your location. Install insulation with the vapor barrier facing out.
Install the insulation
Begin at the furnace and work toward the end of each duct. Avoid compressing the insulation as the trapped air inside is essential for providing the insulating value. Surround the duct with insulation and staple together. Overlap joints at the end of each length of insulation by 2 to 3 inches. Reinforce the joints with pressure-sensitive vapor retarder tape designed for use with ductwork. Use the same tape to seal any tears in the vapor barrier.