Insulating a heating vent will help you keep your building comfortable and reduce your energy bills by improving the efficiency of your central furnace. Duct insulation is highly recommended in the unfinished portions of your house.
Metal duct work is highly conductive and can suck the thermal energy out of hot air before it even reaches the vent grille. Bare ducts can reduce energy efficiency by 10 to 30%. The severity of the energy loss increases with the length of the duct run. Since the state of California made duct insulation mandatory many years ago, most new construction will already have this energy-saving feature. If you have older construction with uninsulated ducts, or your builder-installed insulation has failed, consider installing new insulation.
Step 1 - Determine Material Needs
Trace the duct work throughout your home, including basements, attics, and crawl spaces. All ductwork in unfinished spaces should be insulated. Duct work in finished areas should be insulated if you notice condensation.
Measure the total surface area of duct work that needs to be insulated before purchasing supplies. The amount of insulation you use will depend on the R-value you wish to obtain. R-values of four to eight are sufficient for moderate climates in the United States such as the South, Southwest, and Great Plains regions of the Midwest. R-values of six to 11 are recommended for cold climates in New England, the Great Lakes area and northern Midwest, and for high altitude mountainous regions such as Colorado.
Step 2 - Seal Ductwork with Mastic
Coat all of your duct work with a mastic sealant and allow it to dry for 24 hours. This will prevent energy loss from air leaks at the joints, but not from thermal conduction. Do not attempt to seal leaks with duct tape. Despite its misleading name, most commercially available varieties of duct tape (usually branded with an image of a duck) are unsuitable for sheet metal work.
Step 3 - Wrap Ductwork in Insulation
Fiberglass insulation comes in sheets that are approximately two inches thick. The insulating properties of the fiberglass will be compromised if it is compressed during insulation. Likewise, moisture from condensation can cause the fibers to bunch together. Prevent this by applying a waterproof vapor barrier or choosing waterproof insulation. For example, a type of insulation known as Reflectix is basically a 5/16-inch thick waterproof sandwich of foil and bubble wrap. Cover the duct work in the insulation, using the tape to assist in placement if necessary.
Of course, this job will be easier with older rigid metal duct work rather than newer flexible ducts. Pull on the slack end until the insulation is flush against the duct, but not so hard as to squash it. Staple the ends of the insulation together and trim off any excess. Work down the length of the duct, stapling consecutive sheets together with a slight overlap.