Can You Just Wrap a Metal Duct With Insulation or Will It Condensate?

Duct Work and Condensation

• Because duct work moves air heated or cooled by a furnace, boiler, air conditioner or other central heating and cooling system, the temperature inside the duct work is always significantly cooler or warmer than the air outside the duct work. Such a temperature difference can easily create condensation on the inside or outside of duct work (or both), when air laden with water vapor comes in contact with a colder surface and the vapor condenses into condensation. The presence of excess moisture in any form is always a problem for homeowners, so it is important to control condensation on your duct work before it can damage your home.

Insulation Strategy

• Insulating duct work is the key to preventing this problematic condensation. Insulating duct work is particularly important in attics, crawl spaces, garages or unfinished basements, since these areas are not heated or cooled by the central heating and cooling systems. The U.S. Department of Energy recommends insulating your duct work to achieve an R-value that is ideal for the climate in your geographic area. (An R-value measures insulation's resistance to heat flow.) For warm climates with mild winters (such as coastal/inland California, southeast Texas, southern Louisana and Arkansas), R-4 to R-8 is ideal for duct work. Mixed climates with slightly colder winters and warmer summers (Virginia, West Virginia, Kentucky, Missouri, Nebraska and Oklahoma, for example), R-2 to R-8 is ideal. Finally, for exceptionally cold climates (such as New England, the northern Midwest, the Great Lakes area and mountainous areas), duct work should have R-2 to R-11 values.

Installation Tips

• If you are insulating the duct work on your own rather than hiring a contractor, insulate all the ducts that you can access on your own; a contractor should always be hired to insulate duct work in hard-to-access areas. Wrap the insulation directly on the outside of the duct work, and use enough insulation to achieve the desired R-value. Also make sure that you seal the ducts using mastic sealant or metal tape (never duct tape) to help prevent condensation and control heat loss that diminishes the efficiency of the duct work.

Suggested Insulation Materials

• Rigid fiber board is the most commonly recommended insulation material for insulating duct work. However, really any insulation material can be used so long as it is able to achieve the desired R-value for the climate in your geographic area. As noted, it is most important to insulate duct work in unconditioned rooms of the home, but it is also beneficial to insulate duct work in conditioned parts of the home. Duct work in conditioned areas requires far less insulation than that in unconditioned rooms.