Duct Insulation

With forced-air systems, the ducts that carry the heated or cooled air throughout the home are often located in attics or other unconditioned spaces where temperatures are quite different from that of the air in the ducts. These temperature differences can affect the temperature of the air passing through the ducts and increase heating and air conditioning costs. Insulating ducts in unconditioned areas of the home can help reduce space heating and cooling energy costs and improve indoor comfort. In humid climates, properly installed duct insulation can also reduce moisture condensation on metal ducts.

The recommended amount of duct insulation, shown in the table below, varies depending on your climate and the location of the ducts.

Duct Insulation - Recommended R-values by Climate and Duct Location
ClimateAtticUnheated Basement or Crawlspace
Warm - cooling and minimal heatingR-4 to R-8R-2 to R-4
Mixed - moderate heating and coolingR-4 to R-8R-2 to R-8
ColdR-6 to R-11R-2 to R-11

Insulation Materials

The most common material used for insulating heating and cooling ducts is fiberglass. Fiberglass duct insulation is available in various thicknesses and R-values and in either a flexible or rigid form. The flexible type comes in rolls and is relatively easy to install on either round or rectangular ducts. The rigid type is best suited for rectangular ducts. In some cases, the ducts themselves are constructed of this material (called "duct board"), reducing or eliminating the need for additional insulation. Any duct insulation material should incorporate a foil or vinyl coating to prevent moisture from being absorbed by the insulation. Any existing insulation that has gotten wet should be replaced.

Before Installing

Before installing duct insulation, be sure to seal any leaks or loose joints with either duct mastic or foil tape. Cloth duct tape is not recommended, as it tends to come loose over time. For more information, see "Duct Sealing".

Condensation on Metal Ducts

In humid climates, metal ducts can be prone to moisture condensation, which occurs when warm, humid air contacts a cold surface. Ensuring that the insulation covers the entire surface of the ducts, including the connections to supply registers, can minimize condensation. Any seams should be sealed air tight with foil tape (cloth duct tape is not recommended). A popular alternative in humid climates is the use of pre-insulated flex duct and/or insulated duct board. Both materials reduce the likelihood of condensation because their surface temperature is closer to that of the surrounding air.

Ducts Within the Conditioned Space

Ducts contained within the conditioned space do not need to be insulated. For new construction or situations where ducts are being added to an existing home, enclosing the ducts within the conditioned space can eliminate the need for duct insulation while also preventing heated or cooled air from leaking into unconditioned areas like attics, crawlspaces or basements. The ducts can be hidden from view behind lowered sections of ceiling, spaces above cabinets or other design features of the home.

Related Topics: