What Kind of Insulation to Use for Outside Ductwork?

Insulation Tape

• One option for insulating exterior ductwork is metal foil tape. It is specifically designed for outside ducts. It doesn’t degrade significantly when exposed to air, heat or water, unlike regular tape. The tape comes with an adhesive backing from which you peel off the cover. Apply in concentric circles, slightly overlapping each layer of tape.

High-Density Foam

• High-density foam covers are another option for insulating exterior ductwork. The covers are circular and come spilt down the middle so that they can be fitted around the ductwork. The join is then taped up with heavy-duty tape, and the tubes are secured tightly with strapping.

Fiberglass

• A third alternative is fiberglass insulation. This comes as either a rigid tube, like the foam option, or flexible batts that are wrapped around the ductwork and secured with strapping. If using fiberglass on ducts that convey heat, insulation tape must be used in conjunction with the fiberglass as, while the material won’t ignite, it may melt if exposed to high temperatures.

Mastic

• Mastic is a putty-like substance that is applied over the insulation. It sets slowly, but once set, forms a tough membrane over the insulation. This keeps the insulation in place and provides another layer of insulating protection.

Weatherproof Finish

• Some mastic products are combined with weather-resistant substances. However, many are not and the final part of insulating outside ductwork should be to paint over a weatherproofing finish. Mastic can soften if exposed to continuous moisture, and both fiberglass and foam insulation must be protected from heat and water. Asphalt-based finishes are a commonly used means to provide a moisture barrier.