Duct Wrapping

Heating ducts outside home conditioned spaces, especially supply-side, should be thoroughly insulated. I use R11 Basement Wall Insulation, by Johns Manville. This material has a durable polyethylene-scrim facing that protects insulation, and when sealed tightly, adds to the R-value. Beware of joining tape commonly sold with this material, Venture Tape 1537WMP-VR. Use only 3” width Vapor Tape by Americover. In a recent job I doubled the R11, finding vulnerable unfaced R11 wrap. Half-buried now, and edged with a lot of decking, the second layer with facing, tucked in, with little need of tape.

Where ducts must remain raised, I have learned one can not rely on tape alone. Tapes have weak shear strength. Tape, and then immediately bind with copious wraps of twine. Even the Americover tape fails. I think an air-tight skin is important to achieving claimed R-values, for example R11 with only 1" thickness fiberglass. I suppose someone proved it by test. The same thinking applies with water pipe foam wrap. Without exercising a self-seal or tape, the insulation will be nearly worthless.