What is duct insulationin construction?

Wrap is a foil faced fiberglass blanket applied to the outside of the duct in the field, primarily for thermal performance and condensation control, usually by a mechanical insulation subcontractor. Liner is fiberglass board glued and pinned to the inside of the duct at the sheet metal shop, valued mostly for sound attenuation, and it ships already installed. They are priced by different parties at different times, so a takeoff has to keep them on separate lines.

The energy code and the project's mechanical schedule govern. As a rule, supply and outside air ducts running through unconditioned spaces (attics, plenums above the insulation line, exterior) require insulation at a scheduled R-value, while general exhaust duct typically does not. Return duct varies by jurisdiction and by where it runs. Estimators should take the answer from the duct insulation schedule in the specs rather than assuming by system type.

Because the wrap goes around the outside of the duct, the material has to cover a bigger rectangle than the bare sheet metal. Insulation of thickness t adds 2t to each duct dimension, which adds 8t to the perimeter, so 1.5 in wrap adds a full foot of perimeter to every rectangular duct it covers. On big duct the difference is a few percent; on small duct it can exceed 20 percent, so honest stretch-out math matters.