duct liner vs external wrap: confirm the intended insulation system insulation, liner, vapor barrier & condensation duct liner versus external wrap internal insulation outside insulation confirm whether the duct is designed for internal liner, external insulation, both, or neither before fabrication or installation. liner and wrap solve different acoustic, thermal, and condensation problems and change the finished duct dimensions. mechanical duct schedule, insulation specification, acoustic requirements, approved shop drawings is this run supposed to be internally lined, externally wrapped, or both, and where does the change occur?