A continuous method of making duct liner is provided, comprising the steps of: (i) forming at least one lane of duct liner insulation having a top and bottom surfaces and a pair of side edges, the duct liner insulation having a facing layer on at least a bottom surface thereof, wherein the bottom surface faces downward after the forming step; (ii) flipping the duct liner insulation and facing layer while the duct liner insulation is conveyed, whereby the top surface faces downward and the facing layer faces upward; and (iii) applying a water-resistant coating to the upwardly facing layer, wherein the duct liner insulation is conveyed as part of a continuous manufacturing process from the forming step to the applying step.