ShurMASTIC butyl foil tape used for airtight and moisture-free sealing of Class 1 Flex Duct, sheet metal and duct wrap vapor barriers, and is also approved for subgrade application on PVC-coated duct. A paintable, UL 181B-FX Listed mastic on a roll for indoor or outdoor use, SF 686 is designed with a butyl rubber adhesive system that can withstand wide temperature variances once applied.