ShurMASTIC butyl foil tape is for indoor or outdoor use to seal Class 1 Flex Duct, sheet metal and duct wrap vapor barriers. It is also approved for subgrade application on PVC-coated duct. SF 685 is a linered, paintable foil mastic on a roll that's tested in accordance with UL 723 and delivers permanent, airtight and water-resistant seals in temperatures up to 200°F.