Grease duct joints are held together by way of shaped V clamps and sealed with 3M Fire Barrier 2000+. Grease duct is either routed via a fire rated shaft enclosure or wrapped with an accepted fire insulating blanket that permits for zero clearances, like that manufactured by 3M. Stone wool https://grease-tight-extract-duct34455.blogdomago.com/35681682/detailed-notes-on-grease-tight-extract-ductwork