Grease duct joints are held alongside one another by means of shaped V clamps and sealed with 3M Fire Barrier 2000+. The minimum duct thickness is described by no matter what code jurisdiction that the venture is situated in. Down below are two code references relevant to the expected thickness https://holdenmcrfs.ageeksblog.com/35733044/getting-my-fire-resisting-ductwork-insulation-to-work