Grease duct joints are held together through shaped V clamps and sealed with 3M Fire Barrier 2000+. The minimal duct thickness is outlined by no matter what code jurisdiction which the challenge is found in. Underneath are two code references connected to the expected thickness and product form for grease https://cashfwlan.tinyblogging.com/what-does-fire-resisting-ductwork-insulation-mean-80824289