Where the ductwork leaves the compartment from which it serves (revealed in blue) it have to keep integrity and insulation fire ratings for the same period of time as the compartment through which it passes. Promat has designed self-supporting methods without a metal liner, for instance PROMATECT®-Advert, PROMATECT®-L500 and MASTERBOARD®, https://eduardozqfuh.develop-blog.com/44437070/the-ultimate-guide-to-grease-tight-extract-ductwork