The Specific Double-wall Zero Clearance versions use 3.twenty five” of a singular greater density ceramic fiber insulation between the interior liner and outer jacket. These Zero Clearance versions can also be a two-hour fireplace-rated-resistant enclosure/duct assembly.   Routine cleaning and easy accessibility for grease filters are emphasised in NFPA ninety https://dallashxmbo.blogolenta.com/34002109/5-tips-about-grease-tight-extract-ductwork-you-can-use-today