What must be installed between the hood and the gypsum or cementitious wallboard?

The requirement to install a smooth, cleanable, nonabsorbent, and noncombustible material between the hood and the gypsum or cementitious wallboard is crucial for safety and hygiene in kitchen environments. This type of material helps prevent grease or other residues from accumulating, which could become a fire hazard. Noncombustible materials are essential in areas like kitchens where hot air, steam, and grease-laden vapors are present, ensuring they do not contribute to fire if exposed to high temperatures.

Moreover, the nonabsorbent property is significant as it discourages the growth of bacteria or mold, contributing to a healthier indoor environment. Overall, using such materials promotes safety, cleanliness, and compliance with building codes related to mechanical systems in commercial kitchens.