To exempt the outer coverings of ducts, plenums and other parts of air duct systems from the requirements of Sentence 3.6.5.4.(2), which would otherwise impose more stringent limits with respect to flame-spread rating and smoke-developed classification, and permit higher limits to the flame-spread rating and smoke-developed classification, on the basis that the higher limits do not significantly add to the risk of fire and smoke spread within an assembly permitted to be of combustible construction.
This is to limit the probability that the outer coverings will have inappropriately high flame-spread and smoke-development properties, which could lead to the development of an inappropriate amount of smoke and the spread of fire across the exposed surfaces of the material, which could lead to the spread of fire and smoke to other parts of the building by means of the air duct systems, which could lead to harm to persons.