Any requirement for drywall around a furnace?

For a gas furnace that is installed in the attic, essentially in the eaves behind the kneewalls, is there any requirement that there be drywall between the furnace and the insulation?

In this case, the slopes of the attic (gable style attic) have been insulated with densepack cellulose and covered with a plastic vapor barrier. The furnace would be pretty close to the slopes of the roof. Is it a requirement that the insulation be covered with drywall?

There is a MFG’s requirement for clearance to combustibles that must be followed they do not list flammable material. The dry wall is probably just as flammable as the insulation unless it is fire rated.