Section M1604 addresses ductwork distribution in mechanical systems. Ductwork must be sized to deliver required air flow to all spaces with minimum 6-inch diameter or equivalent rectangular dimensions. Exposed ductwork in conditioned spaces does not require insulation. Ductwork in unconditioned spaces including attics, crawlspaces, and basements must be insulated with minimum R-5 insulation wrapped with vapor barrier. Supply and return ductwork must be balanced to deliver appropriate air flow to conditioned spaces. Duct sealing with mastic or metal-backed tape prevents air leakage losses. Return air ductwork must be sealed and cleaned to remove dust and debris. Flexible ductwork is permitted but should not exceed 8 feet in length in conditioned spaces due to resistance and contamination accumulation.