AF103.4 Entry Routes
Air-handling units in crawl spaces shall be sealed to prevent air from being drawn into the unit.
Exception: Units with gasketed seams or units that are otherwise sealed by the manufacturer to prevent leakage.
Ductwork passing through or beneath a slab shall be of seamless material unless the air-handling system is designed to maintain continuous positive pressure within such ducting. Joints in such ductwork shall be sealed to prevent air leakage.
Ductwork located in crawl spaces shall have all seams and joints sealed by closure systems in accordance with Section M1601.4.1.