mechanical systems using prescriptive and performance-related provisions. The IMC was developed with broad-based principles that make possible the use of new materials, methods and design. Important changes include: Added coverage of pollution control units. A new exception was added to recognize Type I kitchen hoods listed for clearances to combustibles of less than 18 inches. Added coverage for a newer type of non-metallic duct, phenolic duct. New coverage for high volume large diameter fans (HVLD), also referred to as high volume low speed (HVLS) fans. Relaxed requirements for sealing of duct joints and seams for Snap- and Button-lock duct joints located within the thermal envelope.