Abstract: The ductwork utilized in heating and air conditioning systems is provided with internal crossbars secured to the sides of the duct to prevent inward or outward flexing in response to a pressure differential that could otherwise cause collapse of the sides, either inward or outward, and failure of system. The crossbars are spaced along the run of the duct with the opposed ends of each crossbar received within and secured to corresponding sockets mounted internally on opposite walls of the duct.