Intertherm Flue Extension 5in x 16" Outside 5in x 16in Outside. Flue Extension 901935 features an inside diameter of 5" an outside diameter of 8.25" and a length of 16". Extension lengthens the roof jack connection an additional 16". This extension is designed for exterior use and must be installed above the roofline. Connections cannot be installed inside of attics. Features of Intertherm Flue Extension 5in x 16" Outside Exterior use only Install above the roofline connections cannot be installed inside of attics Inside Diameter: 5" Outside Diameter: 8.25" Extension Length: 16" Package Contents: Intertherm Flue Extension 5in x 16" Outside Intertherm 901937: Flue Extension 5in x 16" Outside 5in x 16in Outside