Intertherm Flue Extension 5in x 10" Inside. Flue Extension 901935 features an inside diameter of 5" an outside diameter of 8.25" and a length of 10". Extension lengthens the roof jack connection an additional 10". Make sure the interior extension is below the ceiling connections cannot be installed inside of attics. Features of Intertherm 901935 Flue Extension 5in x 10" Inside Interior use only Install below the ceiling connections cannot be installed inside of attics Inside Diameter: 5" Outside Diameter: 8.25" Extension Length: 10" Package Contents: Intertherm 901935 Flue Extension 5in x 10" Inside Intertherm 901935: Flue Extension 5in x 10" Inside