Intertherm Flue Extension 5in x 18" Inside 5in x 18ininside. Flue Extension 901935 features an inside diameter of 5" an outside diameter of 8.25" and a length of 18". Extension lengthens the roof jack connection an additional 18". Make sure the interior extension is below the ceiling connections cannot be installed inside of attics. Features of Intertherm Flue Extension 5in x 18" Inside Interior use only Install below the ceiling connections cannot be installed inside of attics Inside Diameter: 5" Outside Diameter: 8.25" Extension Length: 18" Package Contents: Intertherm Flue Extension 5in x 18" Inside Intertherm 903107: Flue Extension 5in x 18" Inside 5in x 18ininside