Silvacore Pro by Duraflue 45 degree wall sleeve (for when going through wall): 5", 6", SilverProtects wall when flue passes through. Keeps insulation away from flue. This is a twin wall clip together chimney part used to build a new chimney. Twin wall clip together chimney can be as close as 5cm to combustible materials (7cm if passing through a sealed void). No glue or sealants required. When we say 5 or 6 we speak of internal diameter: outer diameter is 7. 5 and 8. 5 (192mm and 217mm). Detailed technical info see footer of website.