Twin wall components are insulated and can be used as an independent chimney. Twin wall flue can be used internally or externally and is completely weatherproof. Twin Wall flue systems must be used when passing through any wall or roof of a building. Our flues are made from stainless steel 316 inner liner, stainless steel 304 outer liner and has high density insulation between these layers. A minimum gap of 50mm must be maintained between our twin wall flue and any combustible material.