Twin wall flue system design, cavity wall

Cavity wall and twin wall flue systems A cavity wall is common on more modern buildings. Generally there are two brick or block walls seperated by a gap and tied together with metal ties. Often this gap is filled with insulation. We need to know whether your wall is a cavity wall or not because if it is then we need to use a wall sleeve - this is a large tube that you put through the wall that the flue then passes through.