Where the chimney passes through the roof, all combustible materials needs to be kept a minimum of 50mm away. To create an airtight seal non combustible insulation was put around the chimney above the air barrier, then we scribed the shape of the chimney out of aluminium. We sealed this to the airbarrier with airtightness tape. We sealed between the aluminium and the chimney with intumescent mastic.
The twin walled chimney we are using is classified as being allowed to be fully insulated when passing through the floor/roof zone. Some chimneys need to maintain a free airflow around them all the way along them including when passing through floor/roof, so we would not have been able to have insulated above it if this had been the case. When we enclose the lower part of the chimney within the wardrobe space, we will need to ensure that we allow sufficient vents to allow air flow around the rest of the chimney to dissipate any heat.