Twin wall flue boxing in
In Document J, page 24 & 25
When do you have to box in twin wall flue pipe
Where twin wall flue passes through a cupboard, storage space or roof space a guard should be placed around it. This guard should be further away from its designated distance to combustibles. Typically this clearance distance is 50mm or 60mm, but it does vary. Your twin wall flue should have a label with it which will show the distance to combustibles on it. For example, With Clearline AKW you should see a part of the label which says "G50" - this is the distance to combustibles in mm, 50mm in this instance.
The point of boxing in twin wall flue is that it keeps flammable items safely away from the chimney in places where this is likely to happen. A bedroom is a prime example where toys or the contents of the "floordrobe" may easily come to rest against the flue/chimney. Twin wall flue doesn't normally get that hot on the outside so you might not think this is too much of a problem. But if you have a chimney fire then the outside can get very hot when the internal temperature rises up to 1200ºC. It is really this situation that you are preparing for.
In a roof space guarding may often be done with mesh held in a frame around the pipe, and in bedrooms and storage spaces it is more commonly done by boxing the flue in, although some type of mesh is still a way to do it. I much prefer solid boxing in because it means that nothing can accidentally fall into the void and you also need to think about the fact that there is ventilation where the twin wall passes through the floor. A mesh guard would let the noise, smells and more crucially smoke pass from the room below to the one above. Its worth bearing this in mind and to make sure your boxing in is well make and well sealed when you build it.
Most types of twin wall chimney have special components to take you through floors and ceilings. These often have tabs to help build and locate the boxing in and the tabs are placed so that the distance to combustibles is correct. So not only do they make going through the floor or ceiling easy, but they also help with the task of boxing in.