Roofing advice needed

[Antwerpman]

I am about to platerboard the ceiling of my loft and I need advice on what I need to place where

The roof itself is some form of fibre based tile (a bit like the stuff they make some heavy corrugated roofs out of - may even be asbestos based?). These are nailed to battons which in turn are then nailed to the roof joists - there is no underfelt as the house predates this. Between the joists is rockwool insulation with a foil face, which are sealed with tape to create a draft proof barrier (in theory anyway). Because there is no underfelt there is an airgap (probably about 20-30mm) between the tiles and the rockwool to allow air circulation.

Now what I want to do it to place horizontal battons between the joists, fill between the battons with expanded polystyrene boards and then put plater board on to the battons to finish it off. The reason for the polystyrene is to try and add further to the insulating properties and to help a little with noise reduction (it is big roof and there is a reasonably busy road outside)

So firstly, is what I am doing ok, and secondly do I need to place a waterproof membrane somewhere and if so where??

 

[ets80]

no, you can not close off the air space, it is there for ventilation/air flow. If you close this off it will cause condensation and later on formation of mold

 

[Alastairreid]

Check the dates before posting ets, saves confusion.