I recently had insulation installed in my loft. When I bought the house I removed all the original insulation and replaced it with 100mm (4') base layer Rockwool. This basically covered the 4 inch gap (2x4 Joists) between the ceiling board and the top of the joist. I then managed to get a heating grant and when they came to install the loft insulation they put in an additional 8 inches of insulation straight over the top of the joist, which means that I have now lost any packing space I originally had.
The question I have is .... Can I remove the 8 inches of rockwool insulation that was recently added (leaving the original 4 inches between the joists) and place 100mm sheets of polystyrene across the joists? One concern I have is that the house has a cold roof on it (asbestos tiles with no felt) and is well ventilated, so I am a little worried that I will get condensation between the polystyrene and the rockwool. Should I be concerned about this?
Any advice would be appreciated