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?
As you probably know, water vapour is formed in the home by cooking, washing, breathing, it always moves outwards towards the cold. A plasterboard ceiling always lets heat through and water vapour, unless it is painted with laytex or gloss paint. Condensation only forms on a cold surface, that is below the dew point of the surrounding air. If you place the polystyrene between the joists, so that it is in direct contact with the plasterboard, the heat rising through the ceiling will keep its downside surface warm, and there will be no condensation and you will save a lot of your heat. Your suggested position above the fibreglass, will not work.
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