I have just had a kitchen extension which has a sloping roof. The building regs say to use celotex 100mm boards as insulation. The chap who is doing the plasterboarding over the roof space says I am better off using Rafter Roll and this is what he has quoted for. Would that be acceptable and would the building inspector even know once the job is done?
A 2 inch space is required above the insulation for ventilation, how will he manage that?
Also you require a almost water vapour proof membrane below the insulation, what will he use?
You owe it to yourself to have the best possible job done on this roof as the room will be in use for a long time, and this type of roof can leak heat and the room will cost a fortune to heat, it is very difficult to obtain a air tight fit with fibre glass and it is the holes between the fibre glass and the wood that lower its effectiveness.
On the other hand a sloppy fitted solid insulation will be just as bad. Celotex or similar fitted carefully and very tight, no gaps is best.
First of all use a staple gun to fit a waterproof plastic sheet across the underneath of the joists, this need to be sealed with tape and all holes where the light wires will come through sealed as well.
Then hold sheets of polystyrene or similar below plastic sheet and nail through, just to keep them in place.
Then do the same with the plasterboard, except this time use the special galvanized screws made for the job.
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