I have an uninsulated 70s flat roofed extension to an early Victorian Cottage. I suspect that the ceiling contains asbestos. What are my options for insulation? Is it correct that rolled loft insulation can't be used under flat roofs?
It is quite OK to use roll insulation to a flat roof, the problem is getting the correct thickness and then maintaining the 50mm ventilation air gap over the insulation. Unless you have something like 350mm deep joists then you won't have enough space.
The best type of product is a polyurethane board with silver foil both sides, such as celotex or kingspan. With this you can lay some between the joist and maintain the 50mm gap above (this is important and must be ventilated as if it is not it can lead to condensation and thus rot the joists). You can then lay a layer of the insulation board under and across all the joist before fixing plasterboard. This will ensure the correct U-value is achieved. The thickness of boards depend upon what board are used and how much you can get between the joists.