Room in the Roof
If you want to use your loft area as a living space or if it is already being used as a living space, then you need to make sure that all the walls and ceilings between a heated room and an unheated space are insulated also.
Sloping ceilings can be insulated with a layer of plasterboard on the inside of the insulation and vertical walls can be insulated the same way. If you have a flat ceiling it can be insulated like a standard loft.
A flat roof should preferably be insulated from above. A layer of rigid insulation board can be added either on top of the roof’s weatherproof layer or directly on top of the timber roof surface, with a new weatherproof layer on top of the insulation. This is best done when the roof covering needs replacing anyway. If your flat roof needs to be replaced anyway, you must now insulate it to comply with building regulations.
Having flat roof insulation will save you the same amount as with loft insulation.
Insulation stops heat escaping as described earlier which could create or worsen existing damp or condensation problems, it still can be done with well planned ventilation.