My house has got 6 dormer windows but the roof space above each one has not been insulated making all of the bedrooms and bathroom extremely cold. The flat-roof space is not easily accessible and certainly not in a straight line. From the closest accessible point in the main roof space there is only very limited access and possibly only a granular material would suffice. What is the best material to use for insulation? Might I need to strip the roof to achieve the objective?