I'm planning to replace the tired hardwood cladding on my dormers with a layer of 9mm waterproof plywood, a layer of breathable felt and then a layer of UPVC Shiplap cladding. While doing this I want to insulate between skins as there is none at all. Is Kingspan or similar the best thing to use? What thickness should be used ? Should it be installed in contact with the inner skin or in contact with the outer skin or neither. I also need to insulate the flat roofs at some point and was also planning to use kingspan. Any advice at all on insulation / ventilation / cladding of dormer walls and flat roofs would be very much appreciated.