Hi again, roof structure, rafters, purlin ect. now all in place so time to think about insulation. I have decided to go for a vaulted ceiling (warm roof) and am aware that it requires 90mm and 40mm celotex or equivalent. I understand the 90mm goes in between the rafters which are 125mm but I am a little confused as to where the 40mm goes....does it attach to the underside of the rafters?