Hi, I seem to have insulation in the roof of my attic already but not on the floor, I want to insulate the floor now as well and intend doing it myself, I know the thickness should be 270mm, the top layer being 170mm and the bottom layer between the joists 100mm but the DIY stores sell 170mm at a reasonable price but not so much of the 100mm, only expensive rolls, does anyone know where is the best place to shop for best price please