I have a question about my extension of the summer house. I am now going to insulate the joists and wonder if I can use 50mm styrofoam instead of asfaboard. I have nailed pressure-treated boards that these should rest on. The foundation is a pillar foundation. The original house built in 1972 has polystyrene on the underside and shows no signs of moisture damage, however, the insulation will be thicker in the new construction, 170mm compared to 100mm in the old one.
