For fire safety reasons, polystyrene insulation on the inside of house must be covered by an acceptable finish, such as drywall. Preserved Wood Foundations Insulation is normally placed between the studs in preserved wood foundations. The stud cavity should be filled to prevent air pockets and the possibility of convection loops within the cavity, as they would cause heat loss. Insulating Concrete Form Foundations Insulating concrete form (ICF) walls are concrete walls cast in polystyrene forms that remain in place after the concrete has cured (Figure 17). Most ICF walls are “flat” ICF walls, which means that the concrete is of uniform thickness over the width and height of the wall. The NBC requires that any exposed polystyrene be protected by a fire-retardant material, such as gypsum board. 17 Insulated concrete form foundation wall insulated stud wall OSB sheathing spray insulation insulating concrete form (ICF) additional insulation sub-slab insulation fabric footing-forming system