As indicated earlier, a hot rubberized asphalt roof membrane is always installed as part of a PMR system. Extruded polystyrene insulation is the only type of thermal insulation that can be used with a hot rubberized membrane. This insulation is the only type suited for exterior exposure, as it is able to withstand continual freeze thaw cycling without deteriorating. Extruded polystyrene has a very low water absorption rate and has a good resistance to vapour transmission. Boards are produced to 600 mm x 1220 mm (24 in x 48 in) sizes and in thicknesses of 25 mm – 100 mm (1 in – 4 in).

The insulation board is typically loose laid on the membrane and separation sheet. The insulation should have the end joints in the boards staggered by a minimum of 150 mm (6 in) butted tightly together. Fit the insulation tightly to all projections to minimize heat loss. If installing multiple layers of insulation, the joints of the preceding layer should be lapped by the panels of the succeeding layer by a minimum of 150 mm (6 in).

If more than 100 mm (4 in) of insulation is required, use a 100 mm (4 in) board on the bottom layer, and loose lay succeeding layers with the joints in the preceding layer, lapped by a minimum of 150 mm (6 in) by the boards in the top layer on both end and sides.