Composition means what the insulation is made of, its stability and its compatibility with the roof membrane being installed. Thermal characteristics mean the R-value of the material. For instance, polystyrene insulation is not a good choice for use in built-up and modified-bitumen roofing systems as the insulation melts at the temperature of hot asphalt. But it is the best choice for inverted membrane roofs as its insulating value is not affected by water. Mechanically fastened and fully adhered single-ply membranes also are good matches with polystyrene.
Polyisocyanurate insulation is the most common board insulation used because it can be used with almost all types of membrane systems. With hot adhesives, however, it has to be paired with an insulation cover board of another type, such as perlite or wood fiber. Perlite and wood fiber are rarely used by themselves because of their low R-value. When high insulating value is not a requirement, such as unheated warehouses and storage facilities or when insulation board is needed solely to provide a smooth surface to apply a re-cover membrane over an existing roof, their low cost make them attractive. Perlite should not be used with PVC membranes, as the two are incompatible.
Foamed-in-place insulation is the material of choice when a liquid or spray-applied membrane roof is installed. Because the foam application is seamless, there are no insulation board joints for the liquid or spray-applied material to run down into.
Almost any insulation can be used in a tapered insulation system, used when the existing building deck is not structurally sloped to the drain or where deflection in the deck is causing areas of standing water. Perlite is a good choice as the tapered board because it is inexpensive. The insulating value can be met by an underlying layer of polyisocyanurate, leaving the perlite to form the slope. Foamed-in-place insulation is excellent for adding slope to a roof as it can be shaped on the roof to meet the precise requirements of deflected and oddly shaped roofs.
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