The blowing agents in those urethane foams were early members of the Freon family, and the issue of ozone depletion required more environmentally acceptable alternatives. Fortunately, eliminating chlorofluorocarbons (CFCs) and substituting pentane gas as a blowing agent resolved the fate of factory-formed urethane foams. Innovative fire retardants and renamed polyisocyanurate foam (Isoboards) beefed up the fire resistance of urethane. A fire-protecting base layer was no longer needed to meet FM requirements.
With the advent of single-ply roofing, polystyrene roof insulation gained new importance. Whereas a BUR or MB system was typically installed in hot asphalt, solvent-based adhesive or torch application, all of which would melt the styrene foam, loosely laid and ballasted single ply systems were a perfect match.
ASTM Specification C-578 covers the current standards for both types of polystyrene foam (extruded and molded—XEPS and MEPS respectively). Densities of styrene insulation vary from 0.7psf to 3.0psf and compressive strengths as much as 100 psi.
Meanwhile, the oil embargo of the 1970s caused a huge ripple in the availability of petroleum-based asphalt. Up to that time, bituminous built up roof membranes consisted of 3-5 plies of asphalt saturated roofing felt mopped together with hot asphalt and sometimes surfaced with more asphalt to retain the roofing aggregate.
Now, just two or three plies of beefed up glass or polyester mat, factory-coated with polymer-modified bitumen (MB) provided performance equal to or better than the multiple-ply built-up systems. Factory-embedded granules in the cap sheet replaced the material and labor-intensive flood coat and gravel, capturing a significant portion of the low-slope roofing market. Torch application of MB sheets eliminated the logistics of kettles and tankers, providing heat as needed only at the torch itself.
While torching is widely used in Europe, there is not as much danger of fire there because most European roof decks are non-combustible concrete. In the U.S., where steel decks with combustible insulation and cant strips are widely used along with OSB and plywood, several serious rooftop fires have occurred. To address this hazard, the Midwest Roofing Contractors and later the National Roofing Contractors Association developed the Certified Roof Torch Applicator (CERTA) accreditation course.
Innovation is also moving into cold-process application systems for MB, where the adhesive (a mixture of asphalt, petroleum solvent, fibers and fillers, and perhaps polymeric material) is sprayed, rolled on or spread with a squeegee, eliminating the need for hazardous torches. We're moving toward self-adhering membranes to eliminate the odors of solvent-based systems and meet more restrictive VOC regulations. Where just the mineral granules alone on MB cap sheets may not be reflective enough to meet stringent cool roof requirements, factory or post-application of reflective coatings over the granules may comply.
Single-ply roof membranes were already available in the early 1960s, especially PVCs in Europe. The oil embargo jumpstarted the use of these systems as they became competitive with the traditional BUR systems. Virtually all of these first generation materials, such as non-reinforced PVC, chlorinated polyethylene, polyisobutylene, Tedlar, Korad, neoprene, butyl, and chlorobutyl have disappeared.
EPDM rubber, reinforced PVCs, and TPOs now dominate the market. Several generations of TPO have already appeared, and there is some concern that newly appearing problems may cause further reformulation.
The flexible single-ply systems need not be fully adhered. Alternatives include ballast in the form of large river-rounded stones, pavers, vacuum adhered, or mechanically fastened systems. The proper use of ballast and fastened systems was greatly enhanced by the publication of ANSI/SPRI RP-4.