PIR is a thermoset, closed-cell, rigid foam plastic insulation that is manufactured through a controlled chemical reaction. Liquid raw materials expand and are molded into boards, and facers are applied to the top and bottom surfaces. Facers provide strength, improve rigidity, enhance thermal performance, and help limit thermal drift. These facers are usually asphalt-saturated organic and inorganic felts, inorganic glass fiber mats, or aluminum foil and are selected to enhance the end use of the foam (Figs. 11.9 and 11.10).
For example, foil-faced PIR is used most commonly as wall sheathing in residential construction or in masonry cavity wall construction. The facers protect the foam core from UV degradation. Roof applications rely on glass fiber facers or glass fiber-reinforced organic felt facers. Most commonly supplied as 4′ X 8′ or 4′ X 9′ sheets in a range of thickness from V2 to 4", PIR board insulation is
Figure 11.9 Polyisocyanurate core with fiberglass mat. (Celotex)
Figure 11.10 Foil-faced polyisocyanurate sheathing. (Celotex)
moisture-resistant, is classified as a water-repellent panel sheathing, and is resistant to the solvents often found in construction adhesives.
It is also a cost-effective roof insulation product because it is approved for installation directly to the steel roof deck without the need for a thermal barrier. Although not as critical to residential applications, PIR is stable over a large temperature range (-100 to +250°F). It can be used as a component in roof systems that use hot bitumen, but the insulation material will blister if the hot asphalt product is applied directly to it. As mentioned earlier, PIR with glass fiber or organic felt facers will lose thermal resistance over time, which must be taken into account when designing the roofing system.4