Polyisocyanurate ("polyiso") and polyurethane are very similar, closed-cell foam insulation materials. Because both materials offer high R-values (R 5.6 to R 8) per inch of thickness, you can use a thinner foam board to achieve the required thermal resistance. This can be an advantage if you have space limitations.
Polyiso foam board insulation is available in a variety of compressive strengths. Compressive strength refers to the ability of a rigid foam board to resist deformation and maintain its shape when subjected to a force or load. Also, polyiso remains stable over a wide temperature range (-100ºF to +250ºF). This makes it good as roofing insulation. And when used with a laminated aluminum foil facing, polyiso foam board provides an effective moisture or vapor barrier.
These foam boards can also be used to make structural insulated panels (SIPs).