Polyisocyanurate "ISO" Yellow Board

ISO panels have the highest nominal R-value of the three most common types of rigid-foam insulation, R-5.7 to R-6. Its blowing agent is pentane, which has a very low global warming potential. Because ISO starts as liquid foam and has to be sprayed against a substrate to form a rigid panel, all ISO panels are faced. Foil-faced ISO panels are considered vapor impermeable. Paper and fiberglass facers are more permeable and can be used without creating a vapor barrier.  Although the nominal R-value of ISO is the highest of the three major types of rigid foam, its cold weather performance is lower: As the temperature drops, so does the R-value.  ISO is not rated for ground contact.