Phenolic board insulation is an expensive material with a very high R-value per inch. The surface is often coated with kraft paper or foil. Phenolic board insulation is vapor permeable, so it isn't a vapor barrier. It does prevent air movement if the joints are taped.
Phenolic board insulation is less combustible than other plastic insulations such as polyurethane and polyisocyanurate. When phenolic insulation is exposed to water, there is some suspicion that corrosive acids may form, which can corrode metal. This is a topic of some controversy in commercial roofing, where phenolic board insulation was used extensively on metal roof decks.