The three main types of foam rigid insulation boards are expanded polystyrene, extruded polystyrene and polyisocyanurate. Another type is rigid fiber insulation board made from fiberglass or mineral wool, which contractors often use to insulate air ducts or in situations that require high heat tolerance.
Expanded polystyrene is the least expensive of the three types of rigid foam boards and the most vapor-permeable. It has the lowest insulating value per inch of thickness and is commonly used in insulating concrete forms. Expanded polystyrene is usually white and is also found in coffee cups and ice chests. Available in several densities, expanded polystyrene is prone to crumbling and other damage.
Extruded polystyrene is stronger and more resistant to water and has better insulating properties than expanded polystyrene. Due to its strength and water resistance, extruded polystyrene is commonly used to insulate basement walls, foundations and slabs below ground level. It is less vapor-permeable than expanded polystyrene and is considered semi-impermeable.
Polyisocyanurate has the best insulating properties of the three types at warmer temperatures, but as it gets colder, its performance declines. It is the most expensive of the three and is often sold with a foil facing that is easily sealed with foil tape, making it ideal for use in radiant heat situations. Foil-faced polyisocyanurate has low permeability and creates an effective vapor barrier.