The formal, technical name for polyiso is “polyisocyanurate” It is a rigid [foam board insulation](https://homesteadycom/about-5084196-foam-board-insulationhtml) used for roofing and other residential, commercial and institutional applications such as insulation for walls and ceilings. It is composed of various chemicals and synthetic materials that are heated to form a composite material and then cooled to form panels.
Polyiso is categorized as an insulation material with a “closed cell structure” Essentially, this means that the exterior facings of polyiso are protected on both sides. Most manufacturers create the closed cell structure facings by sandwiching the polyiso panel in between exterior layers composed of recycled paper products or fiberglass materials.
As a result, polyiso remains stable over a wide range of temperatures, from minus 100 degrees Fahrenheit to 250 degrees Fahrenheit. This makes polyiso one of the most effective and efficient materials for insulating roofs.
Polystyrene insulation used for roofing is called “XPS,” which stands for extruded polystyrene. Polystyrene XPS is made from polystyrene pellets mixed with various chemicals, which are then liquefied with blowing agents that solidify as beads and are called bead board.
The beads are then heated and injected into molds. When the molds cool, they are cut into panels of various lengths, widths and densities to use as roofing insulation.
The XPS form of polystyrene is stronger, denser and smoother than its “sister” format, EPS, which is more commonly used for walls and ceilings.
Polyiso and polystyrene are popular choices for rigid foam board roofing materials. They are both closed-cell insulation materials, with layers of other materials on the front and back.
The outer layers can be made of paper-based products, fiberglass or even foil. Polyiso and polystyrene XPS insulation are both strong and durable roofing insulation materials and impenetrable by water and moisture.
The R-value of insulation materials is critical when selecting insulation for roofs to prevent heat loss during winter, heat gain during summer and to form a protective insulation barrier from water. The R-value rates an insulation material in terms of its abilities to insulate a structure from heat loss and gain and water penetration.
These capabilities are critical factors in selecting an insulating material for roofing. Polystyrene XPS has an R-value of 5 per inch, while polyiso has an R-value of 56 to 8 per inch.
Of the two, polyiso has the highest R-value.