What Types of Closed Cell Rigid Insulation Are Waterproof?

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Waterproof rigid insulation is used on roofs.

Expanded polystyrene, or XPS, extruded polystyrene, or EPS, and polyisocyanurate, or polyiso, are the three main types of rigid, closed-cell insulation materials. Each type is used in a variety of applications and under conditions where resistance to water and moisture is critical. Rigid insulation is used in homes to insulate roofs and for below-grade applications such as insulating basements. XPS, EPS and polyiso keep homes sealed and protected against everything from rain to snow, ice and hail.


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Polyiso is a rigid foam board insulation used for roofing and for insulating walls and ceilings. It is a composite insulation made from various chemicals and synthetic materials, which are heated and then cooled to form rigid panels. After polyiso panels are manufactured, they are sandwiched in between exterior layers composed of recycled paper products and fiberglass. This creates its closed-cell structure and makes polyiso exceptionally waterproof. The sandwiching also keeps polyiso stable over temperatures as low as minus 100 degrees Fahrenheit to 250 degrees Fahrenheit. Polyiso's capabilities to withstand moisture, heat and cold make it one of the most effective and efficient insulation materials available.



XPS is another waterproof, rigid insulation material. It is made from polystyrene pellets, which are mixed with various chemicals, then heated until the pellets liquefy. When the pellets cool, they turn into tiny beads. Then the beads are heated and injected into flat molds. When the molds cool, the material is cut into panels of various lengths, widths and densities to use as insulation. The final product is rigid, dense and waterproof. As a result, XPS is commonly used for roofing and below grade to insulate cement slabs and foundation walls.


Expanded polystyrene is commonly called beadboard. EPS is used for everything from everyday items, such as coffee cups, to producing insulated concrete forms, called ICFs, used to build concrete walls. While polyiso, XPS and EPS have some of the strongest waterproofing properties of all insulations, EPS is the weakest of the three and absorbs more water. EPS is also susceptible to crumbling on the edges.



The R-value rates an insulation material in terms of its capabilities to insulate a structure from heat loss and gain and water penetration. The R-value of insulation materials is critical when selecting insulation that must withstand moisture and various climatic conditions. EPS ranges in R-values from 3.6 to 4.2 per inch, depending on the density of the rigid panel. Polystyrene has an R-value of 5 per inch, while polyiso has an R-value of 5.6 to 8 per inch, and the highest R-value of all closed-cell rigid insulation materials.



Cheryl Munson

Cheryl Munson has been writing since 1990, with experience as a writer and creative director in the advertising industry. She holds a Bachelor of Arts in journalism with a focus on advertising from the University of Wisconsin in Madison.