What Is Extruded Polystyrene Insulation (XEPS)?

There are four major rigid plastic foam insulations commonly used for residential, commercial and industrial insulation: extruded polystyrene (XEPS), expanded polystyrene (EPS), polyurethane (PUR), and polyisocyanurate (PIR). Each type has individual characteristics and specific advantages and disadvantages for particular building applications.

Nonetheless, the stable properties of polystyrene, when combined with a unique foam extrusion process, produce an exceptionally useful product with benefits for nearly all construction and engineering applications.

Extruded polystyrene has a well established reputation for long-term reliability and superior resistance to the elemental forces of nature: time, water, cold, heat, and pressure.

Consider these forces when choosing insulation types.

Extruded polystyrene foam begins with solid polystyrene crystals. The crystals, along with special additives and a blowing agent, are fed into an extruder. Within the extruder the mixture is combined and melted, under controlled conditions of high temperature and pressure, into a viscous plastic fluid. The hot, thick liquid is then forced in a continuous process through a die. As it emerges from the die it expands to a foam, is shaped, cooled, and trimmed to dimension.

This continuous extrusion process results in a unique foam product with a uniform closed-cell structure, a smooth continuous skin, and consistent product qualities, qualities unequaled by other insulation types.

Comparing Insulation Properties

The ability of an insulation to resist heat flow is usually the principal consideration in comparing insulations. The material's resistance to heat flow is expressed as R-value. The higher the R-value, the greater the insulating power.

However, each insulating material's R-value may vary depending upon the characteristics of its manufacture and conditions of use. In particular, the responsible design professional should consider the long-term aged R-value of installed insulations, as well as the R-value at the average use temperature, when specifying insulations.

The insulation value of most rigid foam boardstock products relies in part on the blowing agent used during their manufacture. The permeation of air, which is more conductive, into the board after manufacture can affect the R-value of the insulation with time. For accurate comparisons and for reliable performance over the life of a building or insulating product, it is important to use appropriate long-term aged R-value data from all insulation manufacturers.

DiversiFoam Products and XEPS industry organizations continue to recommend that design professionals and specifiers use long-term aged values for comparison and design calculations.

Also, the R-values of various insulation types change with different average use temperatures. However, since different types do not vary linearly or proportionally to other types, R-values at representative use temperatures should be used for accurate comparison. Specifically, some insulations with relatively high R-values at warm temperatures lose insulating power as the temperature drops. Note that CertiFoam extruded polystyrene actually has greater insulating power (higher R-value) as the mean temperature drops.

The superior moisture resistance of extruded polystyrene insulation provides outstanding benefits for most construction and engineering applications.

The universal physical and chemical properties of water can have serious consequences for many building materials. Fortunately, the superior moisture resistance of extruded polystyrene foam insulation is well established. Not only is polystyrene naturally hydrophobic (no chemical affinity for water), but its fine closed-cell structure and smooth continuous skin helps the foam resist moisture better than other types of insulating materials.

Temperature fluctuations, above and below freezing, in the presence of moisture have serious consequences for most building materials. However, the characteristic moisture resistance of extruded polystyrene foam, combined with its tough yet resilient nature, results in excellent resistance to freeze/thaw damage.

Extruded polystyrene foam is easy to handle and available in a variety of sizes and compressive strengths to suit varied application requirements.

These and other characteristics also make extruded polystyrene foam insulation the proven product choice for below grade insulation, protected membrane roof or plaza systems and a number of special insulation applications. The uniquely resistant and durable qualities, inherent to extruded polystyrene foam, make CertiFoam one of the most dependable long-term insulation materials available.


CertiFoam brand insulation is recognized as a stable polystyrene plastic foam product and is resistant to many common chemicals such as: acids, bases, water and water-based paints, alcohol and alcohol-based paints, brine or salt water, cement and mortars, asphalt, etc. However, petroleum-based products may attack the foam. Avoid foam contact with concentrations of solvents such as: Aromatic and chlorinated hydrocarbons, gasoline or fuel oil, cut-back asphalts, naphthas or paint thinner, oil-base paint, oil-base insecticides or wood preservatives, polyester resins, etc.

CertiFoam is unaffected by substances normally found in soil (i.e. acids, alkalis, bacteria, etc.) It will not corrode, rot, or support the growth of mold, mildew or soil microorganisms. It has no food value and will not support plant or animal life. Precautions should be taken when insulation is in contact with the soil to prevent hidden entry through the foam by vermin or insect pests.

CertiFoam is not affected by the weather and may be stored outside. Prolonged exposure to UV radiation in sunlight may cause the surface to become pale and dusty. This will have no significant effect on insulating value unless the surface is eroded and thickness is reduced. Surface dust should be brushed off before applying adhesives or finishes. An opaque protective covering should be used for extended outdoor exposure periods.

CertiFoam is not recommended for uses where sustained temperatures will exceed 165F. Avoid contact with chimneys, steam pipes, electrical or sauna heaters or other such surfaces. Intermittent exposure during installation and use should not exceed 180F. Caution should be taken to avoid prolonged excessive temperatures under unballasted roof membranes or during roof construction delays.

Like many construction materials, CertiFoam brand insulation is combustible and may constitute a fire hazard if improperly installed. Although CertiFoam contains a flame-retardant additive to inhibit ignition from small fire sources, it should not be exposed to open flame or other ignition sources during shipping, storage, installation or use. These products should not be left exposed in the interiors of buildings where people reside, work or assemble. An approved thermal barrier, such as one-half inch gypsum board or its equivalent, should be mechanically attached over the foam in interior applications. Recommendations of fire and building codes should also be followed.