Performance Properties of Extruded Polystyrene Foam (XPS) Insulation In general, the physical properties of XPS products are best characterized by using the material standard for rigid, cellular polystyrene foams: ASTM Standard, C578.
Thermal Efficiency (Insulating power)
The thermal efficiency of an insulating material* is expressed as R-value. R is the material’s resistance to heat flow. The higher the R-value, the greater the insulating power. The insulating power of many rigid foam boardstock products relies in part on the hydroflourochlorofluorocarbons used in the blowing agent system. Permeation of air into the board can affect the R-value of the insulation. For accurate comparisons of long-term insulating performance (LTTR) it is important to obtain long-term aged R-value data from all insulation product manufacturers. Figure 5 shows a typical R-value curve of XPS boardstock over a 5 year period.
The closed-cell structure of XPS imparts excellent long-term strength and durability. Products are available in a range of compressive strengths to suit varied application needs.
A critical factor affecting long-term performance is the ability of an insulating material to resist the intrusion of moisture. Moisture can come in contact with insulation not only during construction, but throughout the life of the building. If absorbed, its effect is to drastically reduce thermal efficiency (R-value). The closed-cell structure and lack of voids in XPS helps the foam to resist moisture penetration better than other types of insulating materials. The excellent moisture resistance of XPS foam insulation has been confirmed repeatedly and consistently in laboratory tests and under field use conditions.
Mold, Mildew, Corrosion resistance
Because XPS foam is essentially a plastic material, it will not corrode or rot or support the growth of mold or mildew. It is resistant to microorganisms found in soil and provides no nutrient value to vermin. These properties make it an outstanding insulating material for below grade applications.
XPS is resistant to many types of chemicals. However, petroleum-based solvents will attack the foam. Exercise caution when working with solvent-based materials. If you have an issue, be sure to contact the sponsoring manufacturers for more information.
Exposure to ultraviolet radiation in sunlight for several weeks may cause the surface of XPS foam to become yellow and dusty. This will have no measurable effect on the insulating value. However, an opaque protective covering should be used during outdoor storage, particularly for extended periods.
Use of XPS foam is not recommended where sustained temperatures exceed 165º F. Do not use in direct contact with chimneys, heater vents, steam pipes or other such surfaces. Intermittent heat exposure during installation and use should not exceed 180º F.
Like many construction materials, XPS is combustible. It should not be exposed to flame or other ignition sources. XPS insulation should be covered with a thermal barrier or otherwise installed in accordance with applicable building codes.