EPS is a closed cell, lightweight and resilient, foamed plastic composed of hydrogen and carbon atoms. The mechanical strength of EPS varies with its density. The most important mechanical property of EPS insulation and building products is its resistance to compressive stresses, which increase as the density becomes higher. EPS has a compressive resistance between 10 - 60 psi for most construction applications. Within that range EPS can be produced to meet specific strength requirements.

ASTM C578, Standard Specification for Rigid, Cellular Polystyrene Thermal Insulation is a consensus document that was developed by producers of polystyrene foam, third party testing companies, regulatory agencies and insulation users in the North American region. It covers the types, physical properties, and dimensions of cellular polystyrene used as thermal insulation for temperatures from -65 to 165°F. This Standard covers types of EPS thermal insulation currently available and the minimum requirements for the properties considered most important. Flexural strength and compressive resistance values are included. These values were developed using ASTM C203, Test Method for Breaking Load and Flexural Properties of Block-Type Thermal Insulation and C165, Test Method for Measuring Compressive Properties of Thermal Insulations and/or D1621 for Test Method for Compressive Properties of Rigid Cellular Plastics.

To meet the compressive resistance requirements specified in ASTM C578 polystyrene thermal insulation board must provide the following compressive strengths at 10% deformation when tested in accordance with ASTM D 1621.

Typical Strength Characteristics - EPS Thermal Insulation Board

Property	Units	ASTM test	ASTM C 578 Type
			I	VIII	II	IX
Density range	pcf	C303	0.90-1.14	1.15-1.34	1.35-1.79	1.80-2.19
Flexural Strength	psi	C203	25-30	32-38	40-50	55-75
Compressive Resistance - at yield or 10% deformation	psi	C165 or D1621	10-14	13-18	15-17	25-33

For foundation and wall applications in which the foam insulation bears a minimal load, ASTM C 578 Type I (minimum density of 0.9 pounds per cubic foot) material is adequate. EPS board produced to meet the requirement of Type I EPS has been tested and found to fall within 10 to 14 psi. The resilience of EPS insulation board provides reasonable absorption of building movement without transferring stress to the interior or exterior finish at the joints.

In roofing, Type I EPS material provides the dimensional stability and compressive strength necessary to withstand light roof traffic and equipment weight at reasonably high surface temperatures. EPS foam insulation may experience dimensional and property changes when it is exposed to extreme temperatures greater than 167°F. However, low density EPS not subjected to load will show no noticeable loss of dimensional stability at temperatures up to 184°F. Duration of temperature, external load conditions and density are the variables affecting foam insulation at elevated temperatures. EPS should be adequately protected from temperatures greater than 165°F during installation and may require the use of cover boards, reflective ballast, or a light colored membrane depending upon the roof covering system involved.

Optimum performance of a load-carrying insulation is often related to both strength characteristics and resiliency. Resiliency is the ability of a material to recover its strength following deformation caused by stress. If greater strength and rigidity are needed, compressive resistance up to 60 psi is available by increasing the density of the EPS insulation to meet virtually any compressive strength requirement.

Due to its high resiliency and strength characteristics, expanded polystyrene insulation offers:

• Absorption of substrate and facing movement caused by temperature changes and structural deflections.
• Absorption of substrate irregularities.
• Thickness recovery following excessive construction load exposures.
• Suitable sub-grade reaction for effective load distribution.

Design Considerations

Compressive and flexural strength values for expanded polystyrene are based on short-term load conditions in accordance with typical ASTM test standards. As do most load bearing building materials, EPS foam insulation products creep under long-term, continuous load conditions and, in critical applications, this characteristic must be considered in design calculations. Design professionals should recall that greater strength properties are available from EPS foam by increasing density. Data that reflects deflection resulting from continuous, compressive load exposures for EPS insulation is available.