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Compressive Strength of Polyiso Insulation

May 15, 2001

OMPRESSIVESTRENGTH of P o l y i s o I n s u l a t i o nBy PIMA Headline background: Isocyanurate cells under magnification (Courtesy Koppers Co.)
The ability of rigid foam board to resist deformation or maintain shape when a force or load is applied is due to the physical property known as compressive strength. Although the force may be applied in any direction, it is most often measured in the direction of insulation thickness. The value is defined as a force applied over a unit area. In the customary U.S. measurement system, the value would be expressed in the units of pounds per square inch (psi) or pounds per square foot (psf). In the SI system, the units would be Newton per meter squared (N/m2) or Pascal (Pa).
Measuring Compressive Strength
The American Society for Testing and Materials (ASTM) has two methods which can be used to measure the compressive strength of polyiso foams:

ASTM D1621 — Standard Test Method for CompressiveProperties of Rigid Cellular Plastics.

ASTM C165 — Standard Test Method for MeasuringCompressive Properties of Thermal Insulations.
Both methods require the testing of multiple specimens using
a precision testing machine to determine an average or mean
value. The following procedure is most often used by the majority
of the manufacturers of polyiso board.
From the center of randomlyselected 4’x8′ or 4’x4′ (1220mm
x 2440mm or 1220mm x 1220mm) insulation boards, a 4″ strip is
cut across the width of the board at its center point; this strip is
further cut into 4″ (100mm) pieces. Each alternative piece is a test
specimen for a total of at least five specimens. Sampling the entire
width of the board ensures that the sample is representative and
illustrates uniformity of the product.
After preparation, each specimen is compressed in the testing
machine, and the force required to deform the piece 10 percent
in thickness or until a yield point (whichever occurs first) is
recorded. The compressive strength for a particular product is
defined as the average of at least five test results. Polyiso products
are commonly available in the range of 1625 psi (110172 kPa).
The published values are nominal or typical values due to the variation of manufacturing processes. Most samples should be within 10 percent of the nominal value. Individual companies should be consulted for specific variations. Manufacturers test production materials to ensure continuing quality and determine values for published specifications.
Importance of Compressive Strength
Common construction applications of polyiso foams require compressive strengths adequate for durability during installation and use. Wall application requires the product to support flexible siding materials. In roofing, it must withstand limited installation traffic, support fastener loads, and sustain the total roofing system. The results of the tests provide information about the behavior of polyiso foams under compressive loads and are important to ensure proper performance.
Using Compressive Strength Values
The specific values needed must be determined by the architect, engineer, or designer of the building. The compressive strength value can be used to compare different products or between brands of the same type of product. Polyiso insulation boards are available in a range of compressive strengths. The ASTM Specification for Polyiso Insulations, C1289, indicates that all polyiso products should have a minimum stated compressive strength of 16 psi (110 kPa). Actual compressive strength may vary above the minimum depending on the manufacturer. ■
The Polyisocyanurate Insulation Manufacturers Association (PIMA) is the North American trade association that advances the use of polyisocyanurate (polyiso) insulation. Membership consists of manufacturers of polyiso insulation and suppliers to the industry. This Technical Bulletin 102 is printed with PIMA’s permission.
6 • Interface June 2001