British rigid Urethane Foam Manufacturers' Association

Strength Characteristics

Rigid PIR/PUR foam products have remarkable mechanical strength. High compressive and shear strengths allow low density, highly efficient insulating cores to be faced with thin flexible or rigid materials yet span long distances unsupported.

The strength characteristics of PIR/PUR products can be influenced by many factors, including apparent density, chemical formulation and the physical orientation of the individual cells in the foam structure. Different applications such as spray applied insulation, cavity wall insulation boards and metal faced panels require different structural properties, and these parameters are carefully controlled by the manufacturer to give the optimum balance of properties.

Metal faced panel products have particularly good structural properties due to a complex mix of physical and mechanical properties of the inner and outer facing materials and the inner PIR/PUR core. The bending and torsion resistance of the completed sandwich panel is significantly greater than the sum of the individual components.

The compressive strength of flexible faced insulation boards is carefully controlled to comply with the requirements of BS 4841, which is the British Standard for these products, which sets a minimum value of compressive strength through the major plane of the board. For flat roof applications for example this is set at a minimum of 150 kPa. This is considered sufficiently strong to comply with the imposed loadbearing requirements of BS 6399-1:1996. These products are generally anisotropic, in that the compressive strength through the major plane of the board is greater than that parallel to the major plane of the board. This phenomenon is a factor of the shape of the individual foam cells, which tend to be not spherical but more cylindrical or egg shaped.

For information on how PUR can prevent windstorm damage to roofs click here