Cracking in Exterior Insulation and Finish Systems

Abstract

The outside lamina of an exterior insulation and finish system is typically applied as a plaster using a polymer and portland cement mix, reinforced by a mesh that is usually a glass fiber product. Observations of leakage and cracking are discussed. Engineers sometimes find a particularly severe cracking failure in which the crack splits the system, including the mesh, fully through its thickness, suggesting that in such cases the mesh is of insufficient strength to distribute tensile strains and control cracking. Elementary stress analysis establishes the conditions under which such full splitting will occur. Pilot tests were performed to measure elastic modulus, rupture stress, shrinkage strain, and mesh strength. For the sample system studied, the measured properties confirm the mesh strength to be inadequate for control of cracking, and predict system performance failure by full splitting. Further analysis demonstrates that the tensile strain field producing such failure can be produced solely by shrinkage of the material.