Water Vapor Transmission and Moisture Accumulation in Polyurethane and Polyisocyanurate Foams

Water vapor transport through rigid polyurethane and polyisocyanurate foams was investigated using three test methods—two under isothermal conditions and one in the presence of a thermal gradient. All three methods yielded water vapor transmission coefficients of the materials. It is observed that the magnitude of the coefficients increases rapidly with temperature above 20°C. In one of the isothermal methods called a modified cup method, developed at the Institute for Research in Construction, only the temperature has to be controlled and this is considered a definite advantage over the other method, ASTM Test Methods of Water Vapor Transmission of Materials, (E 96-80) dry cup method, in which both temperature and relative humidity are to be controlled. Further, the modified cup method also allows the determination of moisture accumulated in the test specimen during moisture transport. It was conclusively shown that during isothermal transport processes no moisture accumulates in either the polyurethane or the polyisocyanurate specimens.

The accumulation and distribution of moisture in the presence of a thermal gradient was also investigated. Contrary to the isothermal process, the moisture transport in the presence of a thermal gradient acting in the same direction as the vapor pressure gradient results in the accumulation of large quantities of water in the foams. This phenomenon may be partly attributed to the temperature dependence of the water vapor transmission coefficients.