Wetting of Polystyrene and Urethane Roof Insulations in the Laboratory and on a Protected Membrane Roof

Abstract

When subjected to a sustained temperature gradient in the presence of moisture in laboratory wetting tests, urethane and expanded polystyrene roof insulations accu mulate enough moisture to significantly reduce their insulating ability. Extruded polystyrene is quite resistant to moisture in such tests. But the vapor drive is not as great in actual roofs and it may reverse direction, thereby seasonally drying the msu lation. To determine how well the laboratory tests could predict the wetting rate of insulation in actual protected membrane roofs, extruded and expanded polystyrene and urethane insulations were installed in a protected membrane roof in Hanover, NH. After three years of exposure, little moisture had accumulated in the extruded polystyrene and it still retained essentially all of its initial insulating ability. Moisture progressively accumulated in 16 kg/m³ (1 pcf) and 30 kg/m³ (1.9 pcf) expanded poly styrene and at the end of the test they retained only about 30 and 40 percent of their initial thermal resistance respectively. The urethane accumulated enough moisture to reduce its insulating ability to about 30 percent of its dry value. The laboratory tests provided a valuable indication of the potential long-term moisture gain of these insu lations when installed in protected membrane roofs in cold regions.