Water Absorption Of Insulation In Protected Membrane Roofing Systems

Current methods for evaluation of the moisture absorption of plastic insulations (ASTM-C-272-53 and ASTM-C-355-64) due to vapor pressure gradients or immersion rely on short time periods to predict long term performance. This procedure may not provide accurate information on performance since in practice insulations may absorb more moisture than these tests indicate. A series of tests were conducted on extruded polystyrene roof insulation that had been in place, exposed to environmental moisture and pressure gradients, for a maximum of 36 months. Results indicate that moisture absorption of 1.5% by volume can be expected in the field. (Author).

The repair and maintenance costs of low slope roofs are high and their life expectancy is less than desired. The increasing use of insulation is accompanied by a greater incidence of problems. The experience in cold regions amplifies these experiences. Protected membrane roofs have the prerequisites for better performance and the experience to date is encouraging. The results of performance measurements of three roofs built by the Corps of Engineers verify that the membrane remains at nearly constant temperature, independent of the weather, and that the insulation retains its integrity despite periodic wetting. Moisture absorption is slow and appears to stabilize in time due to the self-drying nature of the roof. Heat losses are increased due to rain, and extra insulation should be added to compensate for these losses. The resistance of protected membrane roofs to fire, traffic, impact, and other adverse forces is superior. So far, the initial cost of protected membrane roofs is at a premium, primarily due to the cost of concrete pavers. The initial cost premium can be justified, however, by the reduced repair and maintenance costs as indicated to date, and by the longer life expectancy of the protected membrane. The high probability of superior performance and cost effectiveness is a compelling reason to incorporate protected membrane roofs increasingly in Government construction.