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Metal buildings generally are constructed with no ventilation between the metal roof sheet and the roof insulation. This situation may result in moisture accumulation in the insulation, if the insulation does not have a satisfactory vapor barrier. The insulation vapor barrier performance is not only a function of the vapor barrier material, but also a function of the vapor barrier seams and any components which must penetrate the vapor barrier. An experimental investigation of a metal building roof insulation system was conducted. The system consisted of faced insulation boards suspended below the purlins with light density insulation installed between the board and the roof sheet. A large scale moisture test facility was used. A 2.44 by 3.67 m (8 by 12 ft) roof test specimen was constructed using actual metal building construction procedures and materials. The interior side of the test specimen was maintained at 21.1°C (70°F) and 50 percent relative humidity, while the temperature above the roof sheet was held below -23.3°C (-10°F). In order to determine the temperature and humidity conditions within the roof plenum, the test specimen was instrumented with thermocouples and humidity gages. The test duration was 3 weeks, after which the test specimen was disassembled for visual observations of the interior portion of the roof section and to obtain specimen weights of the insulation. No significant moisture accumulated in the insulation or the roof cavity.
insulation, vapor barrier, moisture transport, metal buildings, humidity, condensation
Senior engineer, Owens-Corning Fiberglas Corp. Technical Center, Granville, Ohio