Computer Analysis of Wall Constructions in the Moisture Control Handbook

摘要

A computer model, called MOIST, is used to investigate the moisture performance of recommended wall constructions given in the Moisture Control Handbook (1991). These wall constructions are intended to minimize moisture accumulation, thereby preventing material degradation, mold and mildew growth, and loss in thermal performance. For the heating climate (northern United States) and mixed climate (central United States), all the wall constructions in the Moisture Control Handbook were found to perform satisfactorily. That is, when the surface relative humidities of the construction layers were plotted versus time of year, the peak relative humidities were always found to be within acceptable limits that preclude material degradation and mold and mildew growth. For the cooling climate (south-eastern United States), one of the walls had risk of mold and mildew growth behind an interior vapor retarder. During the summer, moisture from the outdoor environment diffused inwardly into this construction. Upon reaching the interior vapor retarder, moisture was significantly retarded and accumulated, thereby causing the surface relative humidity to rise above the critical 80% level for mold and mildew growth. An interesting finding was that moisture accumulated during the winter at exterior layers having low permeability, thereby giving rise to relative humidities above a critical level (80%). However, this moisture accumulation occurred at relatively low outdoor temperatures, which would slow mold and mildew growth. For a cooling climate, a permeable wall (i.e., without vapor retarding layers and lowperrneability materials) was found to perform satisfactorily. During both winter and summer periods, moisture passed through the construction and did not significantly accumulate within construction layers. In this report, a computer model, called MOIST, is used to investigate the performance of all the recommended wall constructions given in the Moisture Control Handbook. Program MOIST predicts the one-dimensioml transfer of heat and moisture in amulti-layer building construction under nonisothermal conditions.