Hygrothermal Modeling of Moisture Accumulation in Composite Roof Decks in High Humidity Environments such as Natatoriums in Cold Climates

摘要

High humidity and corrosive indoor environments in natatoriums in cold climates pose special risks and challenges due to moisture condensation in roofs. Most natatoriums are also echo chambers and therefore require acoustical roofs to minimize reverberation time. Perforated metal decks with acoustical insulations are commonly used as roofs. Without the existence of standards for roof design, based on a simple, steady-state dew-point analysis, the common practice is to employ at least a 0.1 perm vapor retarder on the warm side of the assembly to eliminate the risk of condensation presumably. Nevertheless, rust and corrosion of the metal roofs are a common occurrence. The use of the vapor retarder in such roof assemblies in preventing condensation is not well understood. This study investigates the role of a vapor retarder in a roof assembly for natatoriums by hygrothermal modeling in Climate Zone 7 higher using a well-known hygrothermal simulation model (WUFI). An alternate wood composite roof deck (K-value of＞ 35 hr.ft~2.℉/BTU) that provides acoustics, thermal insulation, and a nailable surface such as oriented stranded board (OSB), with and without a vapor retarder, at different locations in the assembly is used as a roofing candidate for the simulation. The moisture accumulation in the OSB board over the years of exposure is used as an indicator of the roof s performance. In addition, the presence of a dew point to verify the occurrence of condensation in the assembly is monitored. The simulation results show that even without a vapor retarder: 1) The moisture content in the OSB reaches a dynamic steady state in 3-5 years. 2) Maximum moisture content in the OSB does not exceed more than 11.6 (＞20 is considered "risky"). 3) There is no risk of condensation in the assembly at any point in time. 4) With the placement of a vapor retarder on the warm side or close to the OSB, the maximum moisture content is reduced to 11.2. On the other hand, because the vapor retarder also acts as a retarder for drying, condensation is highly likely to occur inside the assembly. The primary reason for these observations is the "self-drying " mechanism due to the influence of the cyclical outside surface temperature as the roof experiences day / night and winter/ summer conditions. 'These observations contradict the simple dew point analysis model that is commonly used to prescribe a vapor retarder for the roof assembly and question the need for one in the roof assemblies.