Drying capabilities of wood frame walls with wood siding

The hygrothermal performance of wood siding and exterior sheathing and the need of cavity ventilation for wood frame wall systems has been investigated. The aim of this preliminary study was to investigate various design strategies to improve the drying performance and capabilities of wood frame wall systems. A moisture engineering approach was undertaken by conducting a combination of laboratory experiments and advanced computer simulations. The intent of the work was to investigate the heat and moisture performance as affected by variations in the wall design. Several different walls systems with wood siding were constructed that included different material layers as exterior sheathing, different insulation materials, and wall systems that incorporated air cavity ventilation and others that did not incorporate a cavity between the wood siding and the exterior sheathing. Laboratory experiments were carried out to examine the hygrothermal performance of the walls exposed to different exterior and interior boundary conditions. The drying capabilities of the walls and their ability to recover from moisture loads caused by vapor convection and diffusion were investigated. The information generated from the laboratory experiments was subsequently analyzed by advanced computer modeling, and additional simulations were performed to determine the response of various wall systems using realistic environmental conditions. The experiments were also numerically simulated for the same laboratory conditions. In general, when comparing the numerical and experimental results, good agreement was observed, both indicating similar trends in the hygrothermal behavior. But at the same time, some anomalous results were also found that were initially believed to be due to anomalies within details of the wall structures but were later found to be caused by spurious values in the prescribed boundary conditions. The different requirements due to actual climatic boundary conditions were addressed by selecting exterior data from a cold climate. The analysis developed preliminary information in terms of guidelines and practices for acceptable thermal and moisture performance of wood frame walls. The hygrothermal performance of exterior sheathing materials, their effect on wall moisture performance, and the ability of the structure to dry out moisture from possible leaks (or initial construction moisture) need further research to establish guidelines applicable for a wider range of climates. Even today, many examples of moisture-related problems exist in the literature, some of which have been attributed to improper design of the cladding system. This paper attempts to shed some light on the issues and concerns of the drying performance of wood frame wall systems exposed to cold climates.