Measurements of Moisture Transport in Wood-Based Materials under Isothermal and Nonisothermal Conditions PDF

Building envelopes in various climates are exposed to a broad range of temperature and moisture conditions. Moisture transfer across the building envelope by diffusion in the hygroscopic range usually involves a thermal gradient as well as a water vapor pressure gradient. While many studies using common building materials have explored the dependence of water vapor permeability on relative humidity (RH), very few measurements have been made to characterize the effect of temperature under isothermal conditions or the effect of a thermal gradient. These effects may be of special importance in wood-based building materials. In our experiments, water vapor permeance is measured by sealing a material between two chambers that are independently controlled; temperature, RH, and the rate of mass transfer are measured on both sides of the specimen. Results are reported for 11-mm-thick plywood at room temperature and compared with other isothermal permeability data from the literature. Three experiments with conditions of nearly constant water vapor pressure in the presence of a thermal gradient show small moisture flows from high RH to low RH (from the cold side to the warm side) that might have been caused by water vapor pressure gradients within the experimental error. These result are discussed in the context of similar experiments in the literature.

Presented at Thermal Performance of Exterior Envelopes of Whole Buildings X – December 2007

Units: SI