Evaluation of Moisture Diffusion Theories in Porous Materials

Abstract

Moisture transport in building materials is directly responsible for structural damage, as well as poor indoor air quality. For these reasons, the need to understand transfer mechanisms and predict moisture transport through building materials has increased over the last couple of decades. Although moisture diffusion phenomenon in the isothermal regime has been studied and explained extensively, there is no universally accepted model for predicting the moisture diffusion in a nonisothermal situation. Several diffusion models in the form of "Fick's Law" including ones based on gradients of water-vapor pressure, chemical potential of water, moisture concentration and activated moisture molecules have been proposed for predicting moisture diffusion through porous materials. However, the lack of reliable experimental results, resulting from the complexity of arranging accurate and repeatable measurement techniques and slow moisture movement, has prevented any model from being universally accepted. The present research addresses this modeling problem by evaluating current diffusion models through a series of experiments performed on oriented strand board (OSB), which is a wood-based material. The present experimental apparatus, developed over the last three years, was designed for the specific purpose of studying and developing an accurate method to measure moisture transfer properties in porous materials under nonisothermal conditions. The apparatus consists of a system of two environmental chambers capable of achieving a wide range of temperatures and relative humidities. Temperature and relative humidity can be independently controlled to within ±0.05°C and ±0.10 per cent R.H. of the set points. This apparatus is an alternative to the ASTM "cup method" which is limited to isothermal conditions and discrete relative humidities that correspond to those for various saturated salt-in-water solutions. Unlike the cup method, the relative humidity within the chambers is controlled by the direct removal and injection of distilled water. The system has forced recirculating flow which reduces the time to reach steady state. The new forced, direct control measurement procedure is denoted "ASHRAE FDC". The results obtained from the ASHRAE FDC experiments, show that moisture diffusion under nonisothermal conditions is governed by the gradient of the water-vapor pressure. The moisture transfer must cease when the diffusion potential is the same on both sides of the material for the validation of the diffusion model. The results show that the water-vapor pressure model meets this necessary and sufficient condition. Furthermore, a plot of the diffusion flux versus vapor-pressure difference was linear, within measurement uncertainty bounds. This observation infers that the permeability is approximately constant over the range of temperatures and humidities used in the investigation. During the ASHRAE FDC experimental procedure a small difference in the static pressure between the chambers was found. This pressure difference which was also observed in ASTM cup tests, is believed to be caused by concurrent air diffusion. The bulk flow of air governed by Darcy's equation balances the diffusion of air in the opposite direction as a result of the gradient in the partial pressure of (dry) air. The air permeability of an OSB specimen was measured and the results presented. The operation and accuracy of the apparatus was validated by comparing results from a series of isothermal tests to previously published results. The results obtained from the isothermal test allowed the permeability to be compared to results obtained from cup tests during the present investigation and to those previously published using the same method. Good agreement was found between the new data from both FDC and cup experiments and previously published results.