Volume 6, Issue 10 (November 2009)
The Effect of Air Cavity Convection on the Wetting and Drying Behavior of Wood-Frame Walls Using a Multi-Physics Approach
The moisture performance of building envelope systems are strongly dependent on the materials used, the workmanship, and the exposure loads from the interior and exterior environments. The authors have long recognized the need to include the effects of exterior cladding ventilation in the predictive capability of software tools used for hygrothermal analysis. Exterior cladding ventilation has been studied, but no conclusive recommendations have been generated until recently (Burnett, E., Straube, J., and Karagiozis, A., “Synthesis Report and Guidelines,” ASHRAE TRP-1091 Report No. 12, Nov. 2004). While the physics describing the thermal and moisture transport in the presence of air convection is understood, the pressure dynamics is still somewhat qualitatively known. With the addition of new literature data and available field generated monitored data, a simplified model for the wall air cavity ventilation was developed. The scientific approach followed initially included the benchmarking of multi-dimensional advanced hygrothermal model with laboratory and field data. The flow was understood for a wide range of exterior loadings, and once this was completed, an attempt to reduce the complex three-dimensional air flow characteristics into a simple one-dimensional analogue was made. The paper describes how this important feature was included into the WUFI-4.1 software. The paper also describes how users may employ this feature in hygrothermal designs to investigate the advantages and disadvantages of cavity ventilation. Results are also presented on the hygrothermal performance of two walls, one ventilated and the other is unvented. Results show that major differences were predicted and the wall with the ventilation cavity dried out nearly five times faster than the wall without the ventilation. Field monitored stucco wall systems with and without cavity ventilation are also included compared to the prediction provided by the hygrothermal model. Good agreement is shown between the field and WUFI 4.1 model.