Empirical model for cavity ventilation and hygrothermal performance assessment of wood frame wall systems: Experimental study

• Published in: Building and environment 2019-06, Vol.157, p.112-126
• Main Authors: Tariku, Fitsum, Iffa, Emishaw
• Format: Article
• Language: English
• Subjects: Air gaps; Air temperature; Cavity ventilation; Construction; Drying potential; Empirical analysis; Empirical model; Hygrothermal performance; Model testing; Moisture content; Moisture control; Moisture performance; Performance assessment; Plywood; Rain; Rain screen wall; Regression analysis; Regression models; Relative humidity; Sheathing; Solar induced flow; Solar radiation; Studs; Temperature effects; Temperature gradients; Ventilated wall; Ventilation; Ventilation drying; Water content; Wind; Wind induced flow; Wind pressure; Wood
• ISSN: 0360-1323; 1873-684X
• DOI: 10.1016/j.buildenv.2019.04.020

In the last few decades, rain-screen wall systems have become common in building enclosure designs and construction for the purpose of mitigating moisture damage risk. Existing literature shows an air cavity in rain-screen wall facilitates moisture control performance. However, it is not clear whether it is just the presence of a capillary break created by the air gap or the cavity ventilation. It is a common practice to use a predefined constant cavity ventilation rate while modelling the thermal and hygrothermal performance assessments of rain-screen wall systems. However, cavity ventilation rates in ventilated wall systems vary with driving forces such as, wind and solar radiation. In this paper, three wall types, namely vented, ventilated, and wall system with no air gap, are manufactured and installed in two opposite orientations of a field-experimental test building. The air velocity, relative humidity and temperature in the cavity, and the moisture content and temperature in plywood and wood frame studs are monitored for 15 months. Based on the monitored data and analysis, a cavity ventilation regression model for the combined wind and solar induced flows is developed. The empirical model is presented as a function of hourly wind pressure and the temperature difference between the outdoor and cavity space. The empirical model is tested with the whole measured data and a good fit (R2 value of 0.912) is achieved between the model and measurement. The long-term hygrothermal performance analysis shows that the wall with no air gap accumulates relatively high moisture content and also has high moisture content changes in a year cycle when compared to the vented and ventilated wall systems. The moisture control performance of vented and ventilated wall systems is comparable thus emphasizing the important parameter for enhanced hygrothermal performance is having a cavity vent not the type of ventilation design (vented or ventilated). The temperature readings of the sheathing boards in the vented and ventilated wall systems are somewhat warmer than that of the wall with no air gap for 85.5% and 73% of the experimental measurement time, respectively. •Long-term field-experimental measurements of air flow in rain-screen wall systems is presented.•Cavity pressure and ventilation rates in rain screen wall system are characterized for different seasons.•An empirical cavity ventilation model is developed for hourly hygrothermal and energy performance analysis.•Hy