b'TECHNICAL ARTICLECAVITY VENTILATION MODELfor Hygrothermal and Energy Performance Analysis of Rain-Screen WallsBy Fitsum Tariku, BCIT Building Science Centre of ExcellenceThis article is a continuation of the discussion presented in An Cavity velocity (m/s)Experimental Study: Hygrothermal performance of vented and ventilated wall systems, an article featured in the Fall/Winter 2020 issue of BCBEC Elements magazine. Readers are referredto the pervious edition for the experimental setup and measurement methods.I n comparison to air and heat transfer, moisture transfer isFIGURE 2: SOLAR-INDUCED CAVITY AIRFLOW IN THE SUMMER, FALL, WINTER a slow phenomena, and use of an average cavity ventilation rate (assumed based on experience or measurements) can be acceptable. However, for transient hourly energy calculation or BIPV cooling analysis, estimation of dynamic airflow ratesAND SPRING SEASONS.in relation to the driving forces (solar and wind) are impor-tant. In this section, an easy-to-use empirical model for estima- Figure 2 presents the solar-driven cavity air flow during the tion of cavity airflow in a ventilated wall system is presentedsummer, fall, winter and spring seasons. The solar effect is repre-based on year-long filed measurements. Figure 1 shows the solarsented by the temperature rise of the air cavity due to solar gain, radiation and the wind pressure on the rain-screen walls wheni.e. the difference between the outdoor incoming air and the airflow in the ventilated cavity is detected. In this figure, the testcavity air (DT). As shown in this figure, the temperature differ-walls on the southeast and northwest orientations are designatedence between the cavity and outside air can be as high asas F3 and F4, respectively. As highlighted in the black and green24 C in all seasons except winter, where the maximum DT is boxes, the majority of cavity ventilations transpire in response to18 C. As DT increases, the cavity ventilation measurements get solar radiation and during calm wind conditions (where surfacemore scattered due to outdoor temperature variations. As such, wind pressure is between -0.5 and 0.5 Pa). The measurement alsoin addition to DT, outdoor temperature is considered an addi-shows that wind-induced cavity ventilations mainly occur whentional independent variable in the regression model (Equation solar radiation is low (as highlighted in the green box). As such,1) developed for a solar-induced cavity ventilation. The second it is possible to relate cavity ventilation with the respective domi- order polynomial regression model, which is developed using nant driving forces separatelysolar and wind pressure using thethe MATLAB Statistics Toolbox (2017), fits the measured data data in the black and the green boxes, respectivelyand comewell with an R2 of 0.995. Multiplying average air velocity, which up with a unified cavity ventilation model. Regression models foris 2/3 of the measured velocity (at mid-depth) by 3,600 and solar- and wind-induced cavity ventilations, along with the finaldividing by the cavity height yields cavity air flow in air-exchange empirical cavity ventilation model, are presented below. per hour (ACH). FIGURE 1: SOLAR RADIATION AND SURFACE WIND PRESSURE ON THE SOUTHEAST (F3) AND NORTHWEST (F4) TEST WALLS DURING THE ONE-YEAR EXPERIMENTAL PERIOD (BASED ON HOURLY AVERAGE SOLAR AND WIND PRESSURE RECORDINGS).10 BCBEC ELEMENTSA BCBEC PUBLICATION'