b'TECHNICAL ARTICLEover 70 per cent of the time. Figure 3 shows the probability distribution of the measured cavity ventilations for the two wall systems. In the ventilated wall system, the cavity ventilation varies from 0 to 650 ACH, and 75 per cent of the time it is higher than 100 ACH. In the case of vented wall system, the cavity ventilation varies from 0 to 160 ACH, and 85 per cent of the time it is below 100 ACH. The average cavity velocities for the ventilated and vented walls are 0.18 m/s and 0.07 m/s, respectively. If one must use a constant cavity ventilation rate in hygro-thermal simulations, 15 ACH to 25 ACH for vented walls and 100 to 120 ACH for ventilated wall systems can be appropriate values. These estimations take into account the fact that there is no airflow in the vented wall and ventilated wall systems for 80 perFIGURE 4: CAVITY VENTILATION IN VENTED AND VENTILATED WALLS.cent and 30 per cent of the time, respec-tively. A typical airflow pattern in the two wall cavities during a sunny week in fall is shown in Figure 4. The cavity ventilation in the ventilated wall is about five times higher than that of the vented wall system. Figure 4 also shows the direct correlation between the cavity ventilations and the solar radiation on the test faades.Figure 5 (pg. 17) shows the moisture contents readings of the three walls during the 15-month experimental period. As the figure shows, the wall with no air gap accumulates relatively high moisture content on the sheathing board when compared to the vented and ventilated wall systems. In general, the hygrothermal responses of the vented and the ventilated wall systems are comparable, but with some slight differences. When compared to that of theFIGURE 6A: EFFECT OF CAVITY VENTILATION ON PLYWOOD TEMPERATURES ON COLD SUNNY DAYS.vented wall system, the moisture content of the plywood in the ventilated wall is slightly lower during the winter period and higher during the fall and the spring seasons. The slightly higher moisture content in the ventilated wall during the fall and the spring is attributed to the enhanced airflow (cavity ventilation) that brings in moist outdoor air into the air cavity and the night sky radiant cooling effect. From a moisture management perspective, its a capillary break, not a higher cavity ventilation, that yields better moisture performance.Figure 6 shows the plywood temperatures in the three test walls during typical cold and hot days. In general, the plywood temperature in the wall with no air gap is much higher than that of the vented and the ventilated wall systems, both in sunny coldFIGURE 6B: EFFECT OF CAVITY VENTILATION ON PLYWOOD TEMPERATURES ON HOT SUMMER DAYS.16 BCBEC ELEMENTSA BCBEC PUBLICATION'