b'THERMAL PERFORMANCEIMPACT OF CAVITY WIDTH ON EXTERIOR WALLS THERMAL PERFORMANCE A study from BCITBy Fitsum TarikuR ain-screen wall assembliestemperature and air velocity fields inprofiles observed, wider cavities have are commonly imple- the air cavity. Figure 1 shows the typicallower temperature gradients both hori-mented for their enhancedvelocity and temperature profiles inzontally and vertically. For example, moisture performance. Thewalls with air gaps ranging from 19the incoming air in the 19-mm air associated cavity ventilationmm to 127 mm during a summercavity increases by 10.5C as it leaves in these wall assemblies day in Vancouver. As can be seen inthe cavity. In contrast, a temperature also has the potential to enhanceFigure 1 (a), the velocity profile in theincrease of only 2.1C is observed in the their thermal performance during the13-mm air cavity is a parabola and127-mm air gap. Following the airflow summer period by reducing heat gainsymmetrical along the cavity centreline.and temperature patterns, the sheathing to the building. One of the factors thatAs the cavity width increases, however,board temperature in general is lower affects cavity ventilation and its impactthe peak velocity shifts to the clad- as cavity width increases (Figure 1 (c)), on thermal performance is air cavityding (more heated surface) and itswhich translates to a reduction in the width. Identification and incorporationmagnitudes decrease, yielding a velocityinward heat transfer. of appropriate cavity width in exteriorprofile that is skewed to the left. The0.7 (a) 1.0 (b)wall design contributes to buildingssimulation results suggest that as the1 Girma, G. and Tariku, F. (2021). Experimental 0.6meeting current, stringent energy- air cavity width increases, the airflowinvestigation of cavity air gap depth for0.8Velocity (m/s) Dimensionless height, y/H [-]efficiency codes. Here, the relationshippattern changes from boundary layerenhanced thermal performance of ventilated 0.5rain-screen walls. Building and Environment,0.6between cavity width and airflow andflow to an isolated plate flow. The0.4V lume 194, 107710, ISSN 0360-1323.its impact on the thermal performanceincreased volume flow associated withoN3-19 mm20.ghana, B., Tariku, F. and Bitsuamlak, of exterior walls are presented basedthe isolated plate flow leads the cavity -19 mm 0.4-25 mm1 G. (2021). Assessing ventilation cavity on field experimentaland computa- air temperature to resemble the outdoor0.2-25 mm-50 mmdesign impact on the energy performance-50 mmtional fluid dynamics (CFD)2 studiesair temperature. Figure 1 (b) shows theof rain-screen wall assemblies: A CFD0.2-76 mm0.1-76 mmconducted at BCIT. CFD allows forair temperature along the centreline ofstudy. Building and Environment, Volume -127 mmbetter insight and visualization ofthe cavities. In line with the velocity0-127 mm 0.0196, 107789, ISSN 0360-1323.0 0.2 0.4 0.6 0.8 1 28 30 32 34 36 38 40 4246 48 50 5244Dimensionless air gap width (-) Temperature, 0.07.7 (a()a) 1.10.0 (b(b) ) 50.0 (c)0.06.6 45.00.08.8 40.0Velocity (m/s) Velocity (m/s) Dimensionless height, y/H [-] Dimensionless height, y/H [-] Temperature ()0.05.50.06.6 35.00.04.430.00.3- 10.3- -19 mm 0.40.4 -91 9m mmm19 mm- 2 25.0 -25m mmm -52 5m mmm0.02.2-25 05 0m mmm 20.0 PwTcMC:-13 mm - 5 5 PwTcMC:-19 mm-05 0m mmm 0.20.2- -76 mm PwTcMC:-25 mm0.01.1- 7 76 mm 15.0-67 6m mmm- 1 PwTcMC:-50 mm - 1 -2172 7m mmm-2172 7m mmm0 0 0 0 0.02.2 0.04.4 0.06.6 0.08.8 1 1 0.0.0 TeTmempepreartautruer,e 10.0 0:00 5:45 11:31 17:16 23:022828303032323434363638384040424244444646484850505252DiDmimenesniosinolnelsess as iar igr agpa pw widitdht h(- )(-) ,Time (hr)FIGURE 1: THE EFFECT OF VARYING THE AIR GAP WIDTH ON THE (A) VELOCITY PROFILE ACROSS THE CAVITY, (B) TEMPERATURE VARIATION ALONG 505.0.0 Figure 1. Heat flow through exterior walls of different cavity widths during a summer week. (c()c) THE CAVITY CENTRELINE AND (C) SHEATHING TEMPERATURE WITH HEIGHT IN A TYPICAL HOT DAY IN VANCOUVER CASE STUDY.454.50.040.040.0 FALL/WINTER 2022 17Temperature () Temperature ()353.50.0303.0.0252.50.0PwPwTcTMcMC:C : - 1-31 3m mmm20.0 PwPwTcTMcMC:C : - 120.0-91 9m mmmPwPwTcTMcMC:C : - 215.0 -52 5m mmm15.0 PwPwTcTMcMC:C : - 5-05 m0 mmm101.0.00:0:000 5:54:545 111:13:131 171:71:616 232:30:202TimeTime (hr) (hr)FFigiguurere 1 1. .H Heaeta tf lfoloww t hthrorouugghh e exxteteriroior rw waalllsl so of fd dififfeferernent tc acvavitiyty w wididthths sd duurirningg a a s usummmmeer rw weekek. .'