b'GREEN ROOF SYSTEMSRAINWATER RETENTION AND RUN-OFF THERMAL PERFORMANCEAs shown in Figure 2, the moisture content in the growingOn a sunny, summer day on August 19, the temperature media of the green roofs fluctuates between 10 per cent andat the growing media middle layer of EPS is 6C and 7C 35 per cent. During wet periods, above 35 per cent growinghigher than GR and RES, respectively. The high temperature media moisture content is observed for all the green roofs andincreases the growing media drying rates. However, when the the rain runoffs are prominent. Higher moisture content fluc- outdoor temperature (To) declines, Figure 4, the tempera-tuation at the growing media of EPS during the rainy monthsture at the growing media middle layer of EPS is 1C and can be associated with the EPS insulation, which absorbs2C lower than GR and RES, respectively. In the winter, the water four per cent by volume and reduce growing mediagrowing media in the EPS is relatively colder.moisture content, and the air spaces created by the groves and holes may also increase air convection and drying of the growing media. In summer, the rainwater retention capacity of the green roofs increases as high as 100 per cent.(A)FIGURE 2DAILY AVERAGE MOISTURE CONTENT AT THE GROWING MEDIA MIDDLE LAYERS OF THE GREEN ROOFS AND DAILY TOTAL RUNOFF FROM SEPTEMBER TO JULY.(B)RES delays the rain runoff compared to other systems, as the demonstrated with the rain event on January 16. Prior to the rainfall event on January 16, the moisture content at the growing media bottom layer of EPS is 34 per cent while the moisture content at GR and RES is 10-per-cent higher than EPS, Figure 3. The rainfall on January 16 increases the growing media moisture content of all the green roof systems but caused runoff from GR and EPS only. RES delays the rain runoff for 27 hours, which can be due to the RES reservoir until its filled. The experimental study also found that the water storage feature of RES to store and release moisture to the growing media in spring and summer to be insignificant.FIGURE 4TEMPERATURES AT THE GROWING MEDIA MIDDLE LAYERS OF THE GREEN ROOFS DURING TYPICAL DAYS IN (A) SUMMER AND (B) WINTER, AND THE CORRESPONDING OUTDOOR TEMPERATURE (TO). On a sunny, summer August 19, the sheathing board temperatures of GR and EPS reach a maximum value of 31C while the sheathing board temperature of RES is 3C lower, Figure 5(a). Although the EPS drainage has a higher thermal resistance, the elevated temperature it created in the growing media increased the downward heat flow and kept the sheathing board warm. In winter, the sheathing board temperature of EPS is up to 4C and 1.5C higher than the FIGURE 3. HOURLY MOISTURE CONTENT AT THE GROWING MEDIA BOTTOM LAYERSGR and RES, Figure 5(b); thereby, EPS reduces the risk of OF THE GREEN ROOFS AND RUNOFF FROM JANUARY 16 TO 22. condensation when compared to GR and RES. 18 BCBEC ELEMENTSA BCBEC PUBLICATION'