At Sedum the daytime peak heat flux was higher

At Sedum, the daytime peak heat flux was higher and occurred later than Control (Fig. 1c and e). The period with heat flux exceeding 40 Wm−2 was 6 h at Control vis-à-vis 12 h at Sedum, comparing with Peanut with only 0.5 h. Adding green roof to building roof furnished the coupled GBR system to enhance thermal mass effect [23] and [24]. The presence of succulent Sedum SR1078 and substrate, both endowed with moisture-holding capacity, increased the thermal capacity of the GBR system. The limited transpiration cooling on the hot summer day due to CAM photosynthetic physiology accentuated the warming effect. Sedum received and stored solar energy to generate an energetic liability in the form of a heat sink [49], which in turn created a thermal gradient and heat flow to creep towards indoor ceiling. The stored heat took a longer period to transmit downward in comparison with Control, with notable heat gain extending well into the evening.
Without BTI, the simple Sedum roof failed to provide a substitute thermal barrier. On the contrary, species permitted more heat to stream downward in both daytime and nighttime in comparison with Control. The relatively more complex Peanut roof furnished a partial thermal barrier which was effective in trimming daytime heat gain. This key function could be attributed mainly to active daytime C3 transpiration. However, in nighttime, it behaved similarly to Sedum to push heat indoor and dampen heat loss. It is noteworthy that both extensive green roofs could increase the thermal-mass effect of the GBR system and capture solar energy, a portion of which could shift indoor. The common belief that green roof can invariably bring interior cooling in the warm months may not always be realized. The notion that green roof installation can do away with BTI [50], partly to achieve cost saving, may not work for simple Sedum roof.