Heat-sink effect and indoor warming imposed by tropical extensive green roof
- The study evaluates differential cooling effect of C3 and CAM herbs on tropical green roof.
- Holistic outdoor-substrate-indoor vertical profile is monitored by temperature sensors.
- Sedum plant surface is heated to high daytime temperature due to CAM stomata closure.
- Sedum and C3 Perennial Peanut plots create green-roof heat-sink effect to warm indoor air.
- Tropical extensive green roof induces positive heat flux to raise indoor cooling load.
The study evaluates diurnal cooling effect of two herbs on tropical green roofs, C3 broadleaved Perennial Peanut (Arachis pintoi) and CAM succulent Mexican Sedum (Sedum mexicanum) with contrasting photosynthesis-transpiration physiology. The holistic outdoor-substrate-indoor vertical temperature profile is evaluated. Control, Sedum and Peanut experimental plots were established on a residential building in Hong Kong, each equipped with temperature sensors at 7–11 vertical positions. On summer sunny day, Control plot displays conspicuous daytime heating at roof surface and 15 cm and 150 cm above it. Indoor air and ceiling temperatures with <2 °C diurnal range indicates effective dampening by building heat-sink effect. Sedum roof displays a vertical thermal sequence: 15 cm > soil > Sedum surface > drainage > 150 cm > tile. Sedum surface is heated to a maximum of 35.4 °C, merely 1–2 °C lower than adjacent soil and 15 cm air, indicating daytime CAM stomata closure to restrict transpiration cooling. Contrary to expectation, sensible heat stored in Sedum roof generates green-roof heat-sink effect (GHE), driving downward heat flux throughout the day to raise indoor temperature by 1–2 °C. Peanut plot's vertical thermal sequence is: 150 cm > 15 cm > soil > rockwool > Peanut surface > drainage > tile. Peanut surface is significantly cooled by C3 transpiration to 28.8 °C, and it remains the coolest among outdoor positions throughout the day. Slightly less heat than Sedum is fluxed downwards to raise indoor temperature. Cooling due to effective C3 transpiration and thicker vegetation and substrate layers is offset by more intensified GHE. The green roofs demonstrate opposite proximal thermal impact which is limited to near-ground (15 cm) air, with warming at Sedum but cooling at Peanut. Both vegetated plots show less heat ingress into indoor space in daytime, but more in nighttime. On summer cloudy and rainy days, both green roofs with GHE brought more heat flux to indoor space throughout the day. Contrary to findings outside the tropics, the tropical extensive green roofs cannot bring net cooling to indoor environment in summer.
- C3 and CAM herbaceous plants;
- Cooling rate sequence;
- Green-roof heat-sink effect;
- Proximal thermal impact;
- Time-lag heating and cooling sequence;
- Tropical green roof
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