Air-conditioning energy consumption due to green roofs with different building thermal insulation

On hot days, green roofs could reduce heat flux into indoor space and air-conditioning energy use. Most thermal-benefit studies estimate energy saving based on temperature measurements. A field experiment on the roofs of two residential buildings in subtropical Hong Kong was designed to measure air-conditioning electricity consumption in relation to three factors: (1) building thermal insulation (BTI): omitted at Block 1 and installed at Block 2; (2) green-roof type: each block had a bare (Control) and two extensive green-roof plots, namely simple Sedum and more complex herbaceous Peanut vegetation; and (3) three summer weather scenarios: sunny, cloudy, and rainy. Air-conditioning electricity consumption of six vacant apartments below the experimental plots was monitored by precision energy loggers. Under all weather conditions, the unshielded Control imposes high cooling load at Block 1, but BTI at Block 2 cuts heat ingress. Sedum reduces more energy consumption than Control at both blocks, with Block 2 better than Block 1. The best effect occurs on sunny day, followed by cloudy and rainy. Sedum roof with BTI enhances thermal benefit. Without BTI, Sedum roof consumes more energy, hence the simple green roof cannot substitute BTI function. Under three weather scenarios, Peanut uses more electricity at Block 2 than Block 1, indicating the joint operation of green-roof heat-sink effect (GHE) and building heat-sink effect (BHE) at Block 2. Thicker substrate with higher moisture-holding capacity generates GHE. Added BTI material layers create BHE, with thermal resistance reduced by moisture penetration and elevated temperature. Their joint effect has raised thermal mass and thermal capacity. A rather steep thermal gradient is formed to induce thermal-insulation breaching to push heat into indoor space. At Block 1, Peanut roof can partly compensate for omission of BTI. At Block 2, however, Peanut coupled with BTI can synergistically increase cooling load. The findings can inform policies and design of green roof and associated BTI in cities with hot summer.