2.2. Green roof installation and experimental design At Block 1, roof BTI layers were omitted to emulate a poorly-insulated building (Fig. 1). At Block 2, normal BTI layers were installed. Both blocks contain normal waterproof membrane resting on a screed layer placed on reinforced concrete slab. The thermal insulation and waterproof design and materials are commonly adopted in Hong Kong. All apartments have main windows facing southeast. To standardize their environmental conditions,small side windows of the terminal unit at each block were shielded by thermal-insulation gypsum boards. Windows and doors were kept closed in the study period to minimize external influence on temperature.Identical experimental designs were adopted at both blocks.Each block’s roof site of about 85 m2 is divided into three roughly equal plots, which correspond to the three apartments lying under-neath (Fig. 2). Plot A (control plot) was left bare to serve as the baseline. Plots B and C were assigned to two green roof types. A proprietary green-roof system (Nophadra in, Kirkrade, The Nether lands) conforming to stringent German specifications (FLL,2008) was installed. The sequence of its multiple materials layers is shown in Fig. 2.