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Urban overheating is a serious environmental issue with a significant impact on cooling loads, peak electricity demand, indoor air temperature, health, pollution, sustainability, and the economy. Cool roofs – which are characterised by high solar reflectance (ρ) or albedo and high thermal emissivity (ε) – are one of the most effective technologies to mitigate the urban overheating effect and its undesirable impacts. The existing literature on the energy impacts of cool roofs in various building types and climate zones is vast but fragmented and predominantly assessed the building-scale and urban-scale benefits separately. Here, we computed the energy conservation potential of cool roofs in 17 building archetypes with different occupation hours, internal heat gains, thermal insulation levels, and building heights under various Australian climates. This study presents a holistic approach to assessing the magnitude and spatial variation of the cooling load savings, the peak electricity demand reductions, and indoor air temperature reductions generated by the building-scale and combined building-scale and urban-scale implementation of cool roofs in all representative building types under various climate conditions in Australia. In Sydney, a city with a warm/mild temperate climate, cool roofs can reduce the cooling loads of a typical low-rise office building without roof insulation by 10.2–13.8 kWh/m2 (37.6–42.0 %) during the summer months of January and February. When cool roofs are implemented in both individual buildings and the whole urban area, the cooling load conservation potential for the same building increases to 14.9–17.4 kWh/m2 (50.3–63.7 %). The corresponding peak electricity demand and indoor air temperature reductions by the combined building-scale and urban-scale application of cool roofs are 53–70 % and 11.2–12.0 °C, respectively. The energy conservation potential of cool roofs can be noticeable even for high-rise buildings with a high level of thermal insulation. We estimated that the cooling conservation potential by combined building-scale and urban-scale application of cool roofs can range between 4.0 and 7.1 kWh/m2 (29.3–48.3 %) in a new high-rise residential building with high levels of thermal insulation to 18.5–24.5 kWh/m2 (23–29.6 %) in an existing low-rise shopping mall centre with low levels of thermal insulation in Sydney. Cool roofs lead to additional savings as the reduced temperature increases the energy efficiency of air conditioning systems, with cooling load savings between 0.6 and 3.7 kWh/m2 (3–11 %) for a typical low-rise office building without roof insulation in Sydney. Also, the heating load penalties induced by cool roofs are negligible compared to the cooling load savings in all building types and Australian cities/climate zones except for non-commercial buildings in the cold climate of Hobart. Policymakers can use the results of this study to obtain a deep knowledge of the expected performance of cool roofs when applied city-wide to mitigate urban overheating and counterbalance its adverse impacts on energy uses and indoor thermal comfort. [ABSTRACT FROM AUTHOR]

Trees are considered to be effective for the mitigation of urban overheating, and the cooling capacity of trees mainly comes from two mechanisms: transpiration and shading. This study explores the transpiration cooling of large trees in urban environments where the sea breeze dominates the climate. In the experiment, sap flow sensors were used to measure the transpiration rate of two large trees located in Sydney over one year. Also, the temperature difference between the inside and outside of the canopy, as well as the vertical temperature distribution below the canopy, were measured during summer. In this experiment, the temperature under the canopies decreased by about 0.5 degrees from a 0.5 m height to a 3.5 m height, and the maximum temperature difference between the inside and outside of the canopy was about 2 degrees. After applying a principal component analysis of multiple variables, we found that when a strong sea breeze is the primary cooling mechanism, the sap flow still makes a considerable contribution to cooling. Further, the sea breeze and the transpiration cooling of trees are complementary. In conclusion, the characteristics of synoptic conditions must be fully considered when planting trees for mitigation purposes. Since the patterns of sea breeze and sap often do not coincide, the transpiration cooling of trees is still effective when the area is dominated by sea breeze. [ABSTRACT FROM AUTHOR]

• Daytime radiative cooling materials are studied in two cities with Urban Heat Island. • Contribution of wavelength band to the ability to achieve sub-ambient temperatures. • Materials studied under highly-insulated and highly-conductive conditions. • Material's behavior strongly depends on the location and the type of application. The objective of this paper is to present the potential of daytime radiative cooling materials as a strategy to mitigate the Urban Heat Island effect. To evaluate the cooling potential of daytime radiative cooling materials, 15 theoretical materials and seven existing materials were simulated: two radiative cooling materials, a coolmaterial, two white paints, a thermochromic paint and a construction material. The novelty of this study is that it shows that the optimal spectral characteristics of radiative cooling materials depending on the climate conditions and the type of application. A sensitivity analysis was performed to evaluate the impact of each wavelength emissivity on the ability to achieve sub-ambient radiative cooling. The sensitivity analysis comprised a total of 90 theoretical materials with 15 different wavelength combinations and 6 emissivity values. The heat transfer model, which includes conduction, convection, and radiation, was developed using a spectrally-selective sky model. Two conditions were considered: a very conductive surface and a highly insulated one. All the materials were simulated in two cities that suffer from the Urban Heat Island effect—Phoenix and Sydney. The mean surface temperature reduction achieved was 5.30 °C in Phoenix and 4.21 °C in Sydney. The results presented suggest that the type of application (active or passive) is a determinant factor in the design of radiative cooling materials. Modifying the spectra of the materials led to a substantial change in the cooling potential. A material that performs well in a dry climate as a passive solution could perform poorly as an active solution. [ABSTRACT FROM AUTHOR]