Black Roof Research Articles

Sustainability of cool and black roofs with high initial construction moisture

This study examines the sustainability of roofs subjected to hot climate of Phoenix and cold climate of Montréal by using a validated numerical model that simultaneously solves the heat, air, and moisture equations. In many situations, the initial conditions needed to solve these equations are not available. This paper provides a procedure for conducting energy and moisture modelling so that using different initial conditions would result in no effect on the final results of roof performance. Additionally, the study focuses on: (a) assessing the moisture performance of black and cool roofs in case of using material layer with high initial construction moisture content (Fibreboard, FB), (b) identifying the time period needed so that the moisture content would reach acceptable limit by the building codes, and (c) determining the amount of energy savings as a result of using sustainable cool roofs instead of black roofs. The results show that the moisture content in the FB decreases to permissible levels after 92.6 days for black roof and 175.7 days for cool roof in Phoenix, while in Montréal, it takes 238.6 days and 1597 days, respectively. The effect of using different initial condition vanishes after 196 days for Phoenix’s black roof, 583 days for its cool roof, and 1247 days for Montréal’s black roof and 4590 days for its cool roof. Mold is detected in FB. In Phoenix, mold disappears after 379 days for black roof and 479 days for cool roof, whereas in Montréal, it takes 708 days and 3143 days, respectively.

Hygrothermal Performance of Roofs with High Initial Construction Moisture Subjected to Hot Climate

Moisture accumulation in the building components/assemblies that form building envelopes can lead to material deterioration and moisture related issues such as mould growth. As a part of the building envelope, this study focusses on assessing the moisture performance and energy performance (i.e., hygrothermal performance) of roofing systems. As roofs can be built with high initial construction moisture, numerical simulations were conducted with and without high initial construction moisture in order to investigate: (a) the hygrothermal performance of cool and black roofs having material layer with high initial construction moisture content, (b) the time needed so that the moisture content reaches acceptable level as per the building code requirements, (c) whether moisture accumulation and mould growth occur in the roofs, and (d) the energy savings as a result of installing white/cool roof instead of black roof. An advanced numerical model is used to conduct the numerical simulations for black and cool roofs when they are subjected to hot climate. This model solves simultaneously the Heat, Air and Moisture (HAM) transport equations in all layers of the building assemblies. The model was extensively validated by comparing its predictions with the experimental data of different building components at various operating conditions. For the roofing systems investigated in this paper, the results showed that mould growth occurred in the black and cool roofs only for the case with high initial construction moisture. The mould has totally disappeared after 378.8 day for the black roof and 479.3 day for the cool roof. The temperatures of the cool roof were much lower than those for the black roof. The total yearly energy load with the black roof was 77% greater than that with the cool roof.

Understanding the hydrological performance of green and grey roofs during winter in cold climate regions

Green and grey roofs have emerged as promising and sustainable measures for effectively managing stormwater in urban catchments. However, there is a gap in the literature in understanding and modelling the hydrological performance of these roofs during winter and snow-covered periods in cold climate regions. The present study attempted to address this gap by validating the use of a snow module in simulating the dynamics of snow accumulation and melting of green and grey roofs. Then, the validated model was used to identify and separate the different events that occur in winter (melt only, rainfall only, rain-on-snow) to assess the hydrological performance of six different configurations of green and grey roofs in Trondheim, Norway. The snow module accurately simulated snow accumulation and melting of green and grey roofs. The results showed that rain-on-snow events in winter have longer duration compared to other events including rainfall events in summer. Consequently, rain-on-snow events yield a higher amount of inflow to the roofs compared to rainfall events in summer, despite summer events having higher intensities. The retention and detention performances of green and grey roofs were found to be lowest for rain-on-snow events compared to other types of events, but still yielding significantly lower peak runoffs when compared to standard black roofs. The decrease in retention and detention performances in winter were attributed to the long duration of events, accumulation effect of snow, freezing of roof surface layers, and reduction of evapotranspiration. The study highlights the importance of considering winter conditions in the design of green and grey roofs in cold climates to enhance stormwater management.

Many Faces of Mediation

Many Faces of Mediation

SUN HATS FOR BAT BOXES: MITIGATING THE RISK OF OVERHEATING AT NORTHERN LATITUDES

At northern latitudes, bats often use roosting structures provided by people. Conventional thinking suggests that bat boxes should be entirely black to absorb heat and assist bats with thermoregulation. However, with long periods of summer daylight in subarctic Canada, we suspected that risk of overheating might occur in black boxes. We investigated whether roost temperatures exceeded 42°C, the upper limit of the thermoneutral zone in Little Brown Myotis (Myotis lucifugus), and whether replacing the black roof with a white one, akin to a sun hat, would alleviate the risk of overheating without compromising roost temperatures especially at night. We also investigated whether bats used the boxes. The internal temperature of black boxes exceeded 42°C on some days in 2 summers. Substituting the black roof with a white one consistently reduced the maximum daily roost temperature to below 42°C, but this was also accompanied by a slightly lower minimum roost temperature at night and a cooler roost temperature regime overall. The reduction in maximum daily roost temperature by the white roof was more pronounced on days with higher maximum daily ambient temperatures. At 5 sites with paired white-roof and black-roof boxes, 4 had bat use of both boxes and 1 had no use of either box. Bats used 10 of 19 (53%) black-roof boxes in 2019, and 16 of 21 (77%) in 2020. Replacing a black roof with a white one can mitigate risk of overheating for boxes in subarctic latitudes, and likely through a range of temperate latitudes. However, completely black boxes are still necessary to enhance the thermal environment for roosting bats through most of the summer. We suggest that providing bats with roosting options, such as paired black-roof and white-roof boxes immediately adjacent to one another, is a better habitat enhancement option than just single black boxes.

“Cool” Roofs as a Heat-Mitigation Measure in Urban Heat Islands: A Comparative Analysis Using Sentinel 2 and Landsat Data

Urban growth, characterized by expansion of impervious at the cost of the natural landscape, causes warming and heat-related distress. Specifically, an increase in the number of buildings within an urban landscape causes intensification of heat islands, necessitating promotion of cool roofs to mitigate Urban Heat Islands (UHI) and associated impacts. In this study, we used the freely available Sentinel 2 and Landsat 8 data to determine the study area’s Land Use Land Covers (LULCs), roof colours and Land Surface Temperature (LST) at a 10-m spatial resolution. Support Vector Machines (SVM) classification algorithm was adopted to derive the study area’s roof colours and proximal LULCs, and the Transformed Divergence Separability Index (TDSI) based on Jeffries Mathussitta distance analysis was used to determine the variability in LULCs and roof colours. To effectively relate the Landsat 8 thermal characteristics to the LULCs and roof colours, the Gram–Schmidt technique was used to pan-sharpen the 30-m Landsat 8 image data to 10 m. Results show that Sentinel 2 mapped LULCs with over 75% accuracy. Pan-sharpening the 30-m-resolution thermal data to 10 m improved the spatial resolution and quality of the Land Surface map and the correlation between LST and Normalized Difference Vegetation Index (NDVI) used as proxy for LULC. Green-colour roofs were the warmest, followed by red roofs, while blue roofs were the coolest. Generally, black roofs in the study area were cool. The study recommends the need to incorporate other roofing properties, such as shape, and further split the colours into different shades. Furthermore, the study recommends the use of very high spatial resolution data to determine roof colour and their respective properties; these include data derived from sensors mounted on aerial platforms such as drones and aircraft. The study concludes that with appropriate analytical techniques, freely available image data can be integrated to determine the implication of roof colouring on urban thermal characteristics, useful for mitigating the effects of Urban Heat Islands and climate change.

Mitigating and adapting to climate change with a taxonomy of smart urban surfaces

Rapid urbanization is replacing natural land with dark, impervious surfaces. This has led to dire urban consequences including rising temperatures and stormwater deluge, resulting in significantly higher energy costs, greater stormwater damage, and associated health and comfort impacts. These issues can be mitigated using smart surfaces, those with high reflectivity and permeability, which can achieve sustainable and regenerative cities. The current literature on the benefits of urban surfaces is very segmented, focusing on either one specific surface type or one property of surfaces. A smart surface taxonomy with correlated heat, and water metrics has been developed to fill this gap. A range of city surfaces in three broad categories - roofs, streets and sidewalks, and parking lots - have been identified with various levels of reflectivity, permeability. Through literature review, the taxonomy reveals surface temperatures that range from 29.7°C for a green roof to 74.3°C for a black roof. Also, the taxonomy reveals Rainfall retention potential ranging from 1.27 mm for impervious pavement to 86.4 mm for bioswales. The development of a smart surface taxonomy with quantified benefits for mitigating or adapting to climate change will be critical for decision-makers to make informed decisions on city surface choices.

Hygrothermal performance of cool roofs with reflective coating material subjected to hot, humid and dusty climate

The measurements for the short-wave solar reflectivity of a Reflective Coating Material (RCM) with various cleaning operations that were obtained in a previous study were used in this study to conduct numerical simulations in order to assess the moisture and energy performance of cool and black roofs when they were subjected to the weather conditions of Saudi Eastern Province and Kuwait City. The results of the numerical simulations showed that black roofs always work with less moisture than cool roofs. Because the highest relative humidity in the different components of the black and cool roofs was well below 80%, there was no risk of condensation and mold growth in these roofs. For both weather conditions of Saudi Eastern Province and Kuwait City, the results showed that installing cool roofs have resulted in increasing the heating energy loads in relation to black roofs. Conversely, the results showed that the decrease in the cooling energy loads due to installing cool roofs were typically much greater than the increase in the heating energy loads. As such, cool roofs have resulted in net energy savings in relation to black roofs. Replacing black roof by cool roof in Saudi climate, the results showed that the annual energy savings in the total energy load was 25% and 34% as a result of installing cool roof with RCM at no cleaning and weekly homemade cleaning, respectively. Additionally, replacing black roof by cool roof in Kuwaiti climate, the results showed that the annual energy savings in the total energy load was 23% and 31% a result of installing cool roof with RCM at no cleaning and weekly homemade cleaning, respectively.

The thermal performance of a green roof on a highly insulated building in a sub-arctic climate

Green roofs are complex systems, with a vegetation layer covering the outermost surface of the building shell. An effective design may confer environmental and energy benefits. Most field studies evaluating green roof performance have been conducted in warmer climates with few studies of full-scale green roofs in cold regions. No study has so far evaluated the energy performance of a green roof in a sub-arctic climate. This study demonstrates the heat flow and thermal effect of an extensive green roof versus a black bare roof area on a highly insulated building in the sub-arctic town of Kiruna, Sweden, for the period from November 2016 to February 2018. Measured temperature and heat flux values were consistently higher and more variable for the black roof than the green roof, except during the snow-covered winter months when the responses were similar. The cumulative heat flux showed that the net heat loss was greater through the black than the green roof, but the values remained low. Overall, the study confirms that the energy benefit of a green roof on a highly insulated building in a subarctic climate is low.

The seasonal microclimate trends of a large scale extensive green roof

At nearly 27,500 m2, the Jacob K. Javits Convention Center (JJCC) located in New York City, hosts one of the largest extensive green roofs in the United States. This paper explores three years of fine scale microclimate data collected at the JJCC green roof and its potential ability to reduce the urban heat island intensity (UHII). Surface energy fluxes and microclimate parameters on four different surfaces are analyzed before and after installation of the southern section of the green roof, offering a unique before/after, test/control study. The results indicate that the temperatures of the air above the green roof, and its exterior surface are different (e.g. lower) than those measured above and on, respectively, the black roof that preceded it. Differences in the maximum daytime air and surface temperature between the black and green roof were 1.80 °C and 18.4 °C, respectively. Installation of the green roof increased evapotranspiration, modifying the roof’s surface energy balance, and reduced the median summer nighttime UHII (compared to the pedestrian level station) by 0.91 °C. Though microclimatic conditions on two sections of the green roofs vary somewhat, the research findings generally support the statement that green roofs are a moderately effective strategy for mitigating the UHI effect.

Investigating the Effect of Dust Accumulation on the Solar Reflectivity of Coating Materials for Cool Roof Applications

Cool roofs use reflective materials or coatings to reflect a portion of the incident solar radiation. This results in a lowering the surface temperature of the cool roof compared to black roofs, and thus helps reduce the cooling energy loads during the summer season. The research reported in this paper was conducted to assess experimentally and numerically the performance of cool and black roofs that were subjected to the hot, humid and dusty climate of Jubail Industrial City (JIC). This paper focused on characterizing one of the important properties of reflective coating material (RCM), which is its solar reflectivity. In this study, the effect of dust/dirt accumulation on the solar reflectivity of the RCM was investigated at different exposure times to the natural weathering conditions of JIC. The test results showed that dust and dirt can significantly contribute in reducing the solar reflectivity of the RCM. As such, a number of cleaning processes were conducted on the surface of the RCM so as to increase its solar reflectivity. The effect of each cleaning process on the solar reflectivity of the RCM was investigated. Finally, this paper provides a test protocol and procedure for characterizing the dust concentration/intensity on the surfaces of the RCM and cleaning this material after different exposure times to a natural and polluted climate.

Evaluation of Applicability of Various Color Space Techniques of UAV Images for Evaluating Cool Roof Performance

Global warming is intensifying worldwide, and urban heat islands are occurring as urbanization progresses. The cool roof method is one alternative for reducing the urban heat island phenomenon and lowering the heat on building roofs for a comfortable indoor environment. In this study, a cool roof evaluation was performed using an unmanned aerial vehicle (UAV) and a red, green and blue (RGB) camera instead of a laser thermometer and a thermal infrared sensor to evaluate existing cool roofs. When using a UAV, an RGB sensor is used instead of expensive infrared sensor. Various color space techniques, namely light-reflectance value, hue saturation value (HSV), hue saturation lightness, and YUV (luma component (Y) and two chrominance components, called U (blue projection) and V (red projection)) derived from RGB images, are applied to evaluate color space techniques suitable for cool roof evaluation. This case study shows the following quantitative results: among various color space techniques investigated herein, the white roof with lowest temperature (average surface temperature: 44.1 °C; average indoor temperature: 33.3 °C) showed highest HSV, while the black roof with the highest temperature (surface temperature average: 73.4 °C; indoor temperature average: 37.1 °C) depicted the lowest HSV. In addition, the HSV showed the highest correlation in both the Pearson correlation coefficient and the linear regression analyses when the correlation among the brightness, surface temperature, and indoor temperature of the four color space techniques was analyzed. This study is considered a valuable reference for using RGB cameras and HSV color space techniques, instead of expensive thermal infrared cameras, when evaluating cool roof performance.

The Effect of Shading and Nutrient Addition on the Microphytobenthos, Macrofauna, and Biogeochemical Properties of Intertidal Flat Sediments

Proliferation of urban structures and mangrove forests in estuaries are altering the shading of intertidal sediments. Urbanisation also tends to increase nutrient loads in estuaries, which can have numerous direct and indirect effects on estuarine flora and fauna. Mangrove canopy shades the sediment and provides nutrients to the ecosystem via leaf litter. Microphytobenthos, macrobenthos, sediment erodibility and various biogeochemical properties of sediments have been shown to differ significantly between unshaded intertidal sediment and nearby sediment under a mangrove canopy. This study tested the effects of experimental manipulation of shading and addition of nutrients on the microphytobenthos, macrobenthos, sediment erodibility and selected biogeochemical properties of exposed mudflat next to the seaward edge of a mangrove forest. In the first of two experiments, shading of plots was increased by the addition of black Perspex roofs for comparison with unshaded plots and plots with clear Perspex roofs; the addition of nutrients was done in an orthogonal design. Clear roofs were originally intended as a procedural control, but due to sedimentation on their upper surface caused intermediate levels of shading. Sediment and benthos were sampled after two weeks. Nutrients were omitted in the second experiment, with plots sampled after two weeks or three months. The only effect of nutrients was a small negative effect on chlorophyll a and colloidal carbohydrate. Intermediate shading (clear roofs) consistently increased measures of microphytobenthos biomass (e.g. Fo and chlorophyll a) and biogeochemical properties associated with microphytobenthos such as colloidal carbohydrate. Substantial shading (black roofs) decreased measures of microphytobenthos biomass and microphytobenthos associated biogeochemical properties. Effects on the fauna were much smaller and inconsistent with previous studies, after 3 months assemblages were different under black roofs compared to clear roofs and control plots, with differences primarily driven by changes in the oligochaetes. Natural or anthropogenic changes in shading at larger spatial scales are likely therefore to directly and indirectly change microphytobenthos, sediment properties, macrofauna and hence ecosystem functions; but any flow-on effects to the fauna are difficult to predict without further experiments to understand the indirect and direct responses of fauna to changing microphytobenthos and properties of intertidal sediment.

Detention-based green roofs for stormwater management under extreme precipitation due to climate change

Abstract Rooftops cover a large percentage of land area in urban areas, which can potentially be used for stormwater purposes. Seeking adaptation strategies, there is an increasing interest in utilising green roofs for stormwater management. However, the impact of extreme rainfall on the hydrological performance of green roofs and their design implications remain challenging to quantify. In this study, a method was developed to assess the detention performance of a detention-based green roof (underlaid with 100 mm of expanded clay) for current and future climate conditions under extreme precipitation using an artificial rainfall generator. The green roof runoff was found to be more sensitive to the initial water content than the hyetograph shape. The green roof outperformed the black roof for performance indicators (time of concentration, centroid delay, T50 or peak attenuation). While the time of concentration for the reference black roof was within 5 minutes independently of rainfall intensity, for the green roof was extrapolated between 30 and 90 minutes with intensity from 0.8 to 2.5 mm/min. Adding a layer of expanded clay under the green roof substrate provided a significant improvement to the detention performance under extreme precipitation in current and future climate conditions.

Smart Roof for Green Building in Small City

A cool roof is defined as the one that reflects the sun’s heat and emits absorbed radiation back into the atmosphere. The roof thus, remains cooler and reduces the amount of heat in the building. It is well known that light colored reflective surfaces heat up less in the sun. Cool roof may be white, but it need not necessarily be white as there are cool colours available which use dark coloured pigments and still are highly reflective. Cool roof can be compared with white shirt; normally one prefers to wear the same reason during summer. Cool roofs reflect more sun lights and absorb less heat than traditional dark-coloured roofs but most of the roofs in the world are dark-coloured. In the heat of the full sun, the surface of the black roof can increase the temperature as much as 50 degree centigrade. Cool roof provides through solar reflectance surfaces which are comparatively cool and thus provides comfortable conditions inside the houses. The advantage of cool roof, properties and cool roof based on the solar reflectance and thermal eminence and describes essential materials for a green roof construction. The paper discusses the importance of green roof in Bangalore city to control the temperature increase. It suggest roof garden as one of the solution to deal with the heat island phenomenon to fight the urban heat island effect.The most important feature of an ideal cool roof is that its surface strongly reflects sunlight. The surface of an ideal cool roof should also efficiently cool itself by emitting thermal radiation. Thus, a cool roof should have both high “solar reflectance” (ability to reflect sunlight, measured on a scale of 0 to 1) and high “thermal emittance” (ability to emit thermal radiation, also measured on a scale of 0 to 1). The solar reflectance and thermal emittance of a surface are called its “radiative” properties because they describe its abilities to reflect solar radiation and emit thermal radiation. For cool roof design parameters like solar reflectance index (SRI) and thermal emittance are defined and the values are obtained for cement, paint and other materials. Based on the discussion it is recommended to have high values of SRI to construct a cool roof in Bangalore.

Optimization of cool roof and night ventilation in office buildings: A case study in Xiamen, China

Increasing roof albedo (using a “cool” roof) and night ventilation are passive cooling technologies that can reduce the cooling loads in buildings, but existing studies have not comprehensively explored the potential benefits of integrating these two technologies. This study combines an experiment in the summer and transition seasons with an annual simulation so as to evaluate the thermal performance, energy savings and thermal comfort improvement that could be obtained by coupling a cool roof with night ventilation. A holistic approach integrating sensitivity analysis and multi-objective optimization is developed to explore key design parameters (roof albedo, night ventilation air change rate, roof insulation level and internal thermal mass level) and optimal design options for the combined application of the cool roof and night ventilation. The proposed approach is validated and demonstrated through studies on a six-storey office building in Xiamen, a cooling-dominated city in southeast China. Simulations show that combining a cool roof with night ventilation can significantly decrease the annual cooling energy consumption by 27% compared to using a black roof without night ventilation and by 13% compared to using a cool roof without night ventilation. Roof albedo is the most influential parameter for both building energy performance and indoor thermal comfort. Optimal use of the cool roof and night ventilation can reduce the annual cooling energy use by 28% during occupied hours when air-conditioners are on and reduce the uncomfortable time slightly during occupied hours when air-conditioners are off.

Test box experiment to assess the impact of waterproofing materials on the energy gain of building roofs in Mexico

In Mexico, the owners of homes use roof coating materials for waterproofing purposes and usually select them just for aesthetic reasons. In this work, four concrete roofs with different waterproofing materials (terracotta, white, gray, and black) were tested under weather conditions of Cuernavaca, Mexico. A set-up with two outdoor test boxes in which the roofs can be exchanged was monitored for six weeks of spring 2015. Three comparative tests (terracotta-black, white-black, and gray-black) were carried out during the period with the highest solar radiation of the year. The results showed that the white roof (WR) had the best thermal performance, under peak solar irradiance it was 29 °C cooler than the black roof (BR) and just 1.5 °C hotter than the outdoor air. Moreover, the average daily energy gain of the WR was 73% smaller than the BR. The terracotta roof (TR) and the gray roof (GR) had peak temperatures of 10 and 14 °C lower than the BR. Moreover, the TR and the GR had daily energy gains 33 and 38% smaller than the BR, respectively, but they are significant compared to one of the WR. Therefore, white waterproofing materials for roofs should be used in Mexico as a passive cooling strategy to prevent solar energy gains of buildings.

Hygrothermal Performance of Cool Roofs Subjected to Saudi Climates

When solar radiation hits a roof surface, a part of solar energy is reflected and another part is absorbed. The absorbed part of solar energy results in an increase of the surface temperature of the roof. Cool (white) roofs use bright surfaces to reflect a significant portion of the incident short-wave solar radiation, which lowers the surface temperature compared to conventional (black) roofs. As such, white roofs help reduce the urban heat island effect during the summer. The questions are: (a) do white roofs lead to moisture-related problems in Saudi climates? (b) what are the amount of energy savings as result of using white roofs instead of black roofs of same insulation amount, and (c) what is the reduction in the amount of insulation in the white roof having the same energy performance level as the black roof? To answer these questions, numerical simulations were conducted for one type of roof that is commonly used in low-rise buildings of Saudi Arabia in order to assess and compare the energy and hygrothermal performance of white and black roofs. The roof was subjected to weather conditions of the Eastern Province of Saudi Arabia. The indoor conditions were taken based ASHRAE recommendations for conditioned space. The main outcome of this study has shown the capabilities of using reflective materials with different short-wave solar absorption coefficients for enhancing the energy performance of roofs and/or reducing the amount of insulation that resulted in same energy performance as black roofs. This study can be used in future for upgrading Saudi Building Code to allow less roof insulation if reflective roof is installed.

Variations of PV Panel Performance Installed over a Vegetated Roof and a Conventional Black Roof

The total worldwide photovoltaic (PV) capacity has been growing from about 1 GW at the beginning of the twenty-first century to over 300 GW in 2016 and is expected to reach 740 GW by 2022. PV panel efficiency is reported by PV manufacturers based on laboratory testing under Standard Testing Condition with a specific temperature of 25 °C and solar irradiation of 1000 W/m2. This research investigated the thermal interactions between the building roof surface and PV panels by examining the differences in PV panel temperature and energy output for those installed over a green roof (PV-Green) and those installed over a black roof (PV-Black). A year-long experimental study was conducted over the roof of an educational building with roof mounted PV panels with a system capacity of 4.3 kW to measure PV underside surface temperature (PV-UST), ambient air temperature between PV panel and building roof (PV-AT), and PV energy production (PV-EP). The results show that during the summer the PV-Green consistently recorded lower PV-UST and PV-AT temperatures and more PV-EP than PV-Black. The average hourly PV-EP difference was about 0.045 kWh while the maximum PV-EP difference was about 0.075 kWh, which represents roughly a 3.3% and 5.3% increase in PV-EP. For the entire study period, EP-Green produced 19.4 kWh more energy, which represents 1.4% more than EP-Black.

Modeling the reduction of urban excess heat by green roofs with respect to different irrigation scenarios

Urban heat reduction by evaporative cooling from extensive green roofs is explored by applying irrigation scenarios to green roofs located in different climate zones using a coupled atmosphere-vegetation-substrate green roof model. The model, which is integrated in the building energy simulation software EnergyPlus, is validated with eddy covariance surface energy fluxes from a green roof in Berlin, Germany. The original model was modified to include interception and an improved runoff calculation. Three irrigation scenarios were defined (no irrigation, sustainable irrigation by harvested runoff water, unrestricted irrigation) to study the heat reduction potential in terms of surface energy partitioning and sensible heat fluxes (QH). The irrigation scenarios are compared to two white roofs (albedo equal to 0.35 and 0.65) and a black roof.High correlation of sensible and latent heat (QE) fluxes between measured and modelled data for the original and the modified version of the green roof model were observed (for the original model, R2 = 0.91 and 0.81 for QH and QE, respectively, while for the modified version R2 = 0.91 and 0.80, respectively). The modified version was applied to study irrigation, due to lower systematic errors for QH, QE and better performance for the substrate moisture content. In comparison to a black roof the green roof reduces urban excess heat by 15%–51% with sustainable irrigation, by 48%–75% with unrestricted irrigation, but drops to 3% for unirrigated roofs in the different cities. Sustainable irrigation can be effective in climates with high annual (or summerly) precipitation.