In the context of global climate change, with the intensification of urban heat island effect, research on urban thermal environment and human thermal comfort is gaining increasing attention. Green roofs in the hot summer-cold winter regions have significant cooling and humidifying effects in summer.

The study is designed to explore the mechanism of green roofs in reducing urban heat island effect and improving human thermal comfort, and provide a scientific basis for the design of urban green roofs.

The garden green roofs of Anhui Institute of Building Research and Design were monitored under a clear and windless weather condition. The ENVI-met 4.4.5 software was used to simulate two different types of roof greening (garden and ground-cover) and three different vegetation layout forms (random, group, and enclosed) based on verifying the accuracy and reliability of the model. The cooling and humidifying effects of green roofs were measured from a "surface" perspective, and the physiological equivalent temperature (PET) was used to evaluate human thermal comfort.

(1) Green roofs had significant cooling and humidifying effects in summer, with a stronger cooling effect during the day than at night and a stronger humidifying effect at night than during the day. (2) Ground-cover and garden roof greening showed similar trends in cooling and humidifying effects. The daily temperature decline of ground-cover and garden green roofs ranged from 0.002℃ to 0.068℃ and from 0.001℃ to 0.141℃, respectively; the humidity increase ranged from 0.367% to 1.952% and from 0.555% to 2.305%, respectively; garden roof greening showed steeper changes. (3) During 19:00-6:00 the next day, i.e., when the air temperature was between 26.462℃ and 28.436℃, the relative humidity was between 54.192% and 66.608%, and PET was between 18-23℃, the human thermal comfort was at its highest level. During 11:00-16:00, i.e., when the air temperature was between 28.557℃ and 30.349℃, the relative humidity was between 49.646% and 57.636%, and PET was >41℃, the human thermal comfort was at its lowest level. During 6:00-11:00 and 16:00-19:00, i.e., when PET was between 23-41℃, the human thermal comfort was at an average level. (4) In the context of same vegetation configuration proportions and different vegetation layouts, the average temperature of the plot from low to high was random layout < group layout < enclosed layout; the temperature was positively correlated with the wind speed and negatively correlated with the relative humidity; the three vegetation layouts showed same heat stress levels at same time points.

In the hot summer-cold winter zones, both ground-cover and garden roof greening have certain cooling and humidifying effects in summer. Associated human thermal comfort levels change in a day. Therefore, increasing green space and urban green volume is important to improve urban microclimate and enhance human thermal comfort.