Influence of blue-green roofs on surface and indoor temperatures on a building scale

Many nature-based solutions are seen as favourable and effective measures to increase urban resilience during more extreme weather events, by for example decrease high temperatures in summer. Since space is often scarce in urban environments, roofs have received increased attention in mitigating the consequences of climate change in urban areas. This resulted in a variety of roof systems of green and blue-green roofs designed as an integral part of the built environment due to their hydrological, insulative and biodiverse capacities. This study examined the impact of blue-green, and conventional roofs on roof surface temperatures, indoor temperatures and insulative properties of the building. Temperature sensors (IButtons) have been used for summer and winter measurements on roofs for early 20th century buildings in the city of Amsterdam (NL).
The results indicate the strongest effect of blue-green roofs on surface temperatures in summer, with significantly lower surface temperatures (2-3°C) than for conventional roofs. During winter days, the surface temperatures were not significantly different on blue-green roofs than on conventional roofs. The measurements in the water crate layers of blue-green roofs show an all year-round temperature buffering effect. During hot summer days, the temperature in the water storage of the blue-green roof was much lower than other measured surfaces (up to 12 °C and 7 °C compared to gravel roofs and the blue-green roof substrate, respectively) and also experienced the least diurnal variation. Similarly, the empty water crate layer showed up to 3 °C higher minimum temperatures during cold winter nights. The measurements also show a small positive systematic effect on the indoor environment under a blue-green roof compared to traditional gravel roof type. The variation in indoor temperature is smaller underneath the blue-green roofs compared to the reference roofs during both warm and cold periods (0.19 – 0.35 °C reduction in STD). This suggests that rooms located under a blue-green roof are less sensitive to the outside air temperature and its natural diurnal variation.
Although the effect on indoor thermal comfort seems to be small, blue-green roofs contribute to overall greening of the city. Second, thanks to the water storage the potential for growing biodiverse vegetation is higher than on extensive green roofs.