Blue-green roofs are emerging as one of the key solutions to combat summer overheating in urban areas. Researchers from the Laboratory for Environmental Technologies in Buildings at the University of Ljubljana, Faculty of Mechanical Engineering, have developed a model that can predict the performance of such roofs even during extreme drought conditions. Their comprehensive numerical model was published in the journal Building and Environment (IF=7,6).
Green roofs are a vital urban solution – they help mitigate heatwaves, retain heavy rainfall, and improve living conditions in densely populated areas. However, their effectiveness is highly dependent on water availability.
To address this, blue-green roofs have been introduced in recent years. These systems combine traditional green roofs with a water reservoir beneath the substrate. The reservoir extends the period during which plants can actively cool their surroundings through evapotranspiration. During dry periods, an air layer also forms in the reservoir – a phenomenon that previous thermal models have failed to account for.
Researchers at the LOTZ laboratory developed a comprehensive numerical model that accurately simulates heat and moisture transport in advanced blue-green roofs, including under challenging drought scenarios. By modelling the heat transfer within the air layer in detail, they significantly improved the model’s predictive accuracy. A key innovation is the introduction of a water extraction coefficient, which quantifies how efficiently plants draw water from the reservoir. An added value of the research is the direct measurement of latent heat flux (evaporation), which allowed validation of the model with field data.
The study is based on several months of on-site measurements conducted at the LOTZ outdoor laboratory.
“With our model, we can accurately predict the behaviour of blue-green roofs under all weather conditions – even extreme heat,” says lead researcher Tej Žižak. “This represents a crucial step towards designing buildings that are resilient to future climate extremes.”
According to the researchers, improved predictions of the thermal and hydrological performance of blue-green roofs will enable optimized design, more efficient use of stored rainwater, and sustainable urban cooling during heatwaves.