Benefits of green spaces in urban areas have proven to have a positive effect on well-being, primarily by improving thermal comfort through vegetations self-cooling mechanisms, such as water transpiration and evaporation—evapotranspiration. Given the limited space in an already built environment, building envelopes offer a solution for greenery placement. The most prevailing green envelope technology are green roofs, particularly the extensive green roof solutions provide benefits with little or no maintenance. For green roofs to effectively cool through evapotranspiration, water availability is crucial. However, during extreme summer conditions, when cooling is most needed, rainwater is scarce, and the city water supply can be limited. The new concept of extensive green roofs with water storage layer (blue-green roofs) offers a solution not only for storm water retention, but also have the ability to store rainwater for extended draught periods. This study is based on an extensive blue-green roof solution with water storage layer, mineral and organic substrate, and sedum vegetation. The experimental setup includes measurements of evapotranspiration using a lysimeter, water storage layer depth, and substrate moisture content. The results present the water content across various layers during an extended drought period and identify the starting point of vegetation water stress conditions. A validated numerical model of heat and mass transfer incorporating evapotranspiration rates is developed. With the numerical model a sustainability study of using only rainwater for irrigation is presented for different locations in Europe. The results indicate that in moderate climates, sustainable year-round rainwater irrigation is feasible during a typical meteorological year, while in some drier regions, additional rainwater storage or irrigation by other means may be needed.