Abstract
Greening structures attract worldwide attention because of their multidisciplinary benefits. Green roofs are considered one of the best ways to eliminate summer overheating, mitigate climate change, or reduce the urban heat island effect. The winter season and its impact on building energy consumption are often overlooked. Common standards do not take a green roof structure into consideration because of possible high water content in their layers. Additional roof layers may have a positive effect during the winter; they help reduce surface overcooling in cloudless winter nights. This paper analyses experimental measurements taken on two different extensive green roofs and compares the results with a single-ply roof (R) with a PVC membrane. Surface overcooling of the R due to radiation reaching up to 10 °C, whereas the green roof membrane is protected. The influence of thermal loss is not so important for the current climate in Central Europe, as the required U-values are lower than 0.1. The temperature difference is reduced from 17 °C on the membrane to 0.7 °C on the top of the concrete slab. The green roof is still advantageous, and the vegetation surface has better thermal stability. The advantage is clearly recognisable in the area of the condensation zone. The difference between these two extensive green roofs is very small in regard to the accuracy of the temperature sensors. The outcome showed the thermal loss reduction compared to the common flat roof; however, after analysis, it was more marginal than expected.