Thermal performance of two different green roofs throughout three winters in Slovakia

This study presents the results of long-term in situ measurements of the thermal response of experimental green roofs during the winter. The in-situ research measurements were done on experimental green roof segments with 120 and 240 mm thick substrate and a comparative reference gravel segment. They were carried out during winters 2020–2023 in Košice Dfb 48°43’N 21°15’E and feature climatic data of the analysed site, including the thickness of the snow layer present on the experimental segments. The presence of green roof layers reduced the negative heat flux (towards ambient) compared to the reference gravel roof, thus reducing heat losses in favour of the green roofs during the entire monitoring period. The biggest potential of heat loss reductions by applying green roofs was observed during a period of water freezing conditions with no snow layer on rooftops, when thermal response of experimental roofs was strongly influenced by latent heat accumulation in the green roof substrates. During cold consecutive days with all-day frosts, almost identical daily heat losses through both green roof segments (≈−57 Wh/m2day) were measured regardless of the outdoor climatic parameters while the losses through the reference segment ranged from −64 to −77 Wh/m2day, representing both green roofs’ daily heat losses reduction of up to 26% compared to the reference roof. Interannual comparison of two consecutive winters showed that heat loss reduction by green roofs varied with winter severity, with roofs featuring 120mm and 240mm substrate layers achieving 9%–14% reduction in the colder winter and 6%–12% in the milder one, compared to the reference gravel roof.