At the World Building Greening Congress in Berlin, an annual gathering for the green infrastructure sector, Nicole Pfoser of Nürtingen-Geislingen University of Applied Sciences presented the revised edition of the guide “Building Greening – Sustainable Solutions for Climate and Cities”. The publication updates the 2013 guide “Buildings, Greening and Energy: Potentials and Interactions”.
After ten years, such an update was necessary, Pfoser stresses. The aim: “To bring knowledge on building greening up to date in terms of performance factors, construction methods, installation and care,” she explains. “But maintenance and the combination with energy-efficient construction were also central. This way, we can make a free guide containing a wealth of knowledge available to everyone.”
Multifunctional roofs introduced
The authors have fundamentally revised the categorisation of green roofs. Instead of the previous division into extensive and intensive greening, the new guide additionally distinguishes between priorities such as weight reduction or multifunctional roofs, which have been introduced as an entirely new category. These include solar green roofs, alongside biodiversity roofs, retention roofs and water green roofs, as well as installations for building-integrated farming. Pfoser explains that the market has shifted accordingly, with demand for such multifunctional roofs seeing a genuine boost.
Criteria for solar green roofs
Alongside façade greening, one of the focal points of the update is the combination of green roofs with photovoltaics. In the guide, the design criteria for building solar green roofs are supplemented by vegetation-related and structural criteria. The common technical options are presented and their respective advantages are set out. The guide describes eight different applications, ranging from flat-plate collectors next to extensive greening, through tilt-optimised solar modules, to semi-transparent photovoltaic systems on pergolas above intensive greening.
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The authors also describe the various benefits of combining photovoltaics with a green roof, including with regard to module performance and yields. The figures given are based on concrete measurements, including a comparison between a green roof and a bitumen roof at the Ufa-Fabrik in Berlin-Tempelhof, as well as studies by Nuremberg Institute of Technology. “The results show that the greening leads to a cooling of the air temperature between the plants and the photovoltaics by up to 1.6 Kelvin compared with the unplanted reference case,” the authors write in the guide.
Synergies in mounting and structural load
Beyond the efficiency gains, the guide describes further structural benefits of the combination. For elevated photovoltaic systems, the green roof can replace the gravel or concrete slabs otherwise required as ballast. This avoids point loads and allows an even distribution of weight. Combined systems are now available on the market as solar green roofs, with substructures for photovoltaic installations as well as flat-roof greening systems.
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The life cycle assessment in the guide does, however, show mixed results. Over a 50-year period, the present value of an extensive solar green roof is around €223 per square metre, while a gravel roof with a solar installation comes in at roughly €168 per square metre. The higher investment and maintenance costs are not fully offset by the additional revenue from electricity generation. Pfoser stresses, however, that soft factors such as the impact on the urban climate, the improvement of the building's appearance and occupant well-being should be taken into account in such assessments.
The full guide Building Greening – Sustainable Solutions for Climate and Cities is available for free download. (su)
