Green roof oasis

Research in focus February 2014

Green or "living" roofs are no longer the preserve of the wealthy or the extravagant creations of modern architects looking to make their mark. Modern ecologists class them as settlement biotopes. They are stipulated in some land use plans as a counterbalance to paved surfaces – and in some parts of Germany they even attract public subsidies. As well as storing rainwater and improving the microclimate in cities, many green roofs are cultivated as rooftop gardens which provide an attractive contrast to surrounding buildings. One of their best-known advocates was the Austrian artist Friedensreich Hundertwasser, who regarded green roofs as an integral part of the reconciliation he sought between mankind and nature. To take just one example of their growing popularity, the German city of Stuttgart saw the creation of 180,000 square meters of green roofs on public and private buildings between 1986 and 2008, and the city’s 2010 land-use plan envisaged a further 1.5 million square meters as minimization or compensation measures for future building projects. A recent survey by the Fachvereinigung Bauwerksbegrünung e.V. (German Professional Green Roof Association – FBB) also reveals that the proportion of German cities with a green roof policy embedded in their land use plans has continued to hold steady, demonstrating that living roofs are now a firmly established phenomenon.

The flip side of the coin, however, is that on-again-off-again roofs – especially fully insulated timber constructions – are suffering damage due to moisture build-up. “One possible cause is that green roofs, like all flat roofs, are insulated on what is effectively the ‘wrong’ side in terms of building physics, in other words on the outside. That means that they can only dry out towards the interior,” says Daniel Zirkelbach, deputy director of the Department of Hygrothermics at the Fraunhofer Institute for Building Physics IBP. In contrast to flat roofs without covering layers, green roofs experience far less warming, as the plant cover means that they cannot dry out properly during the summer months. That’s why green timber frame constructions require such precise and professional planning. This cannot be achieved solely by means of traditional calculations of a building’s moisture balance such as the Glaser method. To develop new ways of improving the planning process, the Federal Institute for Research on Building, Urban Affairs and Spatial Development (BBSR) ran a research project from November 2011 through April 2013 using funds from the “Zukunft Bau” research initiative. The goal of the project was to provide planners and building material manufacturers with the most accurate and reliable basis possible for planning how to protect critical green roof areas against moisture damage. The scientists at Fraunhofer IBP were a good choice to take up this challenge. Within the scope of the research initiative, they succeeded in expanding their transient heat and moisture simulation software WUFI® to create new models for assessing green roofs which take the moisture balance into consideration under realistic conditions. The WUFI ® family of software products – which has received worldwide attention since its development by Fraunhofer IBP – enables the realistic calculation of the changing hygrothermal performance of multi-layered building components under natural climate conditions.
A distinction is generally made between two different variants of green roofs. Extensive green roofs are characterized by a layer of vegetation up to 15 centimeters thick which weighs relatively little and which requires minimal care. This form of vegetation largely takes care of itself and is also suitable for more extreme, localized conditions, though it does not qualify as usable floor space. In contrast, intensive green roofs such as rooftop gardens are typically designed to enable access to and use of the space. The multilayered construction of intensive roofs requires soil depths of between 15 and 100 centimeters, making them significantly thicker and heavier. They require major amounts of irrigation, feeding and other types of care and maintenance. This particular research project focused exclusively on extensive green roofs.
One of the scientists’ key aims was to find a way of reliably calculating the performance of the substructure, since previous research had failed to provide clear mathematical models of the influences of the substrate and vegetation layers due to a lack of generalizable data. “We started by compiling existing measurement results drawn from experimental set-ups and long-term design studies at our Holzkirchen site as well as additional data from Leipzig, Vienna, Kassel and Milan. We also embarked on further new outdoor testing initiatives at Fraunhofer IBP in Holzkirchen,” says Zirkelbach, explaining how they approached the project. To do this, the scientists set up new test areas with different types of substrates and plants on a test roof. These experiments were necessary because previous tests, due to their different focus, had omitted to measure both the moisture conditions in the substrates and the long-wave downward radiation. Both these factors are highly significant when it comes to transferring the calculation model to other climate conditions with different patterns of precipitation and radiation. Sensors were used to measure the temperature in the various types of green roof set-ups. “We were particularly interested to find that we were sometimes getting clearly different temperature readings at two points within a single test section under the same basic conditions. These localized readings varied by up to 6 degrees Celsius, even though we couldn’t ascertain any real difference,” says Zirkelbach. It didn’t take long, however, for the scientists to establish the average conditions which would provide a clear basis for calculations.
The Fraunhofer researchers also showed that the temperatures are significantly influenced by the mass of the substrate and the quantity of water it contains. “This combination results in high thermal inertia. In the summer evaporative cooling means that warming takes longer to occur, while in winter the latent heat counters the cooling effect that would lead to temperatures dropping below freezing. Thanks to a certain degree of self-shading, the plant cover is able to limit warming during the day and prevent excessive cooling caused by the release of heat at night. And on top of that the transfer of heat is lower thanks to the reduced movement of air on the surface,” says Zirkelbach, explaining the results.
The second point that has a major influence on the way the roof functions is the precise configuration of the roof design. Currently the outer design typically involves a timber sheathing sealed off from the substrate which has a fiber insulating material between the rafters, while the inner part features a vapor retarder designed to prevent moisture from the indoor climate penetrating the wood construction. However, in the summer the vapor retarder also prevents the construction from drying out, which means that small pockets of construction moisture or humidity that have already worked his way in can lead to problems. The consequences of this inadequate design have typically been a shorter service life or moisture damage emerging over the short to medium term. And the thicker the insulating layer, the damper the assembly became in winter.
The new series of experiments and resulting calculations have enabled the researchers to come up with a number of promising solutions. One option that can significantly improve the moisture balance is a moisture-variable vapor retarder. In the case of insulation thicknesses in excess of 15 to 20 centimeters, an additional moisture-resistant insulation cover can help prevent damage by significantly reducing the differences in temperature in the timber sheathing and reducing the formation of condensation, especially in the winter months. Although green wooden roofs will always be an advanced form of construction that requires careful planning and execution, the trend towards green rooftop oases can safely be continued as long as certain key aspects are taken into account.
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