Buolus - Building physics design of urban surfaces for sustainable quality of urban living and environments

Graph project goal BUOLUS
© Fraunhofer IBP
The BUOLUS project aims to improve climate resilience and ensure the sustainable development of municipal structures.

Municipalities are faced with the challenge of adapting to climate change. On the one hand, they have to choose effective and sustainable measures, and on the other hand, they have to take into account the interests of residents and act under cost pressure. Improving urban climate resilience alone is not enough to bring about this change successfully, especially when it comes to small and medium-sized municipalities. It is much more a matter of responding to the challenge of climate-resilient urban design with environmentally and financially compatible measures, while at the same time serving growing demands for a better quality of life in cities. This is the integral approach that the BUOLUS project is taking to identify sustainable solutions for a climate-resilient city.

Project goals BUOLUS II

Greened roof  in Rosenheim.
© Fraunhofer IBP
Greened roof on a bicycle garage in Rosenheim.
To collect data, volunteer surveys are being conducted in Rosenheim.
© Fraunhofer IBP
To collect data, volunteer surveys are being conducted at the Salzstadel reality lab in Rosenheim.
3D model of the city Rosenheim
© Fraunhofer IBP
Screenshot of the 3D model of the city of Rosenheim developed in the project, which is now being optimized.

The implementation and stabilization measures in this second project phase will be carried out on three different levels: Building, district and city. The main objective is to facilitate the transfer of theoretical results from the first funding phase into practical implementation and, at the same time, to develop and document implementation procedures and concepts. The implemented measures can also be studied metrologically, and their actual effect evaluated as well as iteratively adapted. The effects of these innovative approaches can also be reproduced using simulation and validation tools so that they can be extended and adapted to meet specific requirements.

BUOLUS takes a sustainable approach in addressing these aims to improve climate resilience and develop municipal structures. In this context, sustainable means taking into account financial constraints, ecological demands and user acceptance in equal measure.


Project focus:

  • Greening: Evaluating the use of greened roofs and façades to reduce heat islands and to improve air quality, retention, biodiversity and quality of life.
  • Cleaning: Development of concepts to prevent pollution and clean the interior of public buildings as well as outdoor spaces.
  • Urban development: Assessment of measures to improve the quality of life in inner city areas.
  • Data management: Use of existing data as a basis for selecting measures and making decisions.
  • Rainwater harvesting and heat island prevention: Assessment of measures to improve the urban climate by using rainwater to cool surfaces.
  • Management: Sustainability analyses based on sustainable development goals (SDG).

Project results from BUOLUS I

The objectives of BUOLUS I were the holistic development, technological expansion and practical testing of the building physics potential of urban surfaces.

As a starting point, the project brought all aspects and actors together for a structured and moderated interdisciplinary exchange. The owners and users of urban surfaces come from all sectors of urban society. Therefore, communication between the relevant stakeholders extended throughout the entire project. The identified urban challenges and demands were analyzed and translated into concrete requirements of the building physical characteristics of urban surfaces. By subsequently comparing existing and new technologies, a portfolio of innovative planning and design solutions was compiled which focuses on the functional area management of urban materials, surfaces and building components. The development of integral processes and instruments for expert planning and municipal participation was directly linked to this approach. Some results are presented below as examples:

Result 1

Diagram on the effect of water storage capacity on soil moisture
© Gößner, D., Mohri, M., & Krespach, J. J. (2021)
Effect of water storage capacity on soil moisture: The retention roof with 3 cm water accumulation, 28.5 L/m² water storage capacity and equipped with capillary bridges and a capillary fleece has on average 10% higher level of moisture in the soil than a comparable natural roof without water accumulation and with a smaller water storage capacity. This shows that water accumulation can act as passive irrigation.

Increased efficiency by greening buildings 

The BUOLUS sub-project “Increased efficiency by greening buildings” dealt with the (micro)climatic effects of greened urban surfaces. Unlike sealed surfaces, water can infiltrate and be retained by these surfaces, thus cooling the environment by evaporation. In the course of the project, these effects were substantiated with concrete and system-specific figures and data. In particular, it was possible to measure the differences between the green roof systems studied in terms of evaporation rate, substrate temperature and air temperature at vegetation level, which were strongly influenced by the capacity of the greened roofs to store water. The collected data are highly important when designing urban spaces, enabling the most efficient and effective greening systems to be used.

Result 2

3D model of city hall in Rosenheim
© Virtual City Systems
Integration of the detailed model of the city hall into the 3D city model of Rosenheim.
Simulation results on temperature in a courtyard with trees.
© Virtual City Systems
Physical simulation results on temperature in a courtyard with trees.

Development of a semantic 3D city model platform

As part of the BUOLUS project, Virtual City Systems developed a semantic 3D city model platform that contains wide-ranging information relevant to building physics calculation methods. This includes the city model of Rosenheim, a 3D tree register, a green space register, and the option to integrate sensor data. Furthermore, a system for mapping IFC data geometrically and semantically onto the CityGML schema was developed and implemented. Thus, the city hall of the city of Rosenheim, which was generated as an IFC model by the company Voxelgrid, was transferred to the open OGC CityGML standard and integrated into the 3D city model platform. Along with the data, geoinformation tools - such as a drawing tool for enriching the model - were also added.  

Development of integrated simulation applications based on the city model platform

Besides the 3D city model platform, a simulation application for estimating the impact of construction measures on the climate was also developed and prototyped. 3D city models serve as a basis since these already exist for many cities and therefore do not have to be modeled in a time-consuming way. Furthermore, existing buildings can be removed and plans imported or drawn in. There are four steps to the procedure. The first step is to select an area in the 3D city model platform and export the relevant objects. The second step is to calculate solar irradiance based on the 3D objects. The third step is CFD simulation using ANSYS Discovery software. In the last step, the results are visualized on the map.


Fraunhofer IBP

Overall coordination of the project and scientific monitoring of all implementation measures

University of Stuttgart, Institute for Acoustics and Building Physics (IABP)

Carbon footprint and life cycle assessment of management processes and urban surfaces.

City of Rosenheim

Implementation partner and platform for the realization measures developed in the project - Rosenheim provides the reality labs “Greened roof on bicycle garage” and the “Salzstadel” square and derives measures for the city from the research results.


As a specialist for greening roofs, Optigrün addresses topics relating to green urban spaces: The aim is to collect microclimate data for urban climate models, to analyze the capacity of green roofs to store CO2, and to optimize the maintenance of greened roofs.

Virtual City Systems

Responsible for geodata management, 3D urban modeling and the development of an interface between the urban model platform and Fraunhofer IBP’s urban climate models.


Generation of a digital twin of the area to be studied using different imaging techniques. Generation of a 3D model of the area to be studied with material properties.

Project sponsors and funding bodies