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

How can the quality of stay at an urban square be improved quickly, temporarily and cost-effectively?

This question was addressed by the Fraunhofer IBP in collaboration with the city of Rosenheim within the framework of the BMBF-funded BUOLUS project. For this purpose, various (temporary) measures were implemented at the real-world laboratory site Salzstadel in Rosenheim, which were accompanied by measurements. In addition, several public surveys were conducted to determine user satisfaction with the quality stay at the square and to derive possible improvement measures. This included, among other things, an online survey in 2021 with 291 participants and a comparative online survey at the end of the project in August/September 2024 with 232 participants. Both surveys were similar in their basic structure, but included additional specific questions on current topics at the square (e.g. the newly constructed seating element, services offered by the city library). This allowed for a comparative analysis of the results, while also considering individual aspects.

The 2024 survey consisted of a total of 34 questions in five categories: 1. demographic data, 2. information about Salzstadel square, 3. building physics variables, 4. urban furniture and 5. the events offered by the Rosenheim City Library.

One notable finding was that, in 2024, Salzstadel Square is used less frequently as a traffic area compared to 2021 and is increasingly being seen as a place for relaxation and social or family interactions (see Figure 1). This indicates an improvement in the quality of stay, as the square is now perceived more as a place to linger and spend time, rather than just a thoroughfare. However, it was also observed that the use of the square for eating and gastronomic offers decreased between 2021 and 2024. One possible reason for this could be the removal of the coffee cart in 2024, which was still present at the square in 2021. This could also be related to the changed frequency of use. While daily and weekly visits to the square have slightly decreased compared to 2021, the monthly visits have increased significantly (see Figure 2). In 2021, many respondents reported using the Salzstadel square for their lunch break or for coffee on weekends. With the absence of the coffee cart, a central meeting place has also been lost.

The satisfaction of respondents with the shading provided at the site has developed positively. While in 2021, 56.7% of respondents were dissatisfied or very dissatisfied with the shading provisions, by 2024, only about a third (30.5%) reported being dissatisfied or very dissatisfied (see Figure 3). This may be due to the growth of tree canopies, which provide more natural shade, as well as the mobile sunshades, which are set up daily by the city library staff during the summer and can be moved as needed.

As a measure to improve the quality of stay, a multifunctional seating element was installed at Salzstadel in November 2023. The design of the seating element was based on the results of previous surveys conducted at the square. The main requirements for the seating element were background noise reduction, a shading option, and greenery. The newly installed seating element received a positive response by a large majority (94.8%). Furthermore, 90.6% of the respondents could imagine similar street furniture on other city squares. Nevertheless, potential for improvement was also identified, such as litter and cigarette butts, which were often mentioned as reasons for dissatisfaction. An additional, clearly visible (cigarette butt) waste bin near the seating area could help address the issue.

The wide range of events offered by the city library is also received very positively, with 87% of users being satisfied or very satisfied with the program. Reasons for dissatisfaction included inconvenient event times or excessive noise levels. Additional suggestions for further enhancing the Salzstadel quare include more greenery, such as large, shady trees and an improved infrastructure for cyclists, such as additional bike racks and e-bike charging stations.

In summary, it should be noted that the survey results are based on convenience and random sampling and are therefore not representative. Nevertheless, they can provide valuable insights into the general public sentiment.

Overall, the Salzstadel has developed into a vibrant city square with a high quality of stay due to the diverse offerings and urban design improvements. However, it remains important to continuously improve the square’s appeal as a place for people to spend time while actively involving users, as they are the ones who bring the square to life.

For a detailed overview of the survey results on the quality of stay at Salzstadel square, please refer to this presentation.

Squares in towns or cities are public places where people can spend time. Providing space for a variety of uses, they are therefore of key importance to the local population. In view of the increasing heat stress in urban areas, such squares need to be designed to be more resilient to the effects of climate change in order to sustainably improve thermal comfort and thus the quality of time spent there. As part of the BUOLUS project, temporary adaptation measures have already been implemented and monitored at the real-world laboratory square “Salzstadel” in Rosenheim. Among others, these include a partially-greened, multifunctional seating element.

To identify further suitable heat adaptation measures for the Salzstadel square, various scenarios were developed using the PALM-4U Urban climate model (see Figure 1). The focus of the climate simulations was on improving thermal comfort during the day and on cooling the square at night.

The relevant parameters considered were air and surface temperature, wind speed and direction and the UTCI comfort index (Universal Thermal Climate Index). The UTCI enables thermal outdoor daytime conditions to be comprehensively assessed and integrates meteorological parameters such as wind, average radiation and air temperature. A total of ten measures were studied. To this end, an original scenario (parking spaces with surfaces that were fully sealed up till 2009) with minimal effect was conceived together with an extreme scenario (simulation with comprehensive redesign) with maximal effect in order to classify the simulated impact of the various measures. This reference context is shown in Figure 2.

The results show that the original scenario with fully sealed surfaces and no trees leads to less favorable climatic conditions, while the extreme scenario shows significant improvements. Design measures on vertical surfaces, such as greening the façades of nearby buildings, had no appreciable effect on the target values considered. This is because the resolution chosen in the simulation program is too coarse to accurately depict the processes. Modifications to horizontal surface properties such as albedo and removal of the surface sealing proved to be more efficient. In particular, introducing water features, changing the use of the streets by removing the road surface, planting new trees and greening helped to improve thermal comfort and nighttime cooling.

It also became apparent that implementing measures throughout the whole square is more effective than implementing “selective” measures only in the center where people tend to gather. In addition, the measures need to be evaluated differently depending on the time of day, as they have different effects. For example, while large groups of trees provide shade and evaporative cooling during the day, they can impair air exchange at night and store heat under their canopy. To continue to design the square to meet the needs of the various user groups, whilst at the same time increasing the quality of the stay, a combination of measures was recommended on the basis of the simulation results, including water features and changing the use of the streets.

The simulations made it possible to test the practical suitability of PALM-4U and to gain valuable insights into the effectiveness of various urban planning measures for improving thermal comfort and night-time cooling. In principle, the measures can be transferred to squares in other towns or cities. However, they must be adapted to the local conditions because every town or city has its own microclimate, which in turn is heavily dependent on the urban structure and local wind conditions.

The results were displayed on posters in the Salzstadel square in the summer.

The following three graphs show a summary of the simulation results, the different scenarios, and more detailed information on the urban climate simulation.

Municipal surfaces are maintained to preserve their functionality. One common practice is the removal of wild weeds, which ensures pedestrian safety and meets aesthetic expectations. However, this maintenance process requires significant resources, generates costs, and contributes to greenhouse gas emissions. Through targeted construction measures, it is possible to prevent weed growth on public surfaces, thereby reducing maintenance demands.

As part of the BUOLUS project, a field study was conducted to examine such a preventive measure. The study focused on evaluating how different joint filling materials influence weed growth and on identifying potential ways to optimize sustainability impacts. The test site was a seating area along the lakeside promenade in the municipality of Rottach-Egern. The surface is paved with granite mosaic stones and was originally filled with crushed sand. Figure 1 shows the degree of weed coverage in the parking bay in May 2023.

The field study began in Rottach-Egern at the beginning of October 2023. The test area was divided into five sections, each treated with selected joint materials. Sections A and B were left unchanged to serve as control areas. The remaining sections were cleared of weeds at the start of the study and then re-jointed using the following materials:

• Section C: Crushed sand
• Section D: Vegetation-suppressing joint sand
• Section E: Crushed sand–lime mixture

Figure 2 shows the implementation of the field study across the five sections.

Figure 3 displays the weed coverage of each section at the end of the study in mid-July 2024. The section filled with the crushed sand–lime mixture showed a significant reduction in weed growth. However, this material may negatively affect the permeability of the entire surface. In contrast, sections filled with crushed sand or vegetation-inhibiting joint sand exhibited substantial weed growth, especially in areas beneath benches or between benches and adjacent green spaces -places that cannot be accessed and are difficult to be cleaned. The study confirmed that, beyond the choice of joint material, factors such as surface accessibility and surrounding vegetation also significantly influence weed growth.

Key Recommendations for Municipal Application:

• Match joint material to use case:
  The selection of joint materials should reflect surface usage intensity and maintenance methods. Priority should be given to maintaining permeability wherever possible.
• Use crushed sand for accessible areas:
  Easily accessible areas, such as those in front of benches, should be jointed with crushed sand. It preserves permeability, is more cost-effective than vegetation-inhibiting sand, and showed no significant difference in weed growth in this study. Re-jointing cleaned paving with crushed sand is also potentially the most environmentally sustainable option.
• Use crushed sand–lime mixture for hard-to-reach areas:
  For areas that are difficult to reach with cleaning machines -such as beneath benches -the crushed sand–lime mixture used in Section E is recommended due to its effective vegetation suppression.
• Edge areas and adjacent vegetation:
  For edge areas of paved surfaces that are less frequently used, or near adjacent vegetation that may promote weed growth, the use of a crushed sand–lime mixture should also be considered.

For more details about the field study, please contact Jakob Richtmann (jakob.richtmann@ibp.fraunhofer.de, study coordination), Kristina Henzler (kristina.henzler@iabp.uni-stuttgart.de, study design) or Daniel Merone (DMerone@rottach-egern.de, study implementation).

As part of the project, a comprehensive study of the Salzstadel city square in Rosenheim was carried out to analyze the urban heat island effect. Various geospatial and material data were collected, and 3D models were developed to support this investigation.

Data collection methods included laser scanning, infrared thermography, and hyperspectral imaging. A detailed 3D RGB model of the study area was created through photogrammetry and supplemented with orthophotos. In addition, infrared and hyperspectral images were taken to gather information on surface temperatures and material properties. These datasets were integrated into a combined model using deep learning techniques, enabling a high-resolution visualization of material distribution and vegetative structures throughout the study area.

The project partner Voxelgrid contributed technical expertise to generate these high-resolution 3D urban models. Thanks to deep learning technologies, these models allow for precise detection of urban surfaces and their material characteristics which is crucial for simulating climatic interactions in urban areas. The resulting 3D model serves as a key tool for analyzing climate conditions in cities and supports the development of urban planning strategies for climate adaptation.

Further information about the project's methodology and specific project results is available via the following link.

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:

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.

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.