**2. Methodology and Approach**

In the context of the investigations for this article, a comparison was made between the cost savings due to less absence through illness and the costs for vertical greening in the interior of office spaces.

The investigations of this article are based, on the one hand, on the measurements of relative air humidity and air temperature in greened and non-greened interiors of three Viennese school buildings, which were equipped with different vertical greening systems within the scope of research projects. All project results can be found in [48,49]. These projects provided extensive long-term measurement data. The evaluations of the hygrothermal comfort for the classrooms in summer and winter have already been published [36]. In addition, recommendations for the dimensioning of vertical indoor greening in classrooms in relation to hygrothermal comfort were developed on the basis of formulas applied [49,50]. For the present study, the hygrothermal measurement data are filtered again and evaluated accordingly. This allows statements about the percentage of improvement of the hygrothermal comfort and thus the improvement of the indoor air quality based on this parameter. The three school buildings investigated differ in their construction method and in the way they are ventilated: A non-insulated old building in brick construction without technical ventilation system, a new building in reinforced concrete construction with a thermal insulation composite system and ventilation system, and one without a technical ventilation system. It is therefore possible to make statements for three different structural situations for these locations. They will be referred to as Building A, B, and C in the following. As an example for building C, Figure 1 shows the three different rooms as they exist in each of the three buildings: A reference room, a green room with the trough system, and a green room with the fleece system. This figure also contains the calculation results obtained. Figure 2 shows the greening with the fleece system as an example from building B.

The measured data of air humidity and air temperature were collected per building in two greened classrooms and one non-greened comparable classroom, which served as reference rooms, over several years in a measuring interval of 5 min. More details on used measurement instruments as well as measurement settings can be equivalently found in [36]. Two different vertical greening systems were used at each of the three locations: A fleece system and a trough system. These two systems were described in detail in [48,49] and shown in Figure 1 in a sketch. The plants used in the vertical greenings were selected within the framework of the research projects by the project partners with many years of expertise in vegetation technology as well as a landscape gardener involved in the project, so that the plant selection is optimally designed for use in vertical indoor greenings. The selection of the plants is attributed a high value, but this should not be the focus of the present investigations, but should always be accompanied by an expert. In order to be able to make statements about the effect of the greening also for office rooms, these measuring data from classrooms were filtered in such a way that only times in winter period when the rooms were not used for teaching were used for the present analysis, so that there is no influence of the presence of the students. In addition, all measured data were checked for plausibility, and data gaps and outliers were processed accordingly. In a further step, it was determined how many workplaces could be arranged in the respective classrooms in

accordance with the Austrian workplace regulations. In this way, the vertical green area per workplace in the respective room under consideration was highlighted.

**Figure 1.** Overview of buildings and rooms with according calculation results.

**Figure 2.** Fleece system in Building B.

The hygrothermal comfort indoors and its criteria have already been examined in detail. In addition to relative humidity and air temperature, detailed analyses also take into account factors such as physical conditions, the activity of the persons, and their clothing. In order to enable statements as general as possible and in accordance with the available data, the definition of hygrothermal comfort according to Frank [51] is used in the present investigations and the measured values are analyzed according to these defined areas. This method is the same as that used in [36] for the analysis. Figure 3 shows these areas. Thus, measured values within the red framed area are in the comfortable range, which means that both the measured air temperature and the relative humidity are in a range that is comfortable for persons present. If a combination of relative humidity and air temperature is within the green framed area but outside of the red area, these measurements are called "still comfortable". Outside of this green area, the existing conditions are considered "not comfortable". This means that the temperature is either too cold or too warm, and the air is too dry or too humid.

**Figure 3.** Hygrothermal comfort according to Frank [51].

The costs for the installation and construction of the greenery for the three locations were summarized and calculated on the basis of the actual costs incurred. Moreover, the costs for operation as well as green care and technical maintenance were collected and presented in values per year for the three locations and the two different greening systems used. Due to the locations as well as the different functioning of the greening systems, these vary. In summary, the costs of the greening systems could be calculated per workplace and year for each greening system used at the three locations.

In a further step, these costs are compared to the costs for the absence due to sickness of one person per day, which were determined based on the explanations in Section 1 and the average annual income of employed persons (including apprentices) and the working hours in hours per year according to Statistik Austria [52].

By improving the hygrothermal comfort in the greened rooms compared to the nongreened rooms, a reduction of the number of sick days is then possible on the basis of the correlations explained in Section 1, which allows a statement about the positive monetary effect of vertical greening in the office space on the saving of salary costs for employees due to fewer sick days.

In a final step, different initial situations addressing the connection between vertical indoor greening and reduced sick leave are considered. These should show in which way the method described in the article can be applied or which statements can be formulated based on the considerations. Situations such as the profitability of greening after a certain number of years or with a certain reduction of sick days per employee are considered before finally a generalization of assumptions based on dimensioning factors is carried out. Due to the compactness of the considerations, the approach is briefly described in the chapter of the actual calculation.

Cost-benefit analyses and also the method of evaluation presented in the context of this article have their limitations and only provide a decision support based on a comparison and do not represent an actual decision. They contain both value estimations as well as uncertainties and contain beyond that in principle no examination of legal defaults.
