**4. Conclusions and Outlook**

The calculations and analyses carried out have shown that an economic consideration of indoor greening in relation to its effects on sick leave in offices is definitely worthwhile due to the improvement of humidity in the room and its impact on human health.

The positive effects of vertical indoor greening on the relative humidity of the indoor air have already been proven worldwide and can also be confirmed for the six greened sample rooms under consideration on the basis of the analyzed measurement data. On this basis, the effects on an office space and the workplaces and employees located in it were derived.

The effects on sick leave were assumed in these studies based on expert literature and only the effect of improved humidity in the form of hygrothermal comfort was considered. Other known positive effects of greening have not been considered so far. In the office environment, these include in particular the improvement of the reverberation time and thus the influence on the room acoustics as well as the possible enhancement of the working environment through greening. It can therefore be assumed that there may be further positive effects on working life and employee satisfaction. In particular, unspecific clinical patterns related to sick building syndrome should be further considered in this context.

A comparison of these effects on the humidity of greenery with conventional air humidification systems or extended possibilities of building technology systems could also be made. However, such systems also cause costs for technical maintenance and, in addition, they only pursue the one benefit of the change in air humidity and, as technical systems, do not achieve any further advantages as is the case with greening.

It should be pointed out that this article is not a financial report, but presents a method to demonstrate and quantify the effects that have not been considered in investment decisions about vertical greening up to now.

Furthermore, it is to be differentiated in connection with the presented method between the macro economical costs of sick leave and the costs for one company, as was already explained in the introductory chapters. So far, only wage costs were considered, and no macroeconomic total calculation were aimed at, which would include also costs of insurance, hospitals, etc. The calculated profitability is therefore a conservative consideration.

Additionally, it must be taken into account that the calculations are subject to uncertainties and, especially when using living, technical systems and when considering the effects on humans, a generalization is not always exactly possible. Uncertainties exist, for example, with regard to personnel costs, prices of greenery, actual reduction of sick leave, as well as in the generalization of sick days and the assumption that the air quality in the office space considered was not optimal before. With regard to a practical application of this method by a specific company, however, it should be pointed out that the formulas presented, with their in-house data for personnel costs and sick leave, provide direct results that are subject to greater certainty.

In a further step, the extension of this presented method to other areas such as outdoor greening is possible. However, this expansion is more complex due to the fact that the effects and responsibilities cannot be clearly assigned. In the presented example of office space and the saving of wage costs, it can be assumed that the person who invests in the greening is also the one who benefits from the savings. Due to the different levels on which vertical greenery is effective in outdoor areas, as shown in [47], the analysis is therefore also much more complex. One instrument that should also be considered in this context is the Cost-Efficiency-Analysis, because it allows the intersection between buildings and urban planning to be made visible, which is also the focus of the greening of buildings.

**Author Contributions:** Conceptualization, J.H. and A.K.; methodology, J.H. and A.K.; validation, J.H. and A.K.; formal analysis, J.H.; investigation, J.H. and A.K.; writing—original draft preparation, J.H.; writing—review and editing, J.H. and A.K.; visualization, J.H.; supervision, A.K. Both authors have read and agreed to the published version of the manuscript.

**Funding:** Open Access Funding by TU Wien.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** The authors acknowledge the TU Wien Bibliothek for financial support humans through its Open Access Funding Program.

**Conflicts of Interest:** The authors declare no conflict of interest.
