In the wake of an ongoing research project the late works of the Austrian architect Konrad Frey are going to be honoured. Konrad Frey can be seen as a pioneer of energy-efficient architecture who adapted the principles of modern solar houses already in the early 1970s. A number of Konrad Frey s key buildings are evaluated and discussed by means of computational performance simulation methods within the nationally funded interdisciplinary research project that is being carried out by TU Graz in cooperation with TU Wien. This master thesis focuses on thermal performance evaluation of Konrad Frey "s "Kindergarten Pachern" (built in the late 1990s, located in Hart, near Graz, Styria) by means of deploying building simulation. The envelope design of "Kindergarten Pachern" follows design approaches that are characteristic of solar houses. On the one hand, the building is embedded in the local landscape's morphology to minimize heat transmission losses and on the other hand, the facade opens up generously towards the south to optimize solar gains. Furthermore the roof is extensively green to offer a passive cooling strategy in the summer and can be used as a huge open air playground. "Kindergarten Pachern" is a single storey building with a total floor area of 600m2 that contains standard facilities for the educational usage type "Kindergarten" such as three group rooms, one gym, two office rooms, sanitary facilities and a kitchen. As a large number of randomly interacting variables combine to dictate the thermal performance of a building, the attempt of this project is to capture these complex natural processes virtually as accurately as possible by means of computational simulation scenarios. Therefore a set of environmental sensors and a weather station are deployed at "Kindergarten Pachern" to examine and support the accuracy of a virtual building model. A research team of TU Wien equipped the "Kindergarten Pachern" with a detailed monitoring system in the summer of 2016. The long term monitoring was carried out for one year to support the energy performance simulation process and the documentation of the building's actual performance. A simulation model of "Kindergarten Pachern" was designed that was calibrated and evaluated by implementation and comparison with monitored data. The aim was to deploy data streams about outdoor and indoor building conditions to both populate the initial simulation model and to maintain its fidelity through a systematic calibration process. Furthermore plan material that was provided by the Municipality Hart and several on sight visits were incorporated in the process of the simulation model generation. To improve the accuracy of the simulation model interviews with the staff at the kindergarten were carried out on a regular basis to get an insight into user behaviour and the daily routine. Based on a calibrated model of the current state of the building, it was tested whether the original intentions of Konrad Frey in terms of thermal performance and comfort were met. As there have been some changes in the building compared to the original design of the building in the 1990s, previous states of the building were virtually reconstructed and evaluated with the calibrated simulation model. In a follow-up project, a "user manual" is going to be designed that addresses the user behavioural aspects and provides suggestions for the staff at "Kindergarten Pachern" on how to improve the thermal building performance by slightly changing their daily routine. On the one hand, this work points out that monitored-based building performance simulation can be considered as a powerful method to predict the "behaviour" of the buildings. As demonstrated in this thesis, the virtual model of "Kindergarten Pachern" reaches a precision level of around 5% (CV(RMSE) - values) difference of simulated and measured indoor air temperatures, which is a satisfying result. On the other hand, weaknesses and inaccuracies of building performance simulation which could be detected in the course of the work are discussed.