The initial interest for the following work corresponds to thermal performance expectations of an existing eco-building in Austria. The building was renovated in 1996 in order to exceed the energy requirements of that time with the aim to design a precursor of ecological and energy efficient buildings by using natural materials and renewable energy sources. This work examined the thermal performance of the building. It explored to which extend the building meets thermal performance expectations of the past in comparison to standard requirements of today. The examination included the energy use and the thermal comfort of the building. In addition, the efficiency of the systems installed in order to use renewable energy sources was investigated. Former noted information of dial counters in the heating system were considered first in order to state the heating demand of the building and the energy provided by the solar thermal system. In the initial phase of this work, a monitoring system was installed in the building to measure actual conditions of energy use and internal climate. This data was used to conclude with the heating demand and the thermal comfort conditions. To put the outcome in relation to standard requirements, a benchmark was set with the generation of energy certificates based on different requirements regarding energy demand of the past and today. In order to detect optimization possibilities to save energy, the building was modeled for simulation purposes. These analyzing methods were compared according to current results of the analysis. In the context of today's standard requirements, the thermal performance of the building reached middle range classification. The analysis showed potential in order to save energy in the building. The heating demand of 65 kWh.m-2.a-1 can be reduced by changing the system's operation which would also improve the thermal comfort in the building. The implementation of systems to use renewable energy sources in a construction that was based on the usage of natural materials showed beneficial results.