The heat storage capacity of building materials can have a significant influence on energy consumption and thermal comfort of buildings. This paper investigates the correlations between heat storage capacity and thermal performance. A test building made of autoclaved aerated concrete (AAC) bricks was constructed in Kloster Lehnin/Emstal (Germany) with the same heat transition coefficient U for the walls, floor, and roof. The heating and ventilation of the building is controlled automatically. Xella and the Department of Building Physics/Low Energy Buildings of Technische Universität Kaiserslautern have been monitoring the test building since April 2017. The thermal performance and comfort of the AAC test building were investigated by measurements and simulations. The results were compared to simulations of a light weight building with the same geometry and heat transition coefficients U. The examinations showed that the heat storage capacity of the AAC has a positive influence on summer heat protection and thermal comfort of buildings in comparison to light weight constructions (LWC). In summer, the increased heat storage capacity of AAC compared to LWC improves thermal comfort. During the heating period, AAC buildings with a higher heat storage capacity in comparison to LWC offer the advantage of a slower cooling down of the indoor air temperature.