The new mixed-use neighbourhood and showcase Vallastaden in Sweden has been designed for social and environmental sustainability, with specific focus on active citizens’ involvement in the early phases of the planning process and the built environment structured to encourage social interaction between the residents. Part of the Vallastaden concept is low energy buildings and passive house designs. All 77 buildings are designed to be 25 % more energy efficient than the requirements in the Swedish building code and 12 residential buildings are designed as passive houses. This research project focus both on the planning, construction and management processes, and the passive house residents’ everyday life. The paper reports from the initial research conducted with a qualitative approach with interviews and diaries with residents and stakeholders. Relational materialism is our theoretical approach to analyse empirical material and guide an understanding of the socio-material as intertwined in assemblages and how different elements and entities are enacted in everyday life for low energy thermal comfort in the passive houses. Conclusions are that building designs and energy system design varies between the different buildings. A passive house in Vallastaden is multiple and comes in different shapes and with many relations between the social and the material. Work is included for both residents and professional groups to reach the desired thermal comfort and this work includes what seems to be endless tinkering with the different parts of the energy system and buildings. Eventually, different practices become established for mundane handling of the thermal comfort.