In the challenge of climate change, energetically improving the existing residential buildings offers a way to reduce the CO2 (carbon dioxide) emissions of the built environment. In the Netherlands, housing associations own and maintain almost a third (2.3 million housing units) of the entire Dutch housing stock and could therefore play a significant role in the reduction of CO2 emissions. To come to this reduction, housing associations first have to choose which housing unit they would like to energetically improve, followed by the decision of when and how. In this decision- making process, they utilize data they have generated themselves to make an informed decision. However, the role of data that is generated by another person or organization is unclear, and even less clear is the role of open data in this decision- making process. Against this background, this thesis explores the role of open data in the decision-making process by housing associations in the energetic improvement of housing units. It studies the approaches housing associations apply in the energetic improvement of housing units, stakeholders of housing associations in the energetic improvement, possibilities and limitations of the available and relevant data to the decision-making process, desirable features of the conventional decision-making support systems and unconventional yet relevant decision-making support systems regarding the energetic improvement by housing associations. The data is collected by reviewing policy documents, conducting and transcribing sixteen semi-structured interviews and observing decision making support systems. The research shows that open data could play an enabling role in the decision-making process regarding the energetic improvement. It informs housing associations about the plans of stakeholders, what is happening in the vicinity of their real estate and explore pairing opportunities. It is very useful to housing associations in a first analysis but might fall short for more extensive analysis. Thus, open data enables housing associations to start using another person or organization’s data in their decision-making process. What is of importance for data analysis is the housing association’s primary data and its data quality. Anyone can analyze open data but analyzing open data in combination with the housing association’s primary data is what makes it valuable in the decision- making process regarding the energetic improvement. In the end, it is not the open data itself which will cause a reduction in CO2 emissions. It is the collaboration between stakeholders in the heat transition which will. The necessary collaboration in the heat transition not only requires open data, but also the acknowledgement and involvement of stakeholders in the decision-making process regarding the energetic improvement.