Key challenges for Germany’s policy on energy performance upgrades in post-war apartment buildings: an interdisciplinary investigation

• Germany wants to energy retrofit its older dwellings of low-income households. • Our qualitative expert interviews build on our quantitative findings in this field. • We confirm that deep energy retrofits seldom pay back through energy savings. • CO 2 emission reduction should be prioritized over excessive energy efficiency. • Government viability calculations should take the prebound effect into account. Germany needs to sharply upgrade the energy performance of its very large stock of post-war apartment buildings, to meet climate goals and improve the thermal comfort of occupants, especially low-income households. The upgrade rate lags far behind what is needed to reach Numbered these goals. The federal government funded a research project in 2022–2024 to explore how the rate and depth of upgrades could be increased affordably. The results of quantitative research in this project are published in 12 peer-reviewed papers. This paper adds a qualitative dimension, by reporting on semi-structured interviews with experts in the field who reflected on the quantitative findings. The interviews mostly affirmed the quantitative findings, but enhanced these with insights into their policy implications, plus a broader social dimension of which actors need to take responsibility for the quality of these buildings and for finance gaps in renovation projects. Policy implications included: acceptance of the financial payback gap, recognition of the prebound effect, subsidy reform, reform of legal fees and taxes in downsizing, and schemes to motivate small private landlords to take more financial responsibility for the energy performance of their properties. The study confirmed the value of interdisciplinary research on a seemingly intractable problem.