CISBAT 2021 took place from 8 to 10 September 2021 online and at the Swiss Tech Convention Center on the campus of the Ecole Polytechnique Fédérale de Lausanne (EPFL) in Switzerland under the heading "Carbon Neutral Buildings - Energy Efficiency & Renewables in the Digital Era". Focused on energy efficiency and the use of renewables in buildings and cities, CISBAT bi-annually offers a dynamic platform for scientific exchange in fields ranging from nanostructured materials for renewable energy applications to urban energy systems. While recent editions have seen machine learning applications for the built environment receive ever greater attention, CISBAT 2021 has further extended its reach to include research promoting circular economy, thus offering insights into a broad range of fields that ultimately have the same target: carbon-free, sustainable buildings and cities.