The scientists wanted to find a way to measure how well a building keeps heat inside while people are living in it, not just during planning. They developed a method to figure out how good a building's walls and windows are at keeping heat in, using data from homes in use. They tested the method on a well-insulated house in Brussels and found promising results. The method could accurately measure how well the building kept heat inside both with and without people inside, showing that it works well. More tests are needed to make sure the method can be trusted for all types of buildings.