I am currently working on a penthouse apartment. The following was communicated to me by the expert. -With external roof insulation, the heat development penetrating the apartment through the roof is reduced. Just like the cold that penetrates from the roof. Likewise, the warmth escaping from the apartment through the roof is reduced, including the cold. -For penthouse apartments, how the roof itself is insulated is less relevant in summer. It does make a difference from 1970s insulation to today's insulation, but the key point for a penthouse apartment is always the windows. -Windows must be as well insulated as possible, meaning triple glazing and especially important is external shading of the windows. If you don’t have suitable external shading for the windows, all insulation measures in summer are practically pointless. However, a penthouse apartment will always be the warmest point in the house. Annoying in summer, nice in winter. Fortunately, here in Germany, for three quarters of the year we have temperatures where the heat bonus is an advantage. My penthouse apartment had 1970s standard insulation, 3 roof windows without external shading and insufficient ventilation options during the hot phase of summer at night 28°C. The new roof will be installed soon, and the hot phase of the summer is approaching as well. I will get 18cm external roof insulation, all roof windows will be removed, dormers with vertically standing windows and external shading will be installed. If I don’t forget, I will gladly report on how the temperatures behave.