The roof is the largest area of heat loss in a house besides the facade. Without adequate insulation, about 30% of household heat is lost through the roof.
As heat naturally rises, it will escape almost unobstructed through the ceiling if the rooms below are heated either rarely or not at all or if they lie directly under inadequately insulated flat roofs.
The best time to think about thermal insulation of the external walls is when the façade is being renovated or when you are considering replacing your windows.
