Passivhaus is a higher standard with thicker insulation and higher standard windows. After two weeks they will be opened and the amount of ice weighed to see how much heat has managed to get into the building. The one with the most ice left will be the more energy efficient. Passivhaus has five principles that reduce the amount of heat loss. They are high-quality thermal insulation, windows with triple glazing, avoidance of thermal bridges, an airtight building envelope, and a ventilation system with heat recovery . It means a Passive House building needs very little energy and they can do without the need for classic building heating systems.