In modern buildings the heat recovery system in the ventilation transfers the heat from the outgoing warm air to the incoming cold air, which saves a lot of ene

In modern buildings the heat recovery system in the ventilation transfers the heat from the outgoing warm air to the incoming cold air, which saves a lot of energy. The savings are big especially in colder climates. (As a bit of trivia, the specific heat capacity of air is pretty close, +- 50%, to common building materials. Explained by the rather low density of air at normal pressure and temperature compared to those.)