[swiftcoder]

In a heating/cooling system that has been specifically designed to improve ventilation, one can pull fun tricks like using the outgoing (stale) air to help heat/cool the incoming (fresh) air. Also in some places houses are built with enough thermal mass that the air within the building doesn't contain the majority of the heat therein.

In general there is likely some level of ventillation that will be worth taking on slightly increased heating/cooling costs.