Passive houses differ from ‘normal’ houses due to the fact that they are built without a conventional heating system. Through their optimised insulation and exceptionally low demand for heating, of under 15 kWk/m²a (normal houses 100 – 180 kWh/m²a, energy saving houses in Vorarlberg approx. 55 kWh/m²a, 3 litre houses approx. 30 kWh/m²a), the required remaining heat can be provided using a controlled aeration and de-airing system with heat recovery.