Airtight Construction in a Passive House
Indoor air has a higher water vapour content (absolute humidity) than outside air - if not
dehumidified. In a cold climate, indoor air is cooled during exfiltration flow inside out. The colder air can not
keep the high amount of water vapour - condensing will occur at a certain place within the construction. This may
lead to serious damage. The process described here is the main reason why external building structure has to be
The key to making a building airtight is the principle of a “continuous, tight building envelope“.
The “level of airtightness“ can be determined by creating high and low pressure inside the building.
Airtightness is an important requirement for any energy-efficient building, however, it is not the most
important one (as sometimes suggested in popular publications - the most important requirement is good
thermal insulation). Example: Passing the blower door test is a necessary prerequisite for the functioning passive
building; yet it is not sufficient. Example: a conventional aircraft shell is airtight (much more airtight than a
Passive House needs to be), but it is not insulated to Passive House quality In aircrafts there is abundant heat
available for cabin heating.