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 built airtight.

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.

Source: http://passipedia.passiv.de/passipedia_en/planning/airtight_construction