Why an Air Tight Enclosure?
The air and windtight building envelope can be compared with clothing. In cold and damp weather, several layers of clothes prove their value ("onion principle").
A woolly sweater protects from cold (= insulation). But in cold, windy weather this sweater is not sufficient; you also need a windtight jacket (= facade membrane or breathable membrane). When this jacket is open, cold wind and moist air enter the sweater and you feel cold. If it is closed, however, which corresponds to the windtight building envelope, wind and weather are kept out and the body remains comfortably warm. This layer is formed by the roofing and the permanently sealed breathable membranes.
More than one-third of the warm air escapes through leaking areas in the building envelope. Consequently an air and windtight layer is the most reliable protection against energy and heating cost loss! Regular, controlled ventilation or a ventilation system provide for healthy indoor air quality. As American Builders develop techniques for low energy buildings, sealing the envelope airtight, combined with super insulation, makes the strategies like Passive House, and the further efforts of Net Zero, or even Plus Houses, possible. Without an air barrier, the heat will leave the envelope in the air leakage.
The Reasons Go Beyond Heat
Mold not only damages the building construction and surfaces, but it may cause serious health consequences for the occupants because of the mold spores in the indoor air, e.g. allergies and respiratory diseases. Children and older people are particularly sensitive to such influences. In new construction, as houses become tighter, it gives us the opportunity to develop new wall and roof systems that will manage the moisture that inevitably leaks into the wall and roof systems.
These new designs are climate zone specific. For the Maritime Climate of the Pacific Northwest for instance, super insulated wall systems are being built that maintain a "vapor open" exterior sheathing. The idea is to allow for the insulated cavity to be able to both "charge" with moisture during high humidity in order to allow it to "dry" to the exterior as the humidity drops.
In this way, these design goals perform like the traditional buildings of the indigenous populations and the modern day "traditional builder" who work with a mixture of timber structures, such as clay, straw bail, and mud or rammed earth infills. All of these "vapor open" materials have successfully harmonized with the flux of the planets atmosphere and remain exceptionally well performing wall and roof constructs today.
In an effort to regulate the indoor humidity, the vapor closed barrier is placed near the inside of the wall, to both keep the humidity from fluctuating beyond the comfort range, and prevent the wall and roof from getting additionally charged with the humidity that we generate from our activity.