Buildings stay dry because their interior space is protected from the exterior elements by a barrier separating inside from outside. This barrier must be "weather-proof" as opposed to "waterproof."
To be effective, and to be suitable for the purpose intended, this barrier must be continuous, and be permanent, either through regular renewal, without need for renewal or replacement, or maintenance. To the extent that this barrier has continuity and weather-proof integrity, the building will be weather-proof. To the extent that this barrier is breached, imperfect in its containment, or not permanent, the building will be subject to problems over its life due to water intrusion.
The system of continuous properly-integrated surfaces that keep the weather out of a building, including the water-resistive barrier, the flashings, the roofing, the roof underlayment, doors and windows and their flashings, comprise the weather envelope. Leaks in buildings occur because one component of the weather envelope was not properly integrated into another component. Determining which component is not properly integrated into the envelope, and what to do about it, is the essence of the analysis of defects involving water intrusion.
It is not uncommon in construction to observe a water-barrier in place with many holes in it. Each hole can permit water penetration, which can create rot, mildew, and reduce the effectiveness of building insulation. Holes in weather-resistive barriers result from careless handling of framing materials, careless installation of the outer skin, or inattention to details before the building is closed up. Before the water-resistive barriers are covered, they should be checked to ensure that the code-mandated standard of "free from holes and breaks" has been accomplished.
The correct method of installation of the water--resistive barrier, is best described as "weatherboard fashion," indicating that lengths of the building wrap are put up on the wall beginning at the bottom in the same nature that weatherboard siding is applied to the wall: from bottom to top with each succeeding weatherboard lapping the top of the preceding weatherboard, and itself being lapped in turn by the next succeeding weatherboard as the process moves up the wall.
This installation method ensures that water running down the wall always drains across the top of the next succeeding weatherboard in line from upper to lower, and eventually to the ground, always on the outside surfaces of the weatherboards. This method always keeps water to the outside of the building, never allowing it to get inside the construction.
All building components to the outside of the water-resistive barrier should be suitable for exterior use, while all building components to the inside of the water-resistive barrier need be suitable only for interior use. Such components are not limited to interior wall coverings, but include insulation, electrical and mechanical services, and structural components such as framed walls, floor framing, headers and beams.
Such components are typically framed with wood or light gauge steel, which are only suitable for interior applications. Such materials must be protected from the elements to maintain permanent structural integrity. Also, structural steel in retaining walls, masonry walls, hangers and supports, can be adversely affected by water penetration unless stopped by an effective, properly installed water-resistive barrier. Finishes such as carpets and ceilings, cabinet work, millwork, furnishings and equipment are all affected by water penetration to the interior through walls.
Moisture penetration through walls also has a profound impact on the livability and utility of a structure. Lack of weather-tight integrity along hallways resulting in wet floors can create slipping hazards, resulting in injuries. Classrooms which are flooded from time to time have limited use in terms of providing a suitable all-weather space for instruction of students. Operational and maintenance costs can be adversely impacted by the need to keep up with not only cosmetic, but functional repairs to all of the components affected by water penetration.
