The ‘Mass’ Wall:
A ‘mass’ wall approach involves building a wall of sufficient mass and thickness that can absorb any water that is not deflected or drained away. The ‘mass’ wall is also known as a ‘storage’ wall for this reason; the wall will hold the water until evaporation occurs. The overall performance of a ‘mass’ wall type depends on how much water the materials can hold as well as how quickly the wall can dry14.
Mass walls are the oldest method of wall construction. Such walls are solid, and continuous elements that are often, but not necessarily masonry. Historically, buildings relied on thick outer walls for structural support, but these types of walls offered other advantages as well. Thick walls tend to absorb the heat from sunlight and slowly radiate that energy when the outside temperature falls. Massive walls also managed vapor and airflow relatively well compared to the leaky windows or doorways of the times15.
The ‘Face Sealed’ Wall:
As improvements in technology and manufacturing came along, so did new building materials, which spurred a new approach to designing walls and controlling water penetration. The industry hoped that these new technologies would make it possible to create a consistent and completely impenetrable exterior surface, called a ‘perfect barrier’ or ‘face seal’, to preclude all water infiltration16.
A wall is not a monolithic structure, but rather compromised of a variety of architectural elements (windows, doors, vents, etc.). ‘Face sealed’ wall systems often require and rely on effective seals between joints in order to maintain a viable barrier. At each joint, opening and transition, the wall may be more prone to leaking than elsewhere, and as the face composition becomes more complex, it becomes more difficult to seal. Every design varies, but most incorporate gaskets, liquid sealant and caulks to fill joints and gaps. Flashing and gutters are also necessary to channel water away from prone areas and to protect them from exposure. Porous surfaces, such as concrete, are often treated with a surface applied sealant to block absorption. A few of the ‘face sealed’ systems currently in use are curtain walls, metal-clad foam panels, and some EIFS systems17.
“Because it is very difficult to build and maintain a perfect barrier wall, most walls are designed as, or perform as, imperfect barrier wall systems of either the mass type or the screened type.”18
The ‘Screened-Drained’ Wall
The third approach works around the imperfections of building materials and the propensity of most types of veneers to leak water by employing better rain control strategies. The screened-drained category encompasses many of the current wall designs used commercially and residentially. The ability for these wall systems to drain and evacuate water that leaks past the veneer and to ventilate is the primary reason why they are preferred and widely used19.
All ‘screened-drained’ systems have the following attributes in common. First, the outside layer or screen is separate from the rest of the wall by an air cavity that creates a capillary break between the outer skin and inner wall, and provides for drainage. The screen also permits air to ventilate the cavity through vents in the screen. The screen is the outer exposed face of the building, and while it is the primary defense against rain, it is not necessary to seal the face perfectly. The drainage cavity allows any water or moisture to evacuate the wall assembly before it damages the structural part of the wall. Also, ventilation keeps the wall assembly and cavity dry20.
Today, most masonry walls are not ‘mass’ type walls, but built as ‘screened-drained’ systems. They have an air and drainage cavity between the outer masonry veneer and the remainder of the wall21. These designs are far superior to ‘mass’ walls in that they are light and can be used on taller buildings while they remain drier despite the lack of storage capacity22. Other very common ‘screened-drained’ systems include lap siding, panel cladding systems, and drained E.I.F.S. or stucco23.
A Rainscreen is a special type of ‘screened-drained’ wall design that moderates air pressure at the face of the wall, and increases the level of ventilation24. Pressure moderation is critical because it the only cause of rain penetration that is not addressed by the other wall types. Rainscreens offer the most comprehensive and practical solution to rain penetration25.