Wind pressure on a large building exerts pressure that flows in two directions. When wind is blowing—pushing at a building assembly—it is considered a positive pressure; when wind draws air away from an assembly, it is referred to as a negative pressure.

Now add water. In a rain storm, water is both pushed and pulled by the wind. On a large building, some areas are more vulnerable than others, due to the prevailing pressures created by air flowing around the geometry of the building: As wind flows around corners, it accelerates, and eddies of air create a high degree of pressure fluctuation. Around the sides and top edges of the building, the air is moving faster, driving rain at a greater velocity against these parts of the wall. At the center of the building, the pressure will be fairly constant, resulting in a “cushion” of high-pressure air. This pressure will be positive (pushing against the building) on the windward side, and negative (drawing air away from the building) on the leeward side. Not as much rain reaches the building in this “dead spot” near the center as compared to the top and corners.

On a tall building, 20 to 50 percent more rain wets the building at the top and corners than at the center area. However, some variability in wetting patterns will occur on multistory buildings. The taller and narrower the building, the more intense the potential for wetting will be. The more corners, bump-outs, parapets, and other differentiations in the building shape, the greater the chances of creating negative pressure flows that are capable of drawing water laterally and even upward through crevices in the façade.

Designing a Greener Building

While the all-glass façade may garner the praises of architectural critics and urban planners hoping for the next architectural splash on the city’s skyline, building performance can suffer. When glazing is overused, the thermal characteristics of glass, which is highly conductive and offers almost no resistance to heat flow, often result in a poor-performing building envelope. “From an energy standpoint, I have my doubts whether a building with floor-to-ceiling glazing on every floor can be a green building,” says building scientist John Straube. “Numerous studies have shown there are no daylighting or energy benefits with exteriors that have very high window-to-wall ratios.” A truly green building reduces energy loads while effectively managing exterior moisture, the bulk of which comes from wind-driven rain. Both energy and daylight performance benefit from a ratio of window-to-wall area between 25 and 40 percent, explains Straube. From a performance standpoint, a façade that combines opaque claddings with glazing will result in a greener building. Today’s design challenge is to make an architectural splash without resorting to simple aesthetics that compromise performance.

 

The type of cladding material used on the building has a big impact as well. Porous masonry exteriors will absorb and hold a high percentage of the water that the building sees, and will release this moisture slowly over time by diffusion (the movement of moisture vapor through a building material by evaporation). On the other hand, with a non-porous cladding such as a glass curtain wall or composite metal panel system, most of the water striking the building will run down the face, and the chances for the wind to drive this water into joints will increase. This suggests that the joints in non-porous claddings will require extra attention, especially near building corners and near the top of the building. Detailing in these wetting areas will be critical to the building's ability to manage wind-driven rain. In most real-world cases, some degree of leakage is inevitable, and even the best, and most carefully installed cladding systems should have some capacity to manage water that penetrates the exterior skin.

General Approaches to Keeping Water Out

There are several approaches to keeping wind-driven rain out of walls:

• Barrier systems attempt to stop all water at a single plane. Typically this plane is the outermost layer of the building envelope—the cladding, sometimes referred to as the outer “leaf.” To stop the water at a single plane, the joints between cladding members must be resistant to the penetration of driven rain over time. Barrier systems are sometimes referred to as “perfect systems” or “perfect walls” or “zero-tolerance wall systems” because the exterior layer must be a perfectly sealed plane. There is no forgiveness in the system, and if that plane leaks, there is typically no accommodation for the water that gets past it. These systems are also sometimes referred to as “face seal” systems. However, the term “face seal” should refer to the seal on the outer surface of a cladding joint. A face seal contrasts with a “drained seal,” “a concealed seal” or a “two-stage seal,” all of which refer to more durable methods of sealing a barrier system.

• Drainable assemblies, often referred to as a “drained cavity wall” or “drained/back-ventilated (D/BV)” system, take into account that some moisture will be driven past the outer skin and allow this moisture to drain harmlessly away. The system consists of two layers separated by an air space. The outer layer is the cladding that sheds the majority of rain water. The inner layer is usually the exterior surface of the structure, and this surface must be protected by a weather barrier to shed any water that does get past the cladding. The air space between the two layers serves as a place for water to drain down to the appropriate base flashings. It also allows air to circulate behind the cladding. This “back ventilation” helps dry the wall by promoting evaporation. Note that a D/BV wall is sometimes called a “rainscreen,” particularly in residential applications. However, it's important to be precise with the term “rainscreen.” Here, the use of “rainscreen” is reserved for a pressure-equalized rainscreen (PER).