10 October 2009

Water Infiltration through Exterior Walls - Pressure, Part 4 of a 4 Part Series

There are four ways that water enters a building:
  1. By Gravity.
  2. By Momentum (kinetic energy).
  3. By Capillarity.
  4. By Pressure.
When you buy a beverage at a fast-food restaurant, it comes with a cap and straw. The cap is useful if the beverage gets tipped, because it retains most of the liquid--perhaps even all the liquid if a sufficient vacuum is formed inside the cup. But when you use the straw to drink it, you create an area of low pressure in your mouth which is sufficient to overcome the gravity and vacuum that is trying to keep the beverage in the container. If the beverage is thick like a milkshake, a greater pressure differential has to be induced or a larger straw used to overcome its greater viscosity, but it is still a pressure differential that enables you to successfully drink your beverage.

Building exteriors work the same way. As long as they are capped and closed in, water tends to stay where it is supposed to stay--outside the building. But they tend to be full of straws of all shapes and sizes due to changes of materials, construction joints, and material deterioration, and they tend to be subject to pressure differentials due to HVAC balancing, stack effects in tall buildings, and wind pressures that rapidly change not only in strength but also from positive to negative.

Just as pressure can overcome a variety of forces in the beverage example, the pressure differential in buildings can and will overcome gravity, momentum, and capillarity to draw water into the building. As there are ways to manage gravity (using flashing to continuously direct water down and out), momentum (blocking the line-of-sight holes), and capillarity (installing capillary breaks), there are ways to manage pressure as a water transport force.

The outstanding concept is to maintain the pressure inside identical to the pressure outside. Then, only the gravity, momentum, and capillary forces are available for water transport. However simple this sounds, in reality it is impossible when dealing with wind loads. The windward side will be under positive pressure and the leeward side will be under negative pressure. The volume inside cannot match both simultaneously.

However, if the exterior skin has an internal cavity that is composed of multiple compartments, each compartment can be pressurized according to the wind pressure in effect at its location, which may be different from a neighboring compartment. The pressure differentials are isolated to the interior wythe only, between the compartments and the interior of the building, rather than spread out through the entire thickness of the exterior enclosure. Although a pressure differential is still available to help transport water to the interior of the building, if the exterior screen has done its job, no water will be available to be transported at the point where the pressure differential takes place.

No assembly that we have can instantaneously and completely equalize a compartment to the exterior. Fortunately an instant response is not necessary because the water transport is also not instant. If there is a small enough time lag and great enough response to changes in pressure, the assembly will be successful. In other words, it is really a case of "pressure moderation." How much moderation is required to be successful is the subject of research. According to AAMA 508-07 "Pressure Equalized Rain Screen Wall Cladding", it should be 50% of the pressure within 0.08 seconds. Many manufacturers have been developing systems that have been tested to this standard.

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