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The Philosophy of Rainscreen:

"The rainscreen principle is a theory governing the design of a building enclosure in such a way as to prevent water penetration due to rain." [1]

In order for water to penetrate a wall, three components must be present:

• water
• an opening
• a force or pressure differential to move water through the opening

The first component, water, will always be present. The traditional construction wall efforts focused on trying to eliminate the second component, the opening, by caulking the joints in the wall in order to diminish any openings for water to penetrate. This is an ineffective solution for many reasons, including:

• Caulking material needs to resist both rain and air (pressure differentials), which requires absolutely no rain leakage paths. Because of this, caulking needs to be perfectly applied in order to be effective, and perfection is impossible to achieve.
• Caulking material is directly exposed to the elements, where the weather causes the material to expand and contract, and finally crack; therefore the caulking needs to be consistently maintained, and this maintenance is costly.
• Caulking material deteriorates and fails when exposed to UV rays.

Since eliminating the opening was rendered an imperfect solution to the problem, efforts turned to the elimination of the third component, the force that drives water through an opening. The forces most difficult to combat in this capacity are air currents and pressure differentials. "Air currents result from differences in wind pressure over the wall surface, or from convection within wall cavities, and these may carry water into the wall. Also, when water is present on one side of an opening, and the air pressure on that side is greater than that on the other side, the water will be moved through the opening, no matter how small, in the direction of the pressure drop."[2]

The rainscreen principle attacks the pressure differential problem by purposely leaving the joints in the façade open. This allows air to move freely between the exterior environment and the interior cavity, and the result is a pressure equalization between the two:

"The only practical solution [to preventing water leakage] is to design the exterior rain-proof finishing so open that no super-pressure can be created over the joints or seams in the finishing. This effect is achieved by providing an air space behind the exterior finishing, but with connection to the outside air. The surges of air pressure created by the gusts of wind will then be equalized on both sides of the exterior finishing."[3]

Air that is continuously flowing through open joints, ensuring a pressure equalization between the exterior and the interior cavity, is also functioning as a ventilation system for the interior cavity. The ventilation of the interior cavity is important because it provides natural air conditioning and prevents heat build-up in warmer climates, and it protects the thermal insulation and vaporizes any penetrating humidity in colder climates.

Most of the buildings in North America were completed using the traditional method, by caulking and sealing the envelope. Professionals in the industry know that this is not an effective method. However, a new trend is evolving which prompts the use of products based on the rainscreen principle -- the rainscreen principle is the superior method for sustainable wall construction.

The European back-ventilated aspect of rainscreen systems: When windows or curtain walls are pressure equalized (often called the Canadian Method), it still requires perfection in building a continuous air barrier. The European or back ventilated version does not require perfection in installation. The open joints at the bottom of the system, allows air to move up and out and water to flow down the back of the panel and out the bottom.