Key Points - The Rain Screen Principle
and Pressure Equalization
- The rain screen principle is a concept requiring, 1) a tight seal at a dry area within the wall system, then 2) equalization of the pressure from this point outward with that of the outside air. This is done by placing slots or openings in the outer face, and additionally, placing a screen behind, or “weep hood” over, such openings to prevent the entrance or blockage effect of rain water.
- For water leakage to occur, three ingredients are essential; 1) water, 2) an opening, and 3) a force to move the water through the opening. If any of the three are missing, there will be no water leakage.
- Proper design starts with tightly sealing the inner contact of a two point contact window frame and ventilator assembly (or similarly, establishing a tight, unbroken inner seal on a wall system). We then “open up” the structure sufficiently, from this point to the exterior, by means of slots, holes or continuous openings.
- The pressure against the exterior face of the window caused by the wind will extend itself through the openings, and produce the same pressure within our window frame as we have on the exterior. Because the pressure within the frame is equal to that on the outside, water will not be drawn or sucked into the frame where it can reach our tight inner seal â€“ which is now the “dry” boundary of the pressure drop.
- In a properly pressure-equalized design utilizing an effective rain screen, pulsed, cyclical, or dynamic water testing methods will seldom produce leakage in systems that have passed static pressure tests.
- Wind velocity “v” creates static pressure (Pressure “P” = 0.00256v2); and further, note that a steady wind velocity pressure will sustain a certain head of water (Water Head “h” = 0.00048v2).
- Most architectural window and curtain wall specifications require a test in which no water leakage will occur through the window or structure when it is subjected for 15 minutes to a water flow of five gallons per hour per square foot, with an outside pressure between 8 psf and 15 psf.
- If curtainwall covers are cut short at ends for drainage, care must be exercised during installation to ensure than adjacent frames are not “set tight”, plugging weep paths. Similarly, if holes or slots are provided on the underside of covers, ensure alignment when looking up at the wall.