The Role of Small Gaps Behind Wall Claddings on Drainage and Drying

A common rain control strategy to help minimize moisture damage is the rain screen wall system. This approach requires a rainscreen, a drainage plane, flashing, weep holes, and a drainage gap. Although this approach to rain control is becoming the most common, and is sometimes even mandated by codes and standards, very little research has been undertaken to define the minimum or optimal gap size required for either drainage or ventilation drying. A test method and experimental program were developed to investigate the gap size required to ensure drainage, and the role of small gaps on ventilation drying. Because of the importance of small geometrical details, full-scale wall systems were tested. The experimental method was able to gravimetrically determine the amount of drainage, storage and drying during and after a simulated wetting event. The test apparatus and method developed were shown to provide repeatable results over multiple tests as well as in an independent laboratory. The experiments to date have conclusively shown that even small gaps (less than 1 mm) can drain more water than would normally be found in a drainage gap. It was also found that in some cases small gaps will store less water than a large drainage gap. It was also found that ventilation drying can play a role in very small gaps of approximately 1 mm, at a pressure difference of only 1 Pa. More research is required to further analyze optimal ventilation gap sizes and compare the laboratory results to hygrothermal modeling