In an article on the topic, “Drain the Rain…On the Plane…the Drainage Plane,”1 notable building scientist Joseph Lstiburek argues there are two reasonable ways to deal with inward vapor drive—a “vapor throttle” or an air gap. With a small air gap (less than 4.7-mm), he writes, there is a “sweet spot” for vapor transmission control of the water control layer. That “sweet spot” is somewhere between 10 and 20 perms. If it is any more permeable, the moisture driven out of the back side of the reservoir cladding into the air space will blow through the layer, through the permeable sheathing and into the wall cavity. Any lower and the outward drying potential of the cavity is compromised.
The evolution of building wraps
Due to their durability and ease of installation, building wraps made of polyethylene or polypropylene fabric have been a popular method of protecting against moisture intrusion since the 1970s. However, as building assemblies have gotten tighter, building wraps have taken on a new function—helping to remove trapped water from the building enclosure. Their unique functionality enables them to block moisture from the outside while also allowing walls to “breathe” to prevent vapor buildup. The very latest innovations in housewrap technology are taking this moisture removal function one step further to incorporate drainage capabilities, as well.
Historically, drainage has been achieved by using furring strips that separate the wrap from the structural sheathing and framing, but today’s advanced building wrap products create integrated drainage gaps through creping, embossing, weaving, or filament spacers. By eliminating the need for furring strips, these wraps can help to reduce material costs and streamline installation.
The drainage efficiency of a building wrap is generally tested in accordance with ASTM E2273. In simple terms, this test involves spraying water onto a wall assembly and measuring its collection over time. However, given the variety of drainable building wraps available, how quickly bulk water is drained can vary significantly. The following is a look at a few methods modern building wraps use to drain water from a wall assembly.
Integrated rainscreens
One method for achieving bulk water drainage is the integration of a rainscreen material onto the building wrap. These products eliminate the cost and time-consuming labor of installing furring strips by creating a gap between the sheathing and the cladding, which facilitates both drainage and continuous airflow. Where wood strapping only vents approximately 85 percent of the wall, building wraps with an integrated rainscreen provide a continuous vented airspace over the entire surface area of the wall, providing greater drainage and more effective drying. Further, since many rainscreen products use a matrix of plastic material to achieve the gap, they are not subject to saturation and decomposition that could compromise wood furring.
Rainscreen products are recommended in areas with wind-driven rain, extreme amounts of rainfall (1016 to 1524 ml [40 to 60 in.] annually), or elevated temperature and humidity. Coastal and hilly areas are prime examples of when this technology would be ideal. In these situations, the importance of creating a drainage plane is heightened when using an absorptive cladding material like wood or fiber cement.
Wraps with spacers or dimples
Another approach gaining in popularity is the incorporation of unique spacers, or dimples, onto the building wrap itself. These drainable building wraps can achieve a 1 mm (0.039 in.) drainage gap through a pattern of integrated spacers and can be as much as 100 times more effective than standard building wraps at removing bulk water from the wall. Most of these wraps can be installed in any direction without affecting performance.
These products are recommended behind wood, fiber cement, vinyl, stone, and stucco cladding, or where budgets do not allow the use of a full rainscreen system.
Integrated fibrous layer
At the forefront of drainable building wrap technology are products that create a drainage gap through a layer of polypropylene fibers. The drainage gap created by this technology has been shown to achieve 94.8 percent drainage efficiency per ASTM E2273 without sacrificing any of the durability and ease of installation benefits builders and contractors have come to expect from premium building wraps.
Like dimpled wraps, these products can also be installed in any direction without affecting performance. They are also vapor permeable, so moisture will not become trapped in the wall assembly and lead to mold or rot issues.
Conclusion
A growing preference for reservoir claddings and new developments in building codes have resulted in specifiers taking a closer look at moisture management and drainage strategies. Each project is unique, of course, and with myriad choices of building wraps, and sheathing and cladding materials, managing water intrusion and vapor drive can be a complex equation to solve. However, the specification process is made easier with a greater understanding of how these components can work together to balance drainage and breathability of the wall, while potentially reducing material costs and helping to keep projects on schedule and on budget.
Bijan Mansouri is the technical manager at TYPAR Construction Products. He has been with Berry Global for nearly 30 years working in different technical capacities. He is responsible for building code requirements, designing and development of new construction products, training builders and architects on application of new and existing products, and creating and educating on the proper practice and installation of building envelope. He can be reached at bijanmansouri@berryglobal.com.
