Draining the rain
Advancements in engineered rainscreen wall systems
By Keith Lolley
A tremendous amount of innovation has occurred in the construction industry over the years. Yet, the ability to achieve a waterproof wall system still eludes us. If the wall is not designed properly, this moisture will remain trapped. This can cause numerous issues, such as the corrosion of structural reinforcing, rot and mold.
Trapped moisture will decrease the effectiveness of certain insulations and decrease the overall life span of the building. Roughly 90 percent of all wall failures are the result of moisture-related issues. Moisture intrusion is a concern that architects and builders need to strongly consider when designing and building.
The cavity wall system is designed to properly “drain the rain.” These systems typically are designed with a backup wall, airspace and outer veneer. Moisture management components such as through-wall flashings, mortar deflections and drainage devices at flashing locations are used to divert moisture that enters the wall back to the outside. Exhibit A is a typical cavity wall detail.
Another important aspect to a successful cavity wall is a clear cavity. Without a clear airspace, moisture will not drain effectively. A two-inch cavity is the industry standard for commercial cavity wall construction; however, with new energy codes, a call for increased insulation exists. This is causing wall space concerns.
We are seeing cavity airspaces range anywhere from two inches all the way up to more than four inches. This increased thickness inflates the overall cost of the wall system, which is making the cavity wall system less price competitive, compared to alternative design methods in the market. What if there were a better way?
Engineered rainscreen wall systems have been around for quite some time. However, the difference between a pressure-equalized rainscreen wall and a cavity wall can be confusing. First, we need to modify the terminology slightly. Pressure equalization is a lofty goal and one that is difficult to truly achieve. In reality, the goal is to create a pressure-moderated wall system. These systems are known as ventilated facades or modified rainscreen walls.
Differential air pressures between the inner wall and outside environment will draw moisture into the wall system’s inner structure. A cavity wall system does nothing to prevent this from happening. But a pressure-moderated rainscreen wall system will reduce significantly the differential air pressures that draw moisture into the building. The is due to the air allowed into the wall system, which neutralizes the air pressure behind the cladding to the air pressure outside of the wall system (see Exhibit B).
A pressure-moderated wall system consists of a backer wall, through-wall flashing, A/M/V barrier, outboard rigid foam insulation, a clear vented airspace with ventilation devices at the top and bottom of walls, and a tough exterior cladding.
Since new energy codes are increasing the overall cavity depth, these wider cavities will take longer to neutralize. The wider the air space, the more air needed to enter said airspace for neutralization. By incorporating an all-wall drainage mat in these rainscreen wall applications, the airspace can be reduced greatly without compromising the functionality of the intended airspace.
For example, Exhibit C shows a 16-inch-wide wall system with two-inch rigid outboard insulation and a 2.75-inch airspace. By installing an all-wall drainage mat as seen in Exhibit D, the airspace can be reduced to 1.75 inches, which will neutralize the air pressure quicker.
More important, it also allows the designer to increase the R-value of the same 16-inch-wide wall system. If increased insulation is not desired, the overall wall system can be reduced greatly and still drain and ventilate as intended. Using an all-wall drainage mat also will reduce the cost of the overall wall system.
The December 2013 issue of “ASK IMI” included research on all-wall drainage mats. What was found was that an air space with a 3/8-inch continuous drainage mat can be just as effective as a two-inch air space, where increased insulation and a minimum wall thickness are design parameters.
All-wall drainage mats typically are made from either a corrugated sheet, dimpled mat, or a random entangled net material. These drainage mats are favorable for these wall applications for a number of reasons.
- Mold and mildew resistant
- Allow multi-directional drainage and ventilation
- Resistant to most known chemicals
- Manufactured from recycled materials
- Class A fire rating (ASTM E84)
- Compatible with freeze-thaw conditions
We are seeing the importance of all-wall drainage mat technology more in our industry. Organizations such as the Building Enclosure Moisture Management Institute (BEMMI) have worked with stakeholders in the industry to establish minimum requirements for engineered rainscreen materials. BEMMI is now in the final stages of obtaining an ASTM standard for polymeric sheet materials used to provide rainscreen functions.
Other organizations are acknowledging the need for all-wall drainage mats, because these mats are not only beneficial in cavity wall applications. Stucco and manufactured stone applications are seeing the value of a drained cavity created by these all-wall drainage mats as well. The cavity created by these drainage mats gives a cavity wall concept without the cost of a true cavity wall system.
Points of consideration for specifying and using an all-wall drainage mat:
- Rainfall totals and frequency
- Wetting and drying cycles
- Wind and storm conditions
- Freeze-thaw conditions
Areas that have high wind content are prime candidates for rainscreen drainage mats. For example, a 50-mph wind will exert 6 pounds of pressure per square inch on a wall’s surface. This is enough pressure to force moisture into cracks of any size.
Originally published in Masonry magazine.
About the Author
Keith Lolley is VP of Advanced Building Products and serves on the board of directors for the Building Enclosure Moisture Management Institute (BEMMI). He can be reached at firstname.lastname@example.org.