Mold is a huge area of concern for owners, architects, and builders. Some attorneys forecast litigation comparable to that resulting from asbestos, and the insurance industry is in turmoil over potential claims. New legislation is proposed at national, state, and local levels to deal with mold problems. Clearly, the construction and real estate industry needs to find ways to combat this new problem in exterior façades and curtainwalls.
Airborne mold spores are virtually everywhere, but they need food and moisture to grow into health problems. Eliminating all mold food sources is nearly impossible, so it is imperative to control the moisture that mold requires to proliferate. Accordingly, a new level of diligence is required to keep exterior wall systems dry. Not only are leaks of heightened concern, but condensation is also a significant factor. Air barriers and rainscreen systems are excellent systems to accomplish both.
Air barriers are a growing technology, but are not yet widely understood. Simply put, an air barrier is a continuous system that limits the movement of air into and out of the building. The barrier must be virtually air-impermeable, permanent, capable of withstanding loads from differential air pressure, accommodating of building movement, and must form an envelope around the entire building from foundation to roof. Many common building materials, such as exterior gypsum sheathing, sealant, and sheet metal flashing, can function as an air barrier. Of particular concern in the installation of air barriers is continuity across joints between various building systems, including windows, doors, penetrations, and the joints at floor and roof lines. The location of the air barrier within the wall assembly dictates whether it should be vapor permeable or a vapor retarder.
Many wall systems unrealistically base performance on a perfect barrier to the weather by sealing every surface, joint, and penetration. Rainscreen systems block water penetration at an outer layer – the rainscreen – by eliminating the forces that could drive water through the inevitable joints and penetrations. For a rainscreen system to function, a vented air cavity and an inner air barrier are required. Under ideal conditions, joint design and cavity air pressure equalization eliminates the passage of water beyond the rainscreen. A practical and prudent approach anticipates that a small amount of water will penetrate the rainscreen. The air barrier, therefore, should be waterproof in addition to its other criteria, and the air cavity should be drained to the exterior. Placing the building insulation in the air cavity and selecting an air barrier that is also a vapor retarder provides a complete system. The air barrier in this application is typically an applied membrane (i.e. self-adhesive, or torched down sheets; spray-applied or toweled mastics), which covers the entire surface and ties into the foundation and roof. This results in an extremely reliable wall system that minimizes water intrusion, controls condensation, and separates the interior environment from moist materials.
In the past, typically only glazed curtainwalls were designed to function as a rainscreen system. Now, common applications include brick veneer, metal panels, wood siding, and stone. The rainscreen and air barrier approach, with inherent safety and redundancy, provides higher-quality wall systems with more resistance to mold problems. The incorporation of these systems into exterior wall design is likely to continue to grow.
For more information on air barrier and rainscreen design:
David Altenhofen AIA, CSI, CCS is chief of Architectural Technology at Kling (www.kling.us), Philadelphia, and chair of the AIA National Building Science Committee.