By Christine Zimmer and David Deress
There are many types of exterior wall materials that can be used to assemble building-envelope systems. These systems can be used together in an infinite number of combinations to create unique exteriors that are limited only by imagination and, for most of us, by cost.
To keep exterior wall systems functioning properly, moisture must be prevented from infiltrating into and through the wall system; otherwise, it can cause mold and other biological growth to proliferate. Moisture exists in two forms: water and water vapor. This article focuses on moisture in liquid state that infiltrates the wall from the exterior and migrates toward the interior.
In designing exterior wall systems, moisture infiltration is controlled following two basic design concepts or strategies; either design strategy can be applied to load-bearing or non-load-bearing walls.
Barrier system. The first design concept is the barrier system, where all moisture is resisted by the exterior surface material (also often called a "surface-sealed system" or a "mass wall system"). In surface-sealed versions, if there is any breach in the exterior surface, infiltration can occur. The system generally consists of the exterior cladding element that may be the only element of the wall, or it may be applied to a structural assembly like wall studs and sheathing with insulation. One of the first generations of exterior insulation and finish systems (EIFS) available in the United States were barrier systems that relied on sealant joints to create a watertight interface between the EIFS and other wall elements (like windows). An example of the mass wall variation on this concept is a solid masonry wall that will absorb some amount of moisture, hold the moisture, and then let the moisture evaporate after the rain stops. In this form, cracking through the depth of the wall can allow infiltration.
Drainage system. The second design concept is the drainage system. This system usually consists of the cladding material that is exposed to view at the exterior with the structural-support components of the wall, insulation, and air and water vapor retarders located to the interior side of the cladding. Protecting the structural portion and other water-sensitive components of the wall from water contact is a weather-resistive barrier that allows for breathability without letting water pass. The cladding material is the first line of defense against rain.
It's assumed in the design and detailing of this type of wall that most moisture will be resisted at the exterior face of the cladding. Behind the cladding and in front of the weather-resistive barrier is a cavity or internal drainage plane that provides a path for the water to drain down to a collection point and out to the exterior. An example of this type of system is a brick veneer with a steel stud or wood back-up wall. The brick veneer is the exposed cladding at the exterior; behind it, there is a cavity to allow drainage. The steel or wood back-up is the exterior wall framing that resists wind loads and, depending on how the floor/roof tie into the exterior wall system, the steel or wood back-up may also resist the loads from the floor and roof above. Additionally, a masonry veneer with a concrete masonry back-up can also be a drainage system if a cavity is provided between the concrete masonry and the masonry veneer. The concrete masonry is the back-up serving as the structural component and may or may not have a weather-resistive barrier applied to it.
A variation on the drainage-system concept is a rain-screen system with pressure equalization that has a cladding component. This cladding serves as a rain screen with compartmentalized cavities behind it that allow the air pressure inside to equalize with outside pressure through the use of vents and openings so that wind-driven rain isn't drawn into the interior side of the cavity. There must be air and weather-resistive barriers (which could be a single component behaving as both) toward the interior side of the cavity for this system to prevent negative air pressure from drawing water into the wall assembly. This is used mostly in curtainwall design. In this type of drainage system (and in all drainage systems), flashings and weeps or weep holes are used to collect and drain water that has found its way into the cavity. The weather-resistive barrier and air barrier must be placed with great care to keep water on the outside face as it flows down the surface to the point where it's collected and drained to the exterior.
Common Moisture-Control Concepts for Exterior Wall Systems
Exterior Wall System Type
Common Moisture-Control Concept
Reinforced and unreinforced cast-in-place concrete
Precast concrete, including tilt-up
Architectural precast concrete, including glass fiber reinforced panels
Solid reinforced and unreinforced masonry
Masonry cavity wall with masonry back-up
Masonry veneer with steel or wood stud back-up
Siding/veneer with steel or wood back-up, such as stucco, fiber cement or vinyl siding, or interlocking metal panels
Aluminum frame curtainwall, including storefront systems
Barrier or Drainage
EIFS designed with no provision for drainage
EIFS designed with a provision for drainage
Panelized rain-screen systems
In poorly designed barrier and drainage systems, sealant joints are relied upon as the primary means to resist water infiltration. Any breach of the sealant joints can trigger leakage to the interior at interruptions in the wall assembly. In most cases, the field of the wall will probably perform very well, but the interruptions in the wall assembly are where water-leakage trouble is usually found.
Penetrations. Besides an entrance/exit, most buildings have additional doors, windows, etc. that create holes or openings in the exterior cladding system. Infiltration through the joint between the wall opening and the element that is set inside it will occur if not properly detailed. As water flows over the outside surface of the wall, there needs to be a means for letting this water continue to flow down when it comes to the top or side of a penetration through the wall. For barrier systems, this is usually the only consideration. For drainage systems, water flowing down and coming in contact with the penetrating element must be considered. Additionally, any water that has entered into the drainage cavity must be able to flow past or around penetrations without migrating through the weather-resistive barrier and flashings. The interface of the penetrating element through the entire depth of the wall has to be protected to keep water from infiltrating further into the wall assembly.
Transitions. Transitions are an interruption in the exterior wall system or a horizontal change in the wall (like an intersecting parapet wall with a taller wall, a ledge, or a soffit). They also include intersecting building elements. These conditions must also be addressed in the design and construction of the exterior wall system to keep water flowing down the wall and draining out of the system.
Terminations. Terminations are the locations where the system ends at the top, bottom, or side. Water should not be allowed to enter an assembly at its top or sides. If it is a drainage system, it should be allowed to drain at its base. If two or more types of wall systems adjoin, control of water infiltration and/or drainage must be integrated or handled independently.
Barrier System Considerations
For a barrier system, sealant is often used to make the joint between the wall and other components watertight. If the sealant isn't completely watertight, water infiltration can occur. This is also true for transitions and terminations of the wall system. Overall, there is little opportunity for redundancy in preventing water infiltration.
In a solid masonry or cast-in-place concrete wall, cracking can be a big problem if the cracks continue through the entire width of the wall. Even though the wall material soaks up some of the water and most just rolls down the face, water that gets into the cracks has an avenue to flow right into the interior.
Identifying sources of infiltration in barrier systems can be fairly straightforward. Many times, a visual inspection of the conditions can identify the problem. To confirm leakage, spray testing can be done in a logical manner using process of elimination to find the leakage source. With walls that are intended to absorb some moisture and let it evaporate later, spray testing can take some time to allow the wall area to become fully saturated.
Drainage System Considerations
Weather-resistive barriers, including flashings, need to be interfaced with elements that penetrate the wall (windows, vents, electrical conduits, pipes, etc.). If this is successfully accomplished, the sealant joints at the exterior surface of the wall system are no longer critical, but still need to be maintained to limit the amount of water that can get to the weather-resistive barrier. Transitions and terminations are then most critical at the weather-resistive barrier. If two adjoining systems both employ drainage concepts, the weather-resistive barrier of both can tie together. If the drainage system abuts a barrier system, the weather-resistive barrier must not let water migrate past it at the point where it ends. This is usually done by sealing it to the end/edge of the barrier system component. This termination must be made with a watertight seal to the barrier component. Terminations in a drainage system must generally keep water from getting in at the top and allow water that has entered the system to drain at the bottom.
Since the assembly of drainage systems is typically more complicated, identifying leakage sources can be difficult. Visual inspection may not reveal all possible sources, so testing is preferred over trying to stop leakage by repairing every suspected leakage source in some manner. Be careful that sealant being added to the system doesn't inadvertently cover an avenue designed for drainage. These kinds of well-intentioned repair efforts can cause bigger problems than the original leak. On the other hand, some exterior wall systems originally designed as drainage systems can be converted into barrier systems through the use of properly applied sealant joints.
Understanding the function of exterior wall systems is key to identifying and correcting problems. A good maintenance program includes regular inspections and interviews with occupants. Proactive maintenance that addresses deteriorating conditions before large-scale failures develop helps minimize damage caused by component failure.
This list features components that undergo weathering, wear and tear, and deterioration, as well as typical conditions to watch for:
- Sealant joints.
- Weatherstripping at the perimeter of operable windows and doors.
- Gaskets between the exterior glazing and window frame.
- Sealers (typically a clear material applied to the surface of the exterior wall system). Since sealers are generally not visible, testing the portions of the exterior wall system where the sealer is known to have been applied may be necessary to ensure it's performing as intended.
- Waterproof coverings such as paints and elastomeric coatings.
- The exterior cladding component itself.
- Debris and biological growth accumulation at drainage points (gutters, downspouts, and flashings).
- Landscaping adjacent to the base of the exterior wall system and foundation. Maintain a minimum distance of 6 inches between the base of the system and the earth, including landscaping build-up along foundations. Also, make sure that sprinklers adjacent to the building are positioned to spray water away from the exterior wall system to prevent moisture infiltration.
- Cracking, deterioration of mortar joints, peeling paint, efflorescence, and any form of distress.
Christine Zimmer is an associate at Wiss, Janney, Elstner Associates Inc. (WJE) (www.wje.com) in Seattle. David A. Deress, a senior consultant and unit manager at the Seattle office, has been with WJE since 1986.
- American Society for Civil Engineers (2000). Guideline for Condition Assessment of the Building Envelope, SEI/ASCE 30-00. (Available for purchase at [www.asce.org/bookstore/book.cfm?book=3990].)
- Baskaran, A. (1992). "Review of Design Guidelines for Pressure Equalized Rainscreen Walls" Internal Report No. 629. (Available at [http://irc.nrc-cnrc.gc.ca/pubs/ir/ir629].)