Numerous reports of accelerated failures in institutional, commercial and residential buildings exterior wall systems attributed to water accumulation in the exterior wall cavities are raising concerns about structural soundness at many properties. MDC has investigated and analyzed numerous failure situations and has reached the following general conclusions for both cement and artificial stucco systems:
Based upon our recent experience, MDC has determined that there are three primary mechanisms which result in water damage to a typical residential or commercial structure. These include:
Some or all of these mechanisms are at work in failure situations and depending on the severity of the condition, the apparent deterioration can be delayed or accelerated.
Exterior Weather Surface
As temperature and humidity change, the exterior surface of a structure continuously contracts or expands. Differential expansion occurs whenever dissimilar materials are in direct contact. A wood door frame, a window, a metal penetration, or an EIFS base coat all expand and contract at different rates. As stated above, the gaps caused by the expansion or contraction of the dissimilar materials are typically controlled by sealants or caulking. Missing or improperly installed caulking will permit water to be drawn by either gravitational or capillary action forces into even the smallest opening in the exterior of the building. It is a simple fact that even structures which were initially properly sealed will eventually leak and fail. Thus, the weakest link for water intrusion in any home is the sealant or caulking used to close these openings. Historically, exterior wall cladding systems have recognized this condition and incorporated provisions to collect and drain this moisture, or allow it to escape to the atmosphere back out through the cladding system as water vapor.
The typical EIFS system consists of (1) an acrylic based adhesive, (2) polystyrene insulation boards, (3) a polymer-modified cement stucco base coat and (4) an acrylic finish stucco coat. This combination of essentially impermeable materials does not allow the evaporation of water which will inevitably penetrate from the outside from the interior of the cladding system.
The typical cement stucco system is also built in layers and consists of a wire mesh with base coat, a following scratch coat and finally the finish coat that imparts the color and texture to the installation.
Any defects in the surface finish that remain open or failures to seal dissimilar materials allow water to enter.
Water Intrusion Pathways
Another physical force acting upon the exterior of buildings that can force water into the interior of the exterior wall system is that of pressure differential. Wind or other weather conditions can create a condition of positive pressure on one side of a building while simultaneously creating negative pressure on the opposite side. Thus water can be simultaneously pushed into the wall surface on one side while it is literally sucked into the wall on the opposite side of the building. Under these conditions, the openings can be very small and virtually undetectable under calm conditions yet allow the intrusion of substantial amounts of water under high wind conditions. Since these pathways for moisture penetration may actually close when the weather changes, it becomes very important that the wall system incorporate a mechanism to collect and drain any such trapped moisture.
This pressure differential penetration condition is not unique to stucco or EIFS clad structures. However, the effect of this pressure differential acts differently on other claddings. More traditional wall coverings such as brick, lap siding, shingles and cement stucco allow air to infiltrate through the entire exterior wall surface rather than a few specific cracks. The positive force being applied to these buildings will be more balanced and less likely to trap rain or water vapor in the structural wall cavity.
The inevitable rain or water vapor from a pressure differential event will penetrate the stucco or EIFS system. The typical stucco system allows for water to drain down through the wall system. However, given the very low permeability of the EIFS materials, any intrusion of moisture caused by pressure differential conditions would result in moisture being trapped behind the EIFS barrier and thereby promote conditions leading to premature wood rot and mold growth in the wood sheathing and wall studs.
Another source of moisture that is equally destructive and is unnoticed - the potential effects of moisture generated inside the structure that is trapped and unable to evaporate into the atmosphere.
As modern buildings have strived to become "tighter" and more energy efficient, there is an increasing concern of poor air quality and the inability of controlling moisture. It is a perverse paradox that the very improvements that enhance the energy efficiency of buildings using modern construction techniques and materials simultaneously set the stage for conditions of interior moisture damage that were previously unknown.
It is well known that warm air can hold more water vapor than cold air. This condition restricts in a higher "vapor pressure." When a vapor-pressure differential exists, suspended water vapor will naturally and inevitably move from areas of high vapor pressure (warm air) to areas of low pressure (cold air) until equilibrium is established. Significantly, this transfer is not dependent upon any air movement but is simply the result of the tendency of water vapor to flow from areas of higher to lower vapor concentration.
In cold weather conditions, the difference in vapor-pressure between the inside (wet) and outside (dry) air will cause water vapor to migrate through every available crack permeable to water vapor. If this water vapor is restrained from permeating to the outside, the water vapor can build up to a point where condensation occurs within the wall cavity.
These facts and observations have serious implications for the useful life of all typical institutional, commercial or residential structures using either natural or artificial stucco systems. Failure conditions will occur; the only question is at what rate the deterioration will advance?
Robert C. McCue, P.E. is Managing Partner of MDC Systems Corp., LLC.
Please call the author at (610) 640-9600 with questions about this article.