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 in many properties. MDCSystems® has investigated and analyzed numerous failure situations and has reached the following general conclusions concerning the cause of the failures in these situations.

Stucco Systems:

• Unsuitable materials are used for the application.
• Construction design details are often ignored in light residential construction.
• Flashing is not properly sealed.
• Penetrations are not properly sealed.
• Materials are not applied within the temperature and humidity limits specified.
• Window and door openings are not properly flashed, sealed, and caulked.
• Other unique design and construction factors result in accelerated failure.
• Moisture is “trapped” within the wall cavity.
• Mold growth and indoor air quality problems can also occur as a result of water intrusion.

Based upon our recent experience, MDCSystems® has determined that there are four primary mechanisms which result in water damage on a typical residential or commercial structure that incorporate EIFS.

• Exterior Joint Penetrations
• Air Pressure Differentials
• Entrapment of Internally Generated Water Vapor Within Structure
• Entrapment of Exterior Source Water Within the Structure
• Material Permeability

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. MDCSystems® has modeled and analyzed these conditions using ASHRAE wall section analysis and WUFI®. As noted above, WUFI® is a proprietary computer program which allows realistic calculation of the transit couples of temperature and moisture levels for one and two dimensional heat and moisture transport in multi-layer building components exposed to natural weather. As temperature and humidity change, the exterior surface of a structure continuously contracts and expands. Differential expansion occurs whenever dissimilar materials are in direct contact eventually leading to moisture intrusion. A wood door frame, a window, a metal penetration or EIFS base coat all expand and contract at different rates.

As discussed above, the introduction of new materials and the changing nature of the components of the stucco itself can lead to moisture penetration issues. Stucco mixes utilizing lime are to a large degree “self-healing” when surface cracks develop over time due to the normal settlement and movement in structures. However, the movement away from lime additives has resulted in open cracks in the stucco surfaces that do not “heal” with time. Differential pressure on the structure caused by HVAC system operation or wind effects draw significant amounts of water through open cracks and this water can become trapped inside the wall cavity leading to moisture related failures.

The use of felt paper (Tar Paper) under the exterior wall surface has been a standard element of traditional building construction. Substitutions of other materials for the felt paper and/or the elimination of the “Tar” from the paper has also contributed to the moisture penetration issues and left the wall section vulnerable to damage from the moisture penetration.