Architects and engineers utilize modeling of complex systems to predict the behavior of systems and components over time. MDCSystems® is experienced in the use of all types of architectural and engineering models including 3-D CADD, 4-D CADD, estimating and cost models, energy consumption models, WUFI® and ASHRAE wall section analysis, financial models and animations of system operations to develop and explain the inter-relationships of components and systems. MDC® also uses critical path method schedules as models to understand and predict the impacts of unplanned events on the outcome of construction projects.
MDC® has modeled and analyzed the following building system conditions using both traditional ASHRAE wall section analysis and WUFI® advanced modeling techniques.
Traditional Design – New Materials; New Opportunities For Failure
While basic building designs have remained relatively constant over time, the introduction of new materials into the traditional building envelope designs has created new moisture penetration and retention problems leading to failures. Water in its various physical states can cause major problems when it appears and remains within building systems. While some materials of construction and assembly systems can tolerate moisture penetration and elevated levels of water for short periods of time without damage, others are easily susceptible to damage over both short and longer timeframes. MDC® has utilized both traditional and advanced analysis techniques to investigate, analyze and correct moisture penetration problems in the following situations.
WUFI® Advanced Modeling Techniques
WUFI® analysis uses actual hourly weather data, building orientation and solar data to create boundary conditions that are realistic of the actual exposure conditions. A WUFI® simulation considers the simultaneous effects of both heat and moisture on the building envelope. Rain and solar radiation are considered on exterior surfaces. Individual hygrothermal properties of each material in the building system are accounted for in each layer of the building section, including properties such as thermal conductivity that vary according to the humidity level in some materials. The resulting data are then displayed in graphical form and provide a time related history of the seasonal variations of the conditions within the building envelope system. WUFI® results have been validated by actual laboratory tests and outdoor weathering tests.
As mentioned above, WUFI® data output produces meaningful graphical time related graphs which can be utilized to explain the hygrothermal phenomena that are occurring in the building system envelope. An example is shown below of a typical home construction wall using 2 inch x 6 inch wood framing members with stucco exterior finish located in the Philadelphia weather area.
Latent Defects Leading to Failure in Structures using EIFS
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. MDC® has investigated and analyzed numerous failure situations and has reached the following general conclusions concerning the cause of the failures in these situations.
Based upon our recent experience, MDC® has determined that there are four primary mechanisms which result in water damage on a typical residential or commercial structure that incorporate EIFS.
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. MDC® 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.
Traditional Cement Stucco
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.
As discussed above in the EIFS failure mechanisms section, these same weather-related conditions, coupled with wind pressure differentials and internally generated moisture conditions, often combine in ways that lead to failure conditions.
Precast Concrete Wall Systems
Recent MDC® experience with precast concrete wall systems has shown that they are also vulnerable to moisture penetration problems. In one unique situation interior humidity conditions combined with exterior weather events and conditions resulted in significant moisture problems internal to the building and within the wall section.