G. Peter Vander Heide, AIA
MDCSystems®
Architect
August 2010

MDCSystems® has performed building exterior envelope investigations for over forty years on all types of residential, commercial and industrial buildings. Some of these investigations have included unique aspects of work concerning:

• Exterior Insulation and Finish Systems (EIFS)
• Traditional hard coat cement stucco repairs/replacements
• Stone veneer failures/replacements
• Shingle roof failures/replacements
• Water penetration due to improperly flashed windows
• Water penetration due to missing/defective sealant applications
• Flashing omissions
• Air and Vapor barrier failures/omissions
• Defective design applications for both new and traditional materials

Many contractors and craftsmen do an excellent job of assembling building systems properly, but with the pace of change of new materials, changes in contractor personnel, training requirements and the pressure to accelerate completion, it seems that some assembly activities are not performed with the required attention to detail and quality concern.

Because we’re creating artificial indoor climates to live in which have to be warmer or colder as well as dryer or wetter than the outside climate most of the time, we have to have an effective envelope to contain our comfortable climate. When something is wrong involving building envelopes, the presenting symptoms are often a combination of energy consumption related issues and condensation or water intrusion related, leading to stains and deterioration of materials and finishes. The complexity of modern HVAC systems and buildings, along with the interdependent behavior of heat and moisture in the atmosphere often leads to interconnectedness in the mechanical systems and the envelope. What looked like a bad mechanical design in the beginning can often reveal itself to be a missing air barrier.

Water vapor moves through most building materials, (with the exception of steel and glass), based on the relative water vapor pressures on either side of and inside the wall/ceiling. Vapor pressure is determined by a combination of heat and humidity. The volume of water in vapor is very low compared to the liquid phase of water, but the process of vapor migration is always working. Since the volumes of water are small, the importance of the vapor migration process has often been overlooked in design and construction. In addition, it may require years after initial construction for problems from vapor migration/drive to show up as symptoms.

One of the best new tools that can be used to screen building envelopes for potential problems is hygro-thermal modeling of the envelope. MDC® is employing an advanced computer program called WUFI® to analyze every building envelope issue that we are presented with. The Fraunhofer Institute of Building Physics in Germany developed the program in cooperation with Oak Ridge National Laboratories in the US. The name comes from the German, “Wärme und Feuchte Instationär”, or in English: dynamic heat and moisture transport. The developers and independent laboratories have done a significant amount of material investigation and system validation work to improve and perfect the predictive properties of the program.

WUFI® modeling offers significantly more information and insight about how building systems/envelopes behave in their actual environments by revealing problems with certain materials and systems, and offering reassurance/validation of adequate drying of the envelope components in other situations. Some important aspects of WUFI® are that the heat and moisture values are coupled, important material properties vary with relative humidity, wind driven rain effects are taken into consideration and solar radiation effects are also considered in the computer driven envelope simulations. The results of multiyear simulations are easily displayed graphically as a feature of the program.

The predictive properties of the program are impressive and using these features MDC® has been able to predict failure locations on properties under investigation and then field verify that these areas are indeed failure locations. However, even though this is the most powerful computer tool available today it must be coupled with insightful architectural and engineering analysis of the entire building system. Cracks, and the air and water that they can introduce into an envelope, are not considered in WUFI® modeling, so visual inspection and traditional investigational analysis is required by experienced investigators. Cracks can be very serious transmitters of water into a wall construction—with a little pressure and a good rain, even a small crack can be a great conduit for water into a wall, leading to failure of the envelope. Breathability of the wall system is important and if the water intrusion can migrate out without permanently affecting the materials in the wall, then no permanent damage may occur. However, even metal wall components deteriorate if wetted frequently and wood or fiberboard with elevated moisture content will evidence accelerated deterioration and rot.

MDCSystems® provides a full range of building envelope investigations and analysis for all types of structures. The wide range of material types available for modern construction present new challenges in assembly and inspection. Conducting a computer simulation for the specific wall in the native environment can lead the way in avoiding problems after occupancy and provide assurance that all the materials utilized are going to perform for their expected life.