Building Envelope Investigations
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I’ve been enjoying the opportunity to do some serious building envelope investigations over the past few years. I’ve been involved with Exterior Finish and Insulation Systems (EIFS) which required replacement, traditional hard coat stucco that required replacement, stone veneer that required replacement, shingle roofs that required replacement, windows improperly flashed, sealant joints missing, required flashings omitted, air barriers omitted, as well as just faulty design.
Many contractors and craftsmen do a fine job of putting buildings together properly, but with the pace of change of new materials, changes in contractor personnel, and the hurry to get work done it seems that some buildings that just don’t go together as they should.
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, sometimes the presenting symptoms seem to be energy consumption related, in other situations water intrusion causes stains or deteriorating materials and finishes. And of course, sometimes both. 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 can turn out to be a missing air barrier.
Water vapor moves through most building materials, not through steel, or glass, but through many common materials based on the relative vapor pressures on either side of and inside the wall. Mainly its heat and humidity together that determine vapor pressures. The volume of water in vapor is very low compared to the liquid phase of water, but the process is always working. Since the volumes are so low the importance of the process has often been overlooked in construction. Also because the volume of water is low, it can take a few years after initial construction for problems from vapor drive to show symptoms.
One of the best new tools to come along in some time to screen building envelopes for potential problems is hygro-thermal modeling of the envelope. There are a couple of computer programs that allow this modeling now. I am now using WUFI® to look at every envelope I’m involved with. The Fraunhofer Institute of Building Physics in Germany developed it 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. And the laboratories have done a lot of validation of results from it.
WUFI® modeling offers significantly more information about how building envelopes behave in their environments. It can reveal problems with some materials, and it can offer reassurance of adequate drying of the envelope components in other situations. Some important aspects of WUFI® are that the heat and moisture values are coupled, that material properties vary with relative humidity, wind driven rain is considered, and radiation effects are considered in envelope simulations.
I’m very impressed with the information I gain from the simulations, but they are not the whole answer. Cracks and the air and water that they can introduce into an envelope are not considered, so that consideration requires more analysis. 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. If it finds its way out again without affecting the rest of the materials in the wall, then it may be no problem. But steel rusts faster if wetted frequently and wood or fiberboard with elevated moisture content will start to rot away all too soon.
Hopefully, your building envelopes are dry and crack free. If you come across some that seem to be a problem give me a call.
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