Exterior siding, flashing, and plumbing can ensure your latest project doesn't have exterior leaks. But many of the moisture problems that have been blamed on leaks can actually be the result of condensation caused by the design of the building envelope—the walls, windows, foundation, floor, ceiling, roof, and insulation.
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| Oak Ridge National Laboratory’s WUFI program uses relative humidity and temperature data collected from regions across the country to effectively predict the performance of walls and air and vapor barriers in different climates. |
The rule of thumb used by most model codes is to require vapor barrier shields on the side of the wall that's warm in the winter. That means that in colder, northern climates, the vapor barrier is installed on the interior side of the wall system; in hotter, southern climates, it's installed on the exterior.
But this rule doesn't address the special needs of marine climates, the dry climates of the Southwest, or any of the mixed climates in between.
Now, computerized climate modeling called WUFI can help predict moisture condensation, air barrier leakage, heat gain, and other weather-related effects specific to the climate in which the project will be built and the materials used.
Originally developed by Hartwig Künzel of the Fraunhofer Institute for Building Physics (IBP) in Holzkirchan, Germany, WUFI (an acronym for the German words for "transient heat and moisture") is a computer program that helps Building Teams realistically calculate the way heat and moisture transfer through all components of a building enclosure when exposed to weather.
Using annually updated data on vapor diffusion in building materials as well as weather data specific to the geographic area in question, WUFI can realistically forecast how your building's components will behave over periods of up to 10 years. ORNL has 15 walls made of various sample building materials in Seattle and 30 walls in South Carolina that it uses to gather data for the program.
WUFI was adapted for the U.S. by Achilles Karagiozis, a senior research engineer at ORNL. The program has been available since 2002. WUFI currently has climate data from 53 U.S. cities. "We have data of temperature, relative humidity, wind speed, building orientation, cloud cover, diffuse horizontal radiation, and rain all collected on an hourly basis," says Karagiozis.
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| WUFI is a Windows-based program that can also take into account factors such as building height and orientation. |
More than 500 users have registered for the full version of the PC software since its introduction. (A free version of WUFI is available from www.ornl.gov. The professional version costs $2,250–6,200, depending on the number of users.) Karagiozis said his team at ORNL is working on new models that test building ventilation and ceiling fenestration; these should be available in test versions by September.
Not a cure-all
"Like any program, it's garbage in, garbage out," said John Straube, PhD, a building science professor at the University of Waterloo, Ont., in a recent Building Design & Construction webcast (available at www.BDCnetwork.com). "But it does take some time to understand the program and what lies underneath it. It takes a fair amount of professional judgment and experience to get the full value. But if nothing else, it's a great starting point."
WUFI calculations are time-consuming, even on a fast computer. Analyzing each specific wall configuration could take anywhere from one to 10 minutes, and several analyses may be required to check different combinations of materials for every orientation of the building to find the optimal design.
Even though ORNL sponsors WUFI workshops several times a year, it still isn't on most Building Teams' radar. But those who have used it like it.
"I can tell you in our practice, we've used it more and more every day and we are seeing the benefits in at least guiding us in choosing building envelope systems," said Kami Farahmandpour, PE, principal and founder of Building Technology Consultants, Arlington Heights, Ill. "You have to make sure your assumptions and your input are going to be correct. You also have to be aware of the limitations of the program. If you have an imperfect vapor retardant that has large holes in it, WUFI cannot predict that."
For more on WUFI, go to: www.ornl.gov/sci/btc/apps/moisture/index.html.
