Is your building envelope design causing mold problems? New computer modeling software can help developers accurately design moisture-resistant apartment communities.

New sophisticated computer modeling programs are turning upside down the old "rules of thumb" about designing for moisture control in the building envelope (walls, floors and ceiling) and raising questions about building code provisions that may cause moisture buildup in some properties.

Controlling moisture became a high priority issue for apartment firms after the mold frenzy broke out several years ago. Because mold is a naturally occurring and ubiquitous organism, it is impossible to completely prevent mold; however, it can be minimized by eliminating moisture sources.

As part of that effort, property owners examined and repaired exterior siding, flashing and plumbing to ensure that they were not causing leaks. But it turns out that many of the moisture problems that property owners blamed on leaks from improper flashing, siding and plumbing may actually be the result of condensation caused by the design of the building envelope.

While those three systems can indeed cause moisture problems, many firms overlook the moisture that can accumulate as a result of air movement and condensation within the building envelope assembly. For example, high outdoor humidity levels will force water vapor through the building envelope by diffusion to an indoor environment with lower humidity levels if the building's design fails to plan for this.

To minimize diffusion of water vapor into wall and roof systems, building designers use vapor barriers, such as kraft paper. Traditionally, local weather conditions and the building and energy codes have determined where vapor barriers are to be placed. The rule of thumb used by the model codes is to require vapor barriers on the "warm-in-winter" side of the wall. That means, in cold climates, the vapor barrier is installed on the interior side of the wan system, and in hot humid climates, the barrier is installed on the exterior side of the wall system.

But new computerized modeling programs demonstrate that following this rule of thumb can result in catastrophic moisture problems, particularly in mixed climates such as North Carolina and Virginia. It is certainly easy to identify the warm-in-winter side in places like Canada where the climate is dominated by heating requirements. But U.S. climates range from the heat-dominated north--where there are minimal or no cooling requirements--to the cooling-dominated south and everything in between.

For this reason, some designers say that the warm-in-winter rule works in Canada, but everywhere else in North America requires specialized design based on local factors.

The rule-of-thumb model doesn't say when to change from placing the vapor barrier on the inside of the insulation to the outside of the insulation. It doesn't say how to handle the special needs of marine climates and the very dry climates of the Southwest. And, it doesn't say what to do if there is more than one layer of insulation, such as insulated sheathing. Does the sheathing or the additional insulation act as a second vapor barrier? Is it better not to install a vapor barrier, or maybe the barrier needs to be a vapor retarder?

Computer Models Provide Answers

To properly control moisture, a building designer needs to determine whether to install a vapor barrier, what kind of vapor barrier to use, where to install it and how it will perform in conjunction with the building's air-barrier and other envelope assembly materials. Until recently, there were no good answers for these questions.

But a new sophisticated computer program called WUFI--an acronym for the German words meaning "transient heat and moisture"--has been imported from Germany and adapted by the U.S. Department of Energy's Oak Ridge National Laboratory to fill this information gap. A free demonstration version with limited capabilities is available at www.ornl.gov/sci/btc/apps/ moisture/ibpe_sof161.htm. The professional version of WUFI costs $2,250.

A south facing wall with sun on it performs differently than a north facing wall that never gets sun. Even east and west orientations have different needs because of their cool temperatures in the early morning and rising temperatures as the sun sets. WUFI takes all of that, and more, into consideration. It can factor in building height and the impact of wind, wind driven rain and wind direction at different elevations. And it can consider the differences between air-conditioned buildings, non air-conditioned buildings and buildings that only occasionally use air conditioning.

However, WUFI is not a quick and easy answer. First, the 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 best design. Second, few architects and even fewer building officials know about WUFI, and there are only a few certified users.

The type of construction being used can determine the importance of conducting a WUFI analysis. In my review of the typical wood frame wall assemblies used in apartments, I have found that it is less important for walls with impervious coverings, such as aluminum of vinyl siding. These generally work better across a wide range of climates, but even with these materials there are exceptions that WUFI can identify.

On the other hand, walls with pervious exteriors, such as masonry, cement board, wood and stucco, perform differently in every climate. Initial WUFI analyses on these walls indicate that wall systems really do need to be designed for the specific application and location. The computer model shows that depending on the specific circumstances of the building, a vapor barrier may be required inside the insulation, outside the insulation of not at all.

The analyses even returned one situation where the vapor barrier needed to be a vapor retarder (i.e., poly) located on the outside of the Oriental Strand Board (OSB) sheathing. This was particularly eye opening for me because I don't know of any apartment building that has been wrapped in poly, especially since earlier attempts to use poly on the inside behind the drywall created major moisture problems.

The potential for moisture collection and high relative humidity leading to mold growth when a vapor retarder (poly) is placed behind the interior dry-wall is the perfect example of how the WUFI program could have been used to predict a moisture problem. Initial runs in several climates with walls having pervious exterior surfaces show major moisture build up on the poly, with moisture collection and high relative humidity in the insulation and OSB board at temperatures supporting major mold growth.

Building Codes Respond

In the 1970s, building codes were amended to address energy consumption. Most of these changes involved "tightening" building envelopes. In essence, the codes promoted energy efficiency at the expense of proper air movement, ventilation, humidity and condensation control, and drainage mechanisms. This is starting to change.

The 2006 edition of the International Code Council (ICC) codes will only require the vapor barriers on the "warm-in-winter" side of the insulation in zones 5 through 8. (See map, page 44). NAA/NMHC are urging the ICC to remove the requirement completely since moisture management requires more specialized designs in virtually every climate. The 2000 and 2003 editions of the ICC codes allow for alternate applications, but require approval of the local building official, which can be time consuming with building officials who are not familiar with the technology.

Although it is a common practice for developers to replicate a standard community design, they should not use one set of architectural or design plans for multiple projects without careful consideration. Different climatic conditions and individual land characteristics necessitate site-specific design and construction. Getting WUFI-informed designs approved will be time consuming until there are more certified users of the software and until the model codes and local building officials become more comfortable with the technology.

Until then, apartment firms need to commit the time and effort required to analyze their building designs. The effort may help them avoid future moisture-related lawsuits.

Ron Nickson is Vice President of Building Codes for the NAA/NMHC Joint Legislative Program. Nickson delivered a presentation on WUFI technology at the 2005 NAA Capitol Conference in March. He can be reached at [e-mail]. Graphic courtesy of the U.S. Department of Energy.