“Do I need a vapor barrier?” It’s a perennial question, but there’s no simple answer: heat and moisture interact with each other and with building assemblies in complicated ways, so the first thing you need is a
vapor profile of the building assembly. This means looking at the vapor permeance of each material and ensuring it can dry should leaks or condensation occur.
You can take vapor profiling to a much more sophisticated level with a modeling tool known as WUFI—short for
Wärme und Feuchte Instationär, or “Transient Heat and Moisture.” WUFI looks at both drying potential
and wetting risk by predicting dynamic heat and moisture flows based on the site, climate, building materials, mechanical systems, and more. Although we’ve gotten by without such tools in the past, modeling hygrothermal performance (the interplay of heat and moisture) has become increasingly urgent as energy flows have radically decreased.
Originally developed by the Fraunhofer Institute for Building Physics for use in central Europe, WUFI was later adapted by the Oak Ridge National Laboratory for North American climates and construction practices. The software simulates the movement of moisture and heat on, into, and through complex building assemblies that you “build” by choosing components from an internal data library.
WUFI also incorporates historical weather data to simulate hygrothermal performance over several years. The software accounts for a wide variety of material properties and how these materials are likely to respond to typical weather events—for example, when wind-driven rain is followed by intense sunlight.
WUFI Pro has all these capabilities and is designed for use by any building professional (with some training). WUFI 2D includes a number of features focused on moisture consequences at joints, corners, and other thermal bridges; users typically have an engineering background. A third version, WUFI Plus, models whole-building hygrothermal performance. For example, it can help assess mold risk based on which type of HVAC system is chosen, determine how interior finish materials affect comfort, or guide decisions about ventilation to handle moisture loads from rare weather events. WUFI Passive models hygrothermal performance specifically for Passive House projects.
Although a free WUFI download is available (WUFI–ORNL), the free version lacks key features and capabilities that serious users will require.