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Evaluating Building Performance Via WUFI Hygrothermal Modeling Software

Overview

Berman & Wright utilizes WUFI hygrothermal modeling software for evaluating moisture conditions in building envelopes to help determine adequacy of design, causes of failure, behavior of the roof/wall systems, risk of mold growth, and any degradation of components over time. WUFI modeling allows the realistic simulation of the transient hygrothermal behavior of multi-layer building wall and roof components exposed to actual weather conditions.

What does “hygrothermal” mean? It essentially is the flow of heat and moisture through materials. Nature abhors a vacuum and will always seek balance – heat will flow to cold, and high moisture concentrations will flow to areas of low concentration. WUFI performs these flow calculations using wind driven rain, the exterior temperature, properties of materials in the building envelope, heating/cooling of the interior spaces, and moisture present on the interior due to human activities. Take as a simplistic example: on a given day rain strikes a masonry wall, WUFI can calculate the portion of that rain will be absorbed by the wall, and subsequently which direction that moisture migrates (inwards or outwards to dry out). Simultaneously, WUFI will evaluate the interior conditions due to mechanical heating/cooling, and calculate the flows of heat and moisture.

Background

Historically, the method for determining the probability of condensation promulgated by the American Society of Heating, Refrigerating, and Air-Conditioning Experts (ASHRAE) was the Glaser method of dew point analysis. The Glaser Method does not allow for either capillary moisture transport or sorption capacity of component materials. Both characteristics reduce the risk of damage due to condensation. Additionally, this method does not incorporate short-term conditions such as rain events and solar radiation. Therefore, the Glaser Method is typically unsuitable or of limited informative value.

In response to a recognized need for an advanced modeling tool for designers and engineers, the Department of Energy’s Oak Ridge National Laboratory (ORNL) initiated an international collaboration with the Institute for Building Physics (IBP) in Germany. WUFI software development had originated in 1994 at the IBP and stands for Wärme Und Feuchte Instationär, meaning the transient flow of heat and moisture. In 2001, the WUFI-ORNL/IBP software model was released. WUFI is kept up to date with data derived from ongoing outdoor and laboratory testing in multiple locations in North America. WUFI takes into account initial moisture content of materials, wind driven rain, weather conditions, solar radiation, long wave radiation, material properties of building components including capillary transport, and condensation.

Inputs

Berman & Wright utilizes their expertise and specialized knowledge to ensure that proper input is provided to yield meaningful results. Specifically models may include:

• Local or regional weather. Berman & Wright runs the simulation on different years of weather data for the location in order to validate findings and provide a more insightful analysis of building performance under varying conditions.
• Different directions or “elevations” of the building and it’s facades. Each direction has varying thermal influences due to solar radiation and wind driven rain.
• Initial moisture content of materials. Berman & Wright customizes the models starting points to account for the additional water content of materials exposed to rain events during construction, and can then model the “dry-in” performance of the assemblies.

Limitations

WUFI models do have some limitations. One is that the model does not address bulk water, meaning that the performance of the flashings, weather resistive barrier and other components to shed water is not simulated. Additionally, air can carry significant amounts of moisture – bulk air infiltration of the building envelope and the related moisture it brings is also not part of the WUFI simulation. However, modeling can still provide valuable information about the conditions to which building components are subjected and isolate what may or may not be a cause of building performance issues.

What WUFI Can Tell You

It is important to understand what results the WUFI modeling can provide, whether it is evaluation of a building design pre-construction or a forensic investigation of an in-service building. These results can help Berman & Wright produce a detailed scope of repair, identify degraded components hidden within the building envelope, and develop an original or retrofit design that allows better long-term building performance.

1. Moisture content of materials over time
   Each component of a roof or wall assembly can be examined in terms of moisture content and how that may fluctuate depending on interior and exterior conditions. Individual “slices” of components can also be separated out to determine if portions of the material are subjected to the moisture and temperature conditions with the potential to cause water damage, degradation of materials, and/or mold growth.

2. Accumulation of water within walls
   Simulations are run over a multiple year span, enabling Berman & Wright to examine whether moisture is accumulating within any of the wall/roof components. This is especially critical with today’s less permeable building envelopes. Degradation due to accumulating moisture may not become evident on the building for years, making WUFI simulations more critical to proactively address these issues.

3. Condensation
   Moisture content and dew point analysis can yield information on conditions where condensation is likely to occur within the roof or wall assemblies. If using WUFI during the design phase, changes to wall components can help a designer choose which materials will result in better performance. For example, a WUFI simulation of a wall assembly of a building in a cold weather climate can provide information on the desired permeability of the interior vapor retarder. A vapor retarder with too low a permeability will trap moisture which can then accumulate within the assembly. Conversely, a vapor retarder with a high permeability may allow too much moisture to enter the wall, and condensation can occur.

4. Video
   WUFI writes a film file during the calculation process which illustrates heat and moisture flow processes in the components. This film is ideal to develop the feel for the reactions of components under different climactic conditions, and provides information on the how the wall or roof system’s conditions change throughout the year.

5. Mold growth
   Building components can be analyzed at interior surfaces for the potential of mold growth. There are also an elevated set of criteria for mold growth potential on mold resistant, chemically treated, or non-organic building materials. WUFI includes a biohygrothermal evaluation tool for interior surfaces which helps evaluate the potential for undesired mold growth.

6. Corrosion
   Metal elements within the building envelope can be evaluated for conditions that can cause excessive corrosion. For example, WUFI can be utilized to evaluate the surface moisture at reinforcement within a concrete wall, or at metal framing within a wall assembly.

The WUFI hygrothermal and biohygrothermal models can be a valuable tool to provide information on the adequacy of building envelope design and performance. With the increased energy efficiency required by building codes, buildings are becoming less permeable or “tighter” which directly affects moisture transport and the ability of these assemblies to shed moisture. WUFI simulation models will continue to play an important role in the evaluation and analysis of building performance for the future.