WUFI Hygrothermic Modeling

WUFI-ORNL/IBP is a software program designed to model the dymanic movement of heat and moisture through building wall and roof assemblies. The software, developed jointly by Oak Ridge National Laboratory (ORNL) and Germany's Fraunhofer Institute of Bauphysics (IBP), is intended as a tool for researchers and building technologists to aid in the analysis and design of building envelope assemblies. This author's first impressions of the software after trying out a freely available research and education version of the program follow.

What is WUFI?

WUFI allows the user to model various building assemblies, run these assemblies through simulations of several years of typical climatic conditions for various locales (see the image at left), and analyze the performance of the assembly in terms of moisture flow, moisture accumulation, and other factors.

WUFI is a sophisticated yet relatively easy to use program. For example, it can simulate climatic conditions for different locals and account for differences in orientation of the building assembly. Assemblies themselves are modeled by selecting and arranging in the desired order components (such as sheetrock, plywood, vapor barrier sheet, etc.) from a database into which the relevant material properties have already been inputted. Once the initial conditions are established, simulations can be run and graphically observed at the push of a button. In this user's experience, an assembly could be modeled and run through a 2-year simulation in less than 5 minutes once the basic mechanics of the program have been mastered.

Some Sample Results

As an example of what WUFI can do, the results of three test scenarios are described below. All three are based on a wall modeled as follows: wood siding at the exterior, building paper, plywood sheathing, glass fiber batt insulation, and gypsum wallboard at the interior. The location is Seattle, Washington, using climate data for a relatively cold winter, with the wall assembly facing to the North. Interior conditions were set to moderate levels of relative humidity. The three scenarios differ in the placement of a polyethylene sheet vapor retarder, either close to the inside of the assembly, close to the outside of the assembly, or not included at all:

Scenario 1: Vapor retarder sheet behind the gypsum wallboard (close to interior, warm side of assembly). This is the conventionally "correct" location for a vapor retarder in the Seattle climate. To the left is a screen shot of the graphic results of the model after a 2-year simulation (click on the image to view a larger version). The exterior side of the assembly is to the left in the charts. The top chart records temperature data. The lower chart records relative humidity and moisture content. For example, the wide green band represents the range of relative humidity conditions encountered in various parts of the wall assembly throughout the two-year simulation. (Results are also presented by the program in tabulated and other graphic formats.)

Scenario 2: In this case, the vapor retarder sheet is (incorrectly) located on the exterior side of the exterior sheathing. In this configuration the vapor retarder is expected to trap condensed moisture in the plywood sheathing. Note the lower blue curve in the lower chart, representing moisture accumulation. As expected, in comparison to scenario 1, this configuration results in greater quantities of moisture accumulated in the exterior sheathing.

Scenario 3: No vapor retarder. In this scenario, the levels of moisture accumulated in the plywood sheathing fall in-between the two previous scenarios. Referring to the tabulated results provided by the software, water accumulated in the plywood sheathing for each of the scenarios is as follows: Scenario 1: 3.6 lb of water per cu. ft. of plywood Scenario 2: 8.4 lb / cu. ft. Scenario 3: 7.6 lb / cu. ft.

Interpreting WUFI Results

The WUFI anayses offer some interesting insights, and also raise additional questions.

For starters, the results seem to agree with conventional wisdom. That is, in a northern climate, a vapor retarder located toward the warm side (interior) of the assembly is beneficial in minimizing moisture accumulation in the wall: In the simulations above, Scenario 1 results in less than half the moisture accumulation of either of the other two assemblies.

A second question relates to the form of the data provided by the simulations. For example, is 3.6 lb of water per cu. ft. of plywood OK? How about 7.6 or 8.4 lb per cu. ft? To answer this question, one would have to convert these numbers to obtain moisture content as a percentage of the wood's oven dry weight, and then compare this to established standards which suggest that wood kept above a moisture content of approximately 15% to 20% is at risk of mold growth and decay. It would also be necessary to look at the moisture content data over time. For example, an occasional excess of moisture might be OK, but a long period of continuous excess moisture might not be.

On another point, this author was somewhat surprised by the results of Scenario 3 (no vapor retarder), which were almost as bad as Scenario 2. In the Seattle region, there is some question among the building technology community as to whether the use of vapor retarders is beneficial in this climate or not. The results of the Scenario 3 simulation seem to indicate that the absence of a vapor retarder is almost as detrimental as deliberately placing the retarder in the wrong part of the assembly. So the WUFI results seem somewhat in conflict with local practice.

Finally, there are some aspects of building assembly behavior that the WUFI program does not address at all. Two important ones are leakage of water due to construction defects and air flow through the assembly. In the real world, these two phenomena may very well be the most severe sources of moisture that an assembly will encounter, potentially contributing an order of magnitude more moisture to the assembly than those phenomena that WUFI does simulate. If so, to what extent are WUFI simulations useful at all? Based on conversations with other building scientists, the answer to this question seems to be that with proper use of the software, these effects can be simulated, and the results of WUFI analysis can be validly applied to actual building practice.

Despite these limitations, for those interested in the science of the building envelope, WUFI appears to be a useful tool for teaching and/or analysis.

More Info

• Oak Ridge National Laboratory WUFI website
• Fraunhofer Institute of Bauphysics WUFI website
• The WUFI Forum