WUFI Without Worries: Doing More Good than Harm with Hygrothermal Modeling
WUFI doesn’t kill buildings. Poor design, specification, and workmanship kill buildings.
Last year, BuildingGreen made a modeling software program one of our Top-10 Green Building Products for the first time—the WUFI hygrothermal modeling software from Fraunhofer IBP and Oak Ridge National Laboratory (see “Using WUFI to Prevent Moisture Problems,” an EBN building science primer). We did this because managing moisture as intensely as we manage energy is key to building durability and indoor air quality (IAQ).
I thought it made sense to reduce that worry by taking Building Science Corporation’s Advanced WUFI one-day workshop. I would like to tell you that today I am less worried about myself and other dilettante users of WUFI—but frankly, I am now more worried than ever.
That’s because, as with any modeling software, getting something wrong in WUFI can lead to wasted materials and money. It’s one thing to use more energy than you expected, though, and quite another to have your building quietly rotting from the inside out. Getting something wrong hygrothermally can be devastating in terms of overall building durability.
No worries here…
There are essentially three ways to use just about any modeling software for buildings, and WUFI is no exception.
1. To better understand the principles underlying building performance: Any of the five different WUFI modeling programs* do a great job of improving users’ understanding of hygrothermal principles with the deep and broad “help” function they all have. The help sections (online) are well organized, generally well written, and fully referenced. And the WUFI-ORNL free version is explicitly set up and characterized as an educational tool. Finally, there is a reasonably active WUFI forum, continually monitored by ORNL professionals.
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2. To conduct sensitivity analyses when substituting materials or changing environmental conditions: WUFI is also nicely set up with the ability to run “cases” within the same project file. You can easily label the cases to represent the variable(s) around which you are doing sensitivity analyses. And then you can check out the quick graphs for total water content and layer-by-layer, and results graphs.
Modeling is a marathon, not a sprint
Once you move from education to analysis, making sense of the results—particularly when comparing cases with targeted different inputs—requires a fairly sophisticated understanding of the hygrothermal properties of individual materials and the overall performance of assemblies.
Users cannot simply sit back and press a “Pass/Fail” output to compare different cases. It can easily take as many as 20 cases to fully explore and identify determinant parameters for the whole building assembly’s performance.
3. Woody Allen and WUFI
Weren’t there supposed to be three ways of using a model in the list above?
Well, yes, but the third—using models for prediction—is where I invoke Woody Allen (famous for comically anxious soliloquies, like this one from Hannah and Her Sisters that lasts for almost a minute-and-a-half).
Using WUFI to predict the performance of various assemblies in different climates can require substantial field experience, actual hygrothermal testing of building material properties, importing site-specific weather files, and even adjustment of hygrothermal data within the WUFI materials database. How many users of WUFI are actually doing this?
I also worry about two more important aspects of the WUFI proposition:
ASHRAE Standard 160 contains standardized conditions designed for use in modeling programs, including WUFI. A tab for ASHRAE 160 conditions is part of all WUFI program settings (for North America). And yet, I meet or talk to building scientists much too often who do little more than scoff at this potential setting and strongly recommend not using it at all. There is real concern that the standard conditions in ASHRAE 160 are unnecessarily complex and overy conservative.
You don’t need a modeling tool to err on the side of caution and substitute materials that cost more. Yet it’s not common knowledge that this aspect of the tool is flawed, so many are probably using it, not realizing its assumptions are questioned by many experts.
Do you get what you pay for? To move from the WUFI-ORNL version (free) to the WUFI Pro 5.2 is €2500 (about $3,350 U.S. or $3,480 Canadian, and this does not include any training). Pretty steep. And although I am not questioning the great coordination and cooperation between German and North American building scientists, I wonder just how focused WUFI is on the needs of our building industry, compared to the needs of the German building industry.
Not only is there a much wider array of climates (including cold-humid and hot-humid) in North America, but also there are major differences in building materials. In Germany, cementitious and masonry materials are much more prevalent; in North America, we are more likely to use light-frame construction, either steel or timber—both of which are harder to model and can be more moisture-sensitive (metal creates thermal bridges where condensation can collect, and wood is porous).
Enough worrying; cut to the recommendations
All building professionals need to understand moisture management as well as they understand energy efficiency. I recommend:
- Mastering use of the vapor profile approach to assessing the qualitative hygrothermal performance of any assembly.
- Deepening your understanding of building assembly hygrothermal performance by downloading the free WUFI-ORNL program, focusing on the Help sections.
- Take an introductory course in building science that includes vapor profile analysis and WUFI-ORNL, confining your use of WUFI to education and sensitivity analysis.
- For any assembly about which you have hygrothermal performance misgivings, sign up with one of the scarily few WUFI masters, or work with the building science professional from one of your trusted product manufacturers to assess your building assemblies. If your trusted manufacturer does not have a WUFI pro on staff, move on to a more trustworthy manufacturer partner.
I can’t emphasize this last one enough: we need to reward building product manufacturers who have:
- invested in the extensive product testing required for their individual materials to be properly populated in WUFI
- acquired or developed staff with the necessary hygrothermal expertise to understand how their products actually work in building assemblies
NOTE: I want to thank two leading building scientists who have generously worked with me on this article, as well as on developing my WUFI understanding and helped me worry less: Achilles Karagiozis of Owens Corning and Chris Schumacher of Building Science Consulting, Inc. The opinions in this article are my own but have been in no small part developed with the insight of both fine gentlemen.
*WUFI-ORNL—the free version; WUFI Pro 5—the 1-D program; WUFI 2D; WUFI Plus—the 3D program; and WUFI Passive—the one designed for North American Passive House projects
(2013, October 1). WUFI Without Worries: Doing More Good than Harm with Hygrothermal Modeling . Retrieved from https://www.buildinggreen.com/blog/wufi-without-worries-doing-more-good-harm-hygrothermal-modeling