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Musings of an Energy Nerd

WUFI Is Driving Me Crazy

This software program provides architects with a dramatic way to experience the ‘garbage in, garbage out’ phenomenon

Is it possible to describe all of the factors that influence heat and moisture movement through a wall during a single day? Perhaps. We could start by listing the outdoor conditions, including air temperature, relative humidity, wind speed, wind direction, the angle of the sun with respect to the wall (its altitude and azimuth), the cloud thickness, the precipitation rate, and the depth of snow on the ground. Needless to say, many of these factors change from minute to minute.

We could describe the indoor conditions, which include the air temperature (remembering, of course, that the air temperature near the floor may be different from air temperature near the ceiling), the relative humidity, and the mean radiant temperature of the surfaces in the room.

We could list the construction details of all of the many layers of the wall assembly, including the vapor permeance of each layer, the R-value of each layer, the air permeance of each layer, the moisture-storage characteristics of each layer, the location and size of the wall’s cracks and holes, the location and size of any windows, the leakiness of the window flashing, the SHGC and U-factors of the window glazing, the distance from the top of each window to the roof overhang, the width of the roof overhang, the depth of the rainscreen gap, the size of the ventilation openings at the base of the rainscreen gap, the size of the ventilation openings at the top of the rainscreen gap, and the orientation of the wall — that is, the cardinal direction it faces.

If we know all of this information, and more, it might be possible to determine how heat and moisture move through the wall — in other words, to describe the wall’s hygrothermal performance.

Then again, it might not.

“Let’s try to model it”

The fact that building assemblies like walls and roofs are complicated hasn’t stopped building scientists (including the developers of WUFI) from trying to model their hygrothermal performance with computer software.

WUFI is a program (actually, a family of programs) developed by Hartwig Künzel, the head of the Division of Hygrothermics at Fraunhofer Institut Bauphysik in Holzkirchen, Germany. (WUFI stands for Wärme Und Feuchtetransport Instationär.) The program was adapted for English-speaking users and further developed by Achilles Karagiozis, a building scientist who formerly worked at Oak Ridge National Laboratory, and now works for Owens Corning in Granville, Ohio.

WUFI was originally developed for use by building science researchers. However, in recent years an increasing number of architects have also started using the program. These architects hope that WUFI can provide answers to some basic design questions, including such perennial stumpers as, “Will this design work?” and “Why did this wall assembly fail?”

The software has been validated

WUFI is a complicated program. It allows all of the factors that I listed above to be entered or adjusted by a researcher or architect using the program. Behind the scenes, the program includes complex algorithms governing heat transfer, adsorption, evaporation, condensation, air movement, and drying rates.

WUFI will tell the user whether the moisture content of any wall components (or other building components) rises over time. This feature is obviously of interest to designers, especially if WUFI can flag proposed building assemblies that stay too wet — that is, wet enough to cause mold or rot.

Like any energy modeling program, the developers of WUFI had to validate the program. One way to do this is to build a test hut using known materials; to measure indoor conditions, outdoor conditions, and conditions inside the walls of the test hut; and to compare the measurements determined by the sensors in the test hut’s walls with WUFI predictions.

If the sensors measure the same conditions as those predicted by WUFI, then Bingo! WUFI was spot on.

If the sensors show that conditions in the test hut differed from WUFI predictions, the software developers would need to change WUFI’s algorithms so that WUFI predictions come into alignment with real-world data.

The software developers repeated this process with a wide array of building assemblies — brick cladding, vinyl cladding, and fiber-cement cladding, for example, as well as fiberglass insulation, cellulose insulation, and spray-foam insulation — and in a wide variety of climate zones, to make sure that WUFI was properly validated and calibrated.

WUFI is useful to researchers

WUFI is immensely useful to building science researchers. Let’s imagine that one of Hartwig Künzel’s test huts in Germany has a wall with mineral wool insulation and brick cladding. After a year of monitoring, Dr. Künzel may be able to say that WUFI predictions and his test hut measurements are in close alignment. At that point, Dr. Künzel can use WUFI to adjust some of the parameters, in hopes of answering questions like:

  • How will this wall perform in a climate with more rainfall?
  • How will this wall perform in a colder climate?
  • What are the differences in brick moisture content if we compare a north wall to a south wall?
  • What happens when we substitute fiberglass batts for mineral wool?

The information gleaned from these WUFI simulations may lead to new ideas for research. Moreover, future field measurements made by researchers can be used to further validate WUFI’s algorithms or, in some case, to improve them.

All designers and builders are deeply indebted to researchers like Künzel and Karagiozis, whose studies have laid the foundations for the field of building science.

Enter the architects

What about architects? An architect who uses WUFI — let’s call him Gehry Jones — isn’t likely to use the program in the same way that Dr. Künzel does. When Jones designs a wall assembly, he may not fully understand all of the program’s many inputs. He’ll probably end up using default values for the various materials in his stack-up — after all, how many projects have a large enough budget to allow architects to order laboratory tests to determine materials properties? — and he’ll probably use climate data for the nearest city, even if that city is 60 miles from the building site. He’ll do his best to estimate future indoor conditions, and then he’ll click “Run.”

After noticing that inexperienced users of WUFI were making real-world decisions based on erroneous WUFI predictions, I posted a comment on the topic on GBA. I wrote, “I was on the phone this week with [building scientist] John Straube, discussing some technical questions, and I exclaimed, ‘WUFI is driving me crazy. Everyone is misusing these simulations.’ John said, ‘Yes! I agree! It’s driving me nuts too.’ I said, ‘Maybe I should write a blog called “WUFI is driving me crazy.”’ John said, ‘Yes. You should write it.’”

As it turns out, a pair of writers at Environmental Building News, Paula Melton and Peter Yost, beat me to it. Their excellent article, “Go with the Flows: The Promise and Peril of Hygrothermal Modeling,” was published on April 2, 2014.

There are lots of ways to screw things up

In their EBN article, Melton and Yost do a terrific job of listing all of the ways that inexperienced users of WUFI can obtain bad results.

One potential problem: bad weather data. The article quotes M. Steven Doggett, the CEO of Built Environments, Inc. “WUFI doesn’t have that many cities in North America,” said Doggett, “and you have to be careful with using data from a nearby city.” The article continues, “Doggett claims he’s gotten unreliable outputs when basing his models on built-in weather files and says his company creates ‘location-specific datasets’ based on measured data — not exactly a job for a casual user.”

Another potential stumbling block is determining interior conditions. The article quotes Dave Bryan, an architect at Third Level Design in Minneapolis. “The results are really sensitive to what you say the interior relative humidity is,” Bryan said. “You have to make conservative assumptions — but you don’t want to be so conservative that you can’t build things. It takes a lot of judgment.”

A third potential issue concerns the material properties of building components. Melton and Yost wrote, “WUFI tools include an extensive material database — of European products. Results rely heavily on the hygric and thermal properties of your materials, and if you choose the wrong products, you haven’t modeled the assembly you meant to. For example, brick can contribute significantly to moisture issues in exterior walls — but different types of brick absorb, transfer, and release moisture quite differently. ‘I’ve had projects go through extensive analysis based on a certain kind of brick, and then the brick is substituted’ for aesthetic reasons, says John Hannum, P.E., of New York-based engineering firm Vidaris, Inc. ‘Unbeknownst to the architect, that can change substantially the moisture activity of the surface.’”

The article also quotes Sean O’Brien, associate principal at Simpson Gumpertz & Heger. “Limestone from a German database developed 20 years ago? The chance that that’s the same as the limestone on your building is very, very small,” O’Brien said. “For a large project looking to insulate an entire [brick masonry] building, if you run the numbers from the German database, they are too wet and fall apart. When you run it with the real properties, everything looks like it turns out fine. That’s not the kind of thing the average user is going to do, but that’s really the only way to do it.”

An American architect who is uncertain of the porosity, thermal conductivity, vapor permeability, and water storage characteristics of a specified material faces daunting challenges. The EBN article quotes Steven Doggett on this point. Doggett said, “I have asked for data from manufacturers for a number of products. I never got any.”

Melton and Yost wrote, “We … heard indications that it’s disturbingly easy to put garbage into WUFI without knowing it. That’s at least partly the tool’s fault, suggests Wagdy Anis, FAIA, principal at Wiss, Janney, Elstner Associates. Contrary to common belief, ‘Good brick doesn’t absorb very much moisture,’ he claims. The material database in WUFI Pro ‘includes the material properties of brick but doesn’t include brick as an assembly with mortar and joints that may be sucking moisture into the wall. That’s a place where a lot of people go wrong — even experienced WUFI users.’ Though he says you can choose to model a brick assembly that includes mortar, you need to know to look for it among many brick options, and many users simply don’t realize that. ‘It’s a really big deal,’ he said, because a wall of straight brick with no mortar would absorb significantly less moisture, leading architects to make decisions based on an absurd assumption.”

Tweaking inputs is essential

Even experienced building scientists are often confounded by WUFI. After interviewing Joseph Lstiburek, a principal at the Building Science Corporation in Massachusetts, Melton and Yost wrote, “Lstiburek still advises against modeling. ‘It took five of us — some of the most skilled people on the planet — an entire day’ to get inputs right to make ‘a simple wall that has had a history of performance’ to run accurately in WUFI in all the major climate zones in North America. His interpretation? ‘What you’re having to do is manipulate the properties of the model to force it into giving you the right answer.’ ”

Needless to say, this anecdote raises serious questions about whether WUFI results can be trusted. As building scientist John Straube told me, “WUFI modeling can guide decision making. But WUFI modeling requires knowledge, comparison to measured data, and real experience.”

Sometimes, reports of WUFI glitches make their way into published scientific papers. In “Moisture Research — Optimizing Wall Assemblies,” researchers Lois Arena and Pallavi Mantha wrote, “WUFI offers several different methods for generating interior temperature and RH [relative humidity] levels. For this study, the interior conditions for all three wall types were generated using the ASHRAE 160-2009 method. … It should be noted that, in all climates, the interior RH levels predicted by this method reach 90% even though cooling was assumed. Using these interior conditions, WUFI predicts that there is the potential for mold growth on the interior surface of the drywall in all climates. Realistically, we know that this is not true.”

I commented on the situation reported by Arena and Mantha in a GBA article published last year. “Here’s the translation: the modeling results don’t pass the sniff test,” I wrote. “Arena and Mantha clearly recognize that fact, and they accurately deduced that the anomalies stem from their use of the ASHRAE 160 values…

“To find out how these unexpected WUFI results may have occurred, I spoke with Anton TenWolde, the building scientist who helped develop ASHRAE 160. TenWolde explained that the indoor moisture values first published in ASHRAE 160 needed to be tweaked. … Since the ASHRAE 160 conditions used in the WUFI modeling performed by Arena and Mantha were flawed, the WUFI results from that study shouldn’t be used to make design decisions. Instead, it’s worth looking at data from monitoring studies of real walls.”

Last year, in a GBA article on WUFI, Allison Bailes warned WUFI users to check whether the program’s results correspond with their real-world experience. Bailes wrote, “The program gives you a lot of power to set everything up with hundreds of inputs. You really have to know what you’re doing to get this right or you could end up modeling a wall that WUFI says will be a disaster even though it’s been working well in the real world for decades. Or you could model a wall, as one architect did, that WUFI says will work fine and then your builder says they won’t build it because it won’t work.”

Melton and Yost reported that Joe Lstiburek calls the use of hygrothermal modeling “ridiculous.” The authors wrote, “Unlike the high-mass stone structures of historic buildings in Europe, where WUFI was invented, ‘North American buildings are hollow and are dominated by complex, three-dimensional airflow networks, which are almost impossible to model.’ [Lstiburek] maintains that because typical hygrothermal models focus on vapor drive rather than unpredictable air or bulk water leaks, it makes a lot more sense as a research tool rather than a decision-making tool for projects.”

You can get any results you want

Back in 2011, in an article on payback calculations, I wrote, “As a cynic might say, all you have to do is tweak your assumptions, and you can prove any conclusion you want.”

Here is the positive corollary to my cynical statement: by tweaking WUFI inputs, a designer can make sure that WUFI predictions match measured data. The negative corollary is, “In the wrong hands, WUFI predictions can be flat-out wrong or deliberately misleading.”

Melton and Yost interviewed Adam Cohen, an architect with Structures Design/Build in Virginia and a Fraunhofer-trained advanced teacher of WUFI. Cohen calls WUFI “an incredibly detailed and robust program.” Cohen went on to note, “There are lots of shiny knobs to touch. Yes, you can really get yourself into trouble. … I can show you in WUFI Pro how to make it look like walls that have been standing for 100 years would melt. … I can [also] show you how to make a wall ‘survive’ when you know it won’t.”

The authors of the EBN article continue, “Any type of modeling can provide counterintuitive results; some are legitimate, and some are not. Anyone who understands the principles behind the model won’t accept these results at face value. ‘If something doesn’t sound right, we investigate further and see if it’s because of user error,’ says SGH’s O’Brien. ‘I’ve gotten plenty of things where I said, “This looks completely wrong.” I don’t stop working at the problem till I can physically explain it.’ If the explanation isn’t clear after more iterations of the model, attempting to isolate what’s driving the iffy result, ‘we distrust it and try a different approach.’”

Alarm bells: Energy consultants are using WUFI in court

As most consultants and lawyers realize, construction defect litigation is a technical minefield. These days, it’s easy for a defendant to find a consultant willing to testify that a case of sheathing rot is caused by driving rain, while the plaintiff’s consultant testifies that the same sheathing rot was definitely caused by condensation of interior moisture. How can juries or judges be expected to sort out this type of contradictory testimony by hired guns?

WUFI to the rescue!

According to Melton and Yost, “Simulation’s value as a forensic tool means it has potential as expert evidence — and it is sometimes referenced by both sides in a case. Employed as a supporting tool combined with other evidence, hygrothermal modeling can be an important part of a legal battle.”

To put it mildly, this legal development is very worrisome news.

Who should use WUFI?

In their EBN article, Melton and Yost interviewed Judd Peterson, AIA, president of Minnesota-based building envelope consultancy Judd Allen Group. “I’m not sure to what degree architects understand what WUFI is telling them,” Peterson said. When WUFI is promoted as a “required tool” on facilities design standards for architectural compliance, Peterson noted, “Some architects simply use the dramatic WUFI output charts and graphs as a marketing tool rather than for its true purpose. … I am concerned that they have no idea there are any limitations or assumption judgments at all. They are simply running the program and relying on the results without critical thought and review.”

Here’s my advice to architects: in general, be very wary of WUFI simulations. Although a handful of engineering companies in the U.S. probably have enough experience to provide useful WUFI results, it can be very difficult for an architect to separate valid WUFI runs from poppycock and horsefeathers.

What makes a good tool?

It’s easy to come up with examples of good tools. A speed square is a good tool; I can use it to verify that the pencil mark on my 2×6 is indeed at 90 degrees to the long dimension of the lumber.

A tape measure is a good tool. I can use it to determine that the exterior wall of my wood shed measures 16 feet 2 inches.

For designers, however, WUFI is a lousy tool. WUFI might tell me, for example, that the OSB sheathing in my planned wall assembly will have a moisture content of 24% one year after the building is occupied. While that result sounds useful, there is a huge difference between what my speed square and tape measure tell me, and what WUFI tells me.

My speed square and my tape measure almost always tell the truth. The same cannot be said, unfortunately, with WUFI.

Martin Holladay’s previous blog: “Does a Home with an HRV Also Need Bath Fans?”

Click here to follow Martin Holladay on Twitter.


  1. User avater GBA Editor
    Allison A. Bailes III, PhD | | #1

    It's a research tool
    We were having a discussion about this just two days ago in Detroit. Joe Lstiburek said the same thing you've written here: WUFI is a research tool, not a design tool. I've taken a 2 day class in WUFI 1-D and have a PhD in physics, and I can't do WUFI modeling with accuracy that anyone should trust.

    I haven't done anything with it since I took the class last October, and I'd probably have to have a month of doing nothing but WUFI under my belt before I'd feel somewhat competent. It's a difficult tool to use correctly because of all the inputs and because you really have to have experience with what kind of assemblies work and which ones don't. That way you have some basis to judge whether or not your WUFI results might be accurate.

    Thanks for writing this, Martin. Architects considering using WUFI in their design business would be wise to reconsider.

  2. User avater
    Armando Cobo | | #2

    Great summary. Thanks!
    Few years ago while pondering about taking a WUFI class, John Straube told me that to perform Dew Point analysis for my house designs, the simplified formula for ASHRAE Fundamentals was good enough. He added, if I wanted to impress an Engineer or a client with 30 reports and charts, then I could learn to use WIFI. Needless to say, I’m still doing my ASHRAE calculations, even with all its limitations..

  3. Rachel Wagner | | #3

    I'm so happy to read this
    Thank you, Martin, once again. I learned to use WUFI during Passive House training a few years ago and wondered then about the issues you raise in this article. In 2011, when I was "trained" on it (not really trained, it was about a 2 hour tutorial, not enough time to even learn the basics), it seemed to me an unlikely tool for designers, and perhaps not even necessary if the designer really understood building science principles and the qualities of the materials being selected for an assembly. As I watch WUFI's popularity with practitioners grow, I worry that it is being used as a replacement for true critical thinking. I would say that utilizing WUFI is not nearly as important to a designer as having a strong fundamental knowledge of building science principles. It seems to me that knowing how to determine which way your wall will dry when it gets wet, and understanding how to craft an assembly with excellent drying potential in the direction you determine it needs to dry, is better insurance than a WUFI model.

  4. User avater
    Robert Swinburne | | #4

    Problem avoidance
    As an architect I prefer to design in a "problem avoidance" manner - simple geometry, simple roofs and simple detailing for new construction. I know enough to stay away from WUFI myself but I will occasionally solicit professional help with WUFI for renovation jobs to help with less than ideal situations. For example: are these clapboards with many layers of paint going to effectively stop outward drying potential on this wall? Which is perhaps a more experience based decision than a WUFI decision anyway.

  5. Dan Kolbert | | #5

    Good blog
    As someone who is essentially an idiot when it comes to engineering and software, I am simultaneously awed and suspicious of modeling. GIGO to the nth degree. I guess what I've tried to use it for is more as a tool for comparison - not assuming it's accurate but seeing what happens when you tweak one variable.

  6. Ron Keagle | | #6

    Implications of using WUFI
    If WUFI is too unreliable to use as a design tool, how can it be reliable enough for research? Research leads to design and execution. It also leads to regulation. Martin mentions the worry about using it as a legal tool. What about the dangers of using it as a regulatory tool?

  7. User avater GBA Editor
    Martin Holladay | | #7

    Response to Ron Keagle
    I explained in my article how WUFI is used by reputable building science researchers. It is an excellent tool for exploring "What if?" questions, and for developing ideas for field testing.

    Building scientists understand the need to verify WUFI predictions with field measurements. While modeling studies are often published, they are considered to be no more and no less than they claim to be -- modeling studies.

    There are many forms of research. All over North American, scientists have built test huts and are monitoring conditions in these huts. Other researchers have embedded sensors in occupied houses to study field conditions.

    Researchers need a variety of tools -- both modeling tools and data-acquisition tools for field measurements.

    Concerning your worry that WUFI might be used as a "regulatory tool," I can only say that I can't think of any examples where that has happened.

  8. User avater GBA Editor
    Allison A. Bailes III, PhD | | #8

    Response to Ron Keagle
    Ron, you raise an excellent point. In fact, researchers also have trouble getting it right. The recent brouhaha over open cell spray foam and rotting roofs is exhibit A.

  9. Matt Dirksen | | #9

    So, is it a good "idea" of a good tool?
    Thank you for this. Like many others I suspect, I have played around with it just enough to know that I don't feel I am at all capable of using WUFI anywhere close to the way it is intended.

    And just like a tape measure or speed square, is there anyone out there improving on WUFI to possibly make a tool that can ultimately be more reliable, and less prone to erroneous results? An i-WUFI ? WUFI 2.0?
    Or should it simply stay close in the arms of the brilliant research scientists out there, where it should remain for the foreseeable future?

  10. User avater GBA Editor
    Martin Holladay | | #10

    Response to Matt Dirksen
    The reason that I don't think that we will soon see an i-WUFI that can be used by architects, builders, and 8th graders is that there are so many variables that affect the hygrothermal performance of building assemblies. I listed some of these variables in the first three paragraphs of my article, and it's a daunting list.

    For the time being, I hope that WUFI remains "in the arms of the brilliant research scientists," as you put is, rather than on the computer screens of designers and architects. One of the brilliant research scientists who disagrees with me is Achilles Karagiozis, who just telephoned me to explain his objections to my article. (I urged him to post his comments here, or to write a guest blog for GBA, and I hope he does so.) Karagiozis believes that architects are smart enough to learn how to use WUFI; that, if they are diligent, they can track down the materials properties they need to know to provide the inputs that WUFI requires; and that erroneous WUFI results won't happen if only architects take enough training courses to become experts in the program.

    I'm willing to change my view in the future. But I've heard today from Allison Bailes, who has a PhD in physics and took a 2-day WUFI course with Karagiozis, and from Jesse Thomspon, a very smart architect in Maine who has used WUFI, and both of these smart people told me that they don't know enough to be sure that their WUFI results are accurate.

    Jesse Thompson tweeted, "WUFI made me more nervous than any other software I've tried. Huge variation of results from the same starting point."

  11. Chris West | | #11

    The Geist of the Moment
    Thank you Martin. As many others have commented we in the Passive House movement in Vermont have been wrestling with this very issue. We live in a the dangerous Cold Wet climate zone 6 and are constantly running into the issue of how to ensure a super insulated wall that is durable.

    After reading your article on moisture and dew points in walls, and the associated articles by Joe Lstiburek, I did what you, Joe and another comment poster suggested. I did the Dew Point Analysis for the Average Temps in my specific area.

    What we came up with was a minimum of 40% of the total insulation value outside the sheathing to keep the dew point away from the sheathing (with 35% indoor RH and 70F). To make it easy we are going with a 1:1 ratio.

    Many of us in the Passive House movement are working with new ideas and we all want the assurance of as much science as we can get to support our decisions. We all want to build durable structures.

    GBA and Building Science Corp's papers are invaluable in this work.

    Thanks again for a cogent and critical look at this important piece of the puzzle.

    I wonder if we (designers, architects and builders) will ever get a tool we can use to model these assemblies to the level of rigor we would like.

  12. User avater
    Paul Eldrenkamp | | #12

    short-term need
    I think builders and architects feel a need to try to use WUFI only because in the US we don't quite have all the materials and experience we need to design and build with confidence wood-frame buildings with very thick walls and roofs.

    I don't think we're too many years from having all the materials and experience we need, at which point WUFI will return to the domain of researchers and engineers trying to develop new building products to sell us.

  13. User avater GBA Editor
    Martin Holladay | | #13

    Reponse to Paul Eldrenkamp
    I think your point is an important one. Builders want to choose a robust wall system that works well in their climate. Ideally, this wall system will be resilient enough to work, even if homeowners maintain high relative humidity indoors, and even if the wall is exposed to significant wind-driven rain. Right now, not all of our wall assemblies are like that.

    If builders are confident that they understand what works and what doesn't in the climate zone where they build, they won't need to run simulations every time they switch from a smart vapor retarder to a system that depends on vapor-retarder paint. That means that builders and homeowners need to accept the value of materials that provide robustness -- that keep us away from the cliff of failure -- even if these materials raise costs somewhat.

    As you say, once we have the experience to trust what types of assemblies fit the bill and work well, we won't feel the need for tools like WUFI.

  14. Aj Builder, Upstate NY Zone 6a | | #14

    Chris West Bruce Brownell goes 100% exterior
    Bruce Brownell outsulates 100% and has been for decades. Check out his work.

    Google Bruce Brownell

    One way to not have rotting OSB in a structure is not build with OSB. It is cheap and it is crap.

  15. User avater GBA Editor
    Martin Holladay | | #15

    Taking a week off
    I'll be taking a week off (May 5-9). If anyone posts questions for me during the coming week, answers will have to await my return. In the meantime, the GBA publishing schedule is in the capable hands of Rob Wotzak.

  16. Dan Whitmore | | #16

    With a complex vehicle, manage your expectations before dirving
    Martin- Thanks for writing this article but let’s not throw out the bathtub because there’s been a lot of peeing in the bathwater.

    Many of your statements and the comments above ring true for me, I’ve attended a number of classes on the program and the science behind it (Achilles is a great teacher) and also came to the same conclusion that this is a stunningly complex computer program which can readily be run off the tracks if not used and applied with experience and due knowledge.

    Would I trust results that didn’t pass the sniff test? No more than I trust a suspicious assembly that has been proven to function when it’s constructed perfectly in a lab. Great, now I need to get my crew to perfectly replicate lab conditions. Would I trust my personal hygrothermal modeling on a suspect assembly? Nope, not without supervision. Would I trust a partially-trained carpenter to install a critical air-sealing detail without supervision? Nope.

    For me, knowledge of how the tool functions has notably helped my understanding of the Building Science behind it all; so when an architect or client comes up with a new, unfamiliar or worrisome detail we can have an intelligent conversation and, if need be, watch some cool WUFI movies for effect. I know a handful of people I thoroughly trust to run the program and all of them state clearly that the results are not an answer of what will work and what won’t, instead it’s a measure of relative risk.

    To echo Paul E’s comments, there’s renewed effort to building more efficient and durable structures yet we don’t have a robust set of assemblies which we trust for each of our climates, much less each of our regional preferences (Brick? You mean those expensive rectangular things that only get put on the big institutional/commercial buildings around here?) But we want to get there and not have too many failures along the way. If only every client showed up with the funds to build a great, hygrothermally-sound, super-insulated building then we could all rest easy, in the meantime I’ve got to struggle explaining why I really don’t want to build a 14” Double-Stud Wall on the edge of Puget Sound and WUFI has occasionally helped.

  17. Katy Hollbacher | | #17

    Martin nailed the title
    THANK YOU. I've been patiently waiting for your article, and now I can just direct people here rather than sound like a broken record in my response to "hmm, can you run a WUFI model on this"?

    My company does mainly new buildings, so we don't encounter the moisture-related concerns that insulating old buildings can introduce. WUFI has its place, absolutely, but is not for amateurs. In a WUFI class I took from Achilles & crew more than 3yrs ago, various red flags were raised in just those 2 days, and here are just a couple of those I still remember:

    1) you pick a bulk water "leakage rate" from some kind of hole through a wall or roof; how about 1%? sure, sounds as good as anything, let's try that (numerous other similarly esoteric selections exist; and with some you can barely change an assumption and find totally different conclusions)

    2) you build a wall assembly layered up as directed by the instructor. one layer is exterior polyiso. I notice in puling it from the database that the perm rating is something like 3. hmm? has anyone in the US used or even seen polyiso that isn't foil-faced on one or both sides? so I ask about this, and it is agreed that we need to include an additional foil material in this buildup. cool, but what about users who don't have an intuitive sense of what a perm rating should be for a certain material?
    In prior dabbling with WUFI, my strategy had been to test my own instincts on how I expected a certain assembly should fare in a certain climate under various assumptions. For basic “known” assemblies I got to feeling as confident as I thought possible, short of working with WUFI regularly and testing it with a continuous flow of data from building failures as my former colleagues at WJE are doing. When serious WUFI modeling is needed, I’d go straight to the real experts like WJE.

    I still don't trust myself enough with WUFI to give my thumbs-up to an assembly I’m otherwise uneasy about. But why on earth what I want to? This does not need to be so complicated. At my company, we recommend assemblies that leave little question about long-term durability and resilience. Generally speaking, we already know what works or have other respected resources to rely on (even building codes sometimes get it right, horror!) and don’t need to reinvent the wheel and waste time and money. I feel SO indebted to forums like gba and resources like Building Science Corp to do the heavy lifting and allow me not to run WUFI models all day. Thank you, again.

  18. User avater GBA Editor
    Martin Holladay | | #18

    Response to Katy Hollbacher
    Thanks for the feedback. And thanks for sharing the illuminating examples from your WUFI training -- concerning the need to guess the amount of water that gets past cladding when performing a WUFI run, and the traps for the unwary concerning polyiso permeance.

    It sounds like you have come to the right conclusions concerning the uses (and potential dangers) of WUFI in an architectural office.

  19. AJ Rao | | #19

    It's not just WUFI
    Hi Martin,

    I echo Dan Whitmore's statements in that no one should trust results that do not pass the sniff test, and I would say that a lot of what you wrote about using WUFI applies equally well to pretty much all of the modeling tools out there (EnergyPlus, anyone?).

    Despite its challenges (and there are many), I think WUFI remains one of the best hygrothermal modeling tools currently available. There's definitely a lot that remains to be done, especially on the material properties front, but used correctly, it can be a powerful tool.

    On the issue of how WUFI would fare in the hands of a typical architect, I think this largely reflects a lot on the state of building science education in the US. But that’s a topic for another conversation....


  20. User avater GBA Editor
    Martin Holladay | | #20

    The sniff test
    AJ Rao,
    You and Dan Whitmore suggest the following method: use WUFI, and sort all WUFI results into two categories -- those that pass the sniff test, and those that don't. The idea is to only use WUFI results if they smell OK, but the throw away WUFI results when they stink.

    As far as it goes, this is good advice. But it raises an important question: which tool is more useful, WUFI or your nose?

    And if WUFI results aren't trustworthy, to what extent is it a useful tool?

    You wrote, "I think WUFI remains one of the best hygrothermal modeling tools currently available." I agree. But it is useful for building science researchers, not designers.

    The basic problem was accurately noted by Melton and Yost: “Any type of modeling can provide counterintuitive results; some are legitimate, and some are not." Since this is true, it's very hard to know when oddball WUFI results are meaningful.

  21. Hartwig Künzel | | #21

    On the growing use of hygrothermal simulation tools
    I guess you have touched a sensitive spot with your blog. I agree that performing hygrothermal simulations isn’t as straightforward as using a ruler. We are aware of the fact that things can go wrong if you don’t know what you do. However, structural engineers are using simulation tools for a long time and there is no doubt that errors in this domain have far more severe consequences than erroneous WUFI results. Nevertheless, it is true that performing hygrothermal simulations is not part of every architect’s education. Therefore, we decided to issue together with ORNL a non-commercial version of WUFI (WUFI ORNL) free of charge, to give everybody the opportunity to get a feel for the moisture transport processes in building assemblies without any consequences. Together with our colleagues from ORNL we thought that understanding hygrothermal phenomena in building assemblies is the first step to design more durable and healthy energy efficient buildings. The professional version of WUFI is rather expensive compared to other hygrothermal simulation tools, such as HygricIRC, because we want only real professionals to use it.

    The growing application of hygrothermal simulation tools worldwide can be explained by a growing number of moisture problems. To reduce carbon dioxide emissions we need more energy efficient buildings. However, improved energy efficiency may result in moisture problems if moisture transfer is not controlled appropriately. Therefore, standards for adequate moisture control design have been developed in Europe and the North America (e.g. ASHRAE Std. 160). These standard require the application of hygrothermal simulation tools especially if your design or intended building operation deviates from standard practice.

    Martin, I hope your blog helps to raise the awareness of the importance of appropriate moisture control design. Your point that everyone performing hygrothermal simulations should know what they are doing is very valuable and justified. I am sure your intention has not been to scare people away from designing energy efficient buildings, because the moisture control issues are perceived to be too complicated; I rather hope that more people get educated and become hygrothermal and energy efficiency experts!

    - Hartwig Künzel

  22. User avater GBA Editor
    Martin Holladay | | #22

    Response to Hartwig Künzel
    Dr. Künzel,
    I think we agree on most points.

    I agree that performing hygrothermal simulations isn’t as straightforward as using a ruler.

    I agree that everyone performing hygrothermal simulations should know what they are doing.

    You guessed correctly: it's not my intention to scare people away from designing energy-efficient buildings. However, it is my intention to scare people away from relying on WUFI results that were produced by inexperienced architects or designers.

  23. Daniel Lu | | #23

    A Call to Action

    You have well delineated how the input uncertainties and real world sensitivities make WUFI an inaccurate tool. But are these fixed facts of life? Or are they areas for improvement? Can software engineers improve WUFI to account for uncertainties like quality of construction and microclimate conditions? Can researchers seek a better understanding of moisture performance’s sensitivities to those uncertainties? And most importantly, can engineers take extra steps to better account for local factors like microclimate? For a tool to become accurate, to "tell the truth," responsibilities lie in both the tool’s makers and the tool’s users.

    While a bulk of the responsibilities lie with WUFI researchers and developers, I wonder if users can improve their inputs. Take a week's worth of climate measurements at the building site, instead of weather data from an airport tower, for instance.

    Hygrothermal performance of walls is inherently complex. But we can still take proactive measures to capture that complexity.

  24. User avater GBA Editor
    Martin Holladay | | #24

    Response to Daniel Lu
    Q. "Can software engineers improve WUFI to account for uncertainties like quality of construction and microclimate conditions? Can researchers seek a better understanding of moisture performance’s sensitivities to those uncertainties? And most importantly, can engineers take extra steps to better account for local factors like microclimate?"

    A. Yes, yes, and yes. However, even if all of these steps are taken, it's highly unlikely that WUFI would ever be a useful tool for builders.

    Your example illuminates the dilemma. You wrote, "Take a week's worth of climate measurements at the building site, instead of weather data from an airport tower, for instance." It would, of course require far more than a week of monitoring to gather enough data to adjust the data recorded at a weather station situated (for example) ten miles away. But even if a week's worth of data were adequate, how many builders or designers will do that? They just won't.

  25. sethw | | #25

    I've just come across this [somewhat old, now] posting. All I can say is - USE IT PROBABLISTICALLY! WUFI as well as any other toll should be used in a real context of other actual knowns to develop knowledge - not as a whizbang quick definitive answer.

  26. User avater GBA Editor
    Martin Holladay | | #26

    I think we're in agreement. However, I'm not sure I understand your intended meaning when you advise WUFI users to "use it probablistically." The central question of this article is whether WUFI is a useful tool for designers and architects (rather than researchers).

    Designers and architects want useful tools, not tools that provide "probablistic" answers. If the answer isn't useful, neither is the tool.

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