GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted
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.

GBA Prime

This article is only available to GBA Prime Members

Sign up for a free trial and get instant access to this article as well as GBA’s complete library of premium articles and construction details.

Start Free Trial


  1. 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. Expert Member
    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. JustHousing | | #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. 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. dankolbert | | #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. user-1140531 | | #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. 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. 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. user-831308 | | #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. 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. user-1078381 | | #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. 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. 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. wjrobinson | | #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. 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. user-3382061 | | #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. user-731553 | | #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. 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. AJRao | | #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. 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 Kuenzel | | #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. 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. ika2315 | | #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. 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. 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.

Log in or become a member to post a comment.



Recent Questions and Replies

  • |
  • |
  • |
  • |