If you have any involvement with the world of building science, you may have heard about something called WUFI and wondered what the heck it is. Maybe you’ve heard that it’s a piece of software (several pieces, actually) that does hygrothermal modeling. Well, today’s your lucky day because I recently went through a two-day class on WUFI 1-D with Dr. Achilles Karagiozisâ€ and Mr. Mikael Salonvaara of Owens Corning, and I’ll give you a brief explanation of what it’s all about.
And I’ll give it to you the way we learned about Guy Pearce’s character in the movie Memento; that is, I’ll start with the results and work my way back to the inputs.
Step 5: WUFI Bio tells you if your assembly will grow mold.
Once you’ve done all the modeling, you can take your results to the add-on program, WUFI Bio, to see if your assembly will be wet enough to grow mold. The graph below shows a material that doesn’t dry out and ends up being a good mold factory. Note the red light at top left.
Step 4: WUFI shows how your the water content of the materials over time.
The graph below shows that the OSB sheathing in this wall assembly doesn’t dry sufficiently over the three year period of the simulation. Ideally, you’d like to see it stay below 15-16% (right axis).
Step 3: Run the simulation and watch what happens in the different layers.
The graph below is one screen shot of the animation you can watch as WUFI goes through all the calculations for moisture and heat flow, hour by hour. The top graph shows the temperature profile, from outdoors on the left to indoors on the right. The red line shows the temperature at that particular hour of the simulation. The pink area shows all the temperatures reached in the various layers, including the minimum and maximum.
The graph below with the green and blue lines and areas shows the moisture analysis. Relative humidity is in green and water content is in blue.
This screenshot of the WUFI analysis animation shows only one point in the hourly simulation. One of the outputs of WUFI is a video showing how the moisture and temperature change over time in the different layers.
Step 2: Set up your assembly and enter all the inputs.
The screen shot below shows the main screen of WUFI 1-D. The first thing you do is set up the layers of your assembly. The one you see below has brick on the outside, a 1-inch air space, building paper, oriented strand board, fiberglass insulation, and drywall. You choose and adjust the parameters to match the actual materials being used.
You also set a lot of other parameters that control how the simulation will run: orientation, coatings, permeance of paint, climate, wind-driven rain, and initial conditions. 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.
Remember the GIGO principle: Garbage in, garbage out.
Entering the assembly layers and their properties is the most critical part of using WUFI. There are many entries, and it’s easy to mess things up here.
Step 1: Solve the partial differential equations for the flow of heat and moisture.
The equations are shown below. WUFI solves them numerically so you don’t have to.
Now let me cover some of the basics. First, you may be wondering what the WUFI acronym stands for, and it is WÃ¤rme- Und Feuchtetransport InstationÃ¤r. According to Google Translate, that’s German for transient heat and moisture transport.
Recall what I said about it being a hygrothermal modeling tool, so it basically looks at the movement—or accumulation—of moisture and heat through an assembly over a period of time. The Fraunhofer Institute in Germany originally created the software, but Oak Ridge National Laboratory (where Dr. Achilles Karagiozis used to work) and Owens Corning (where Achilles now works) are involved in its continued development.
The version of WUFI that I’m discussing here is the one-dimensional WUFI 1-D. Other versions include WUFI 2-D, WUFI Pro, WUFI Passive, and WUFI Bio (mentioned in step 1 above). They have different capabilities and aims but all address the flow and accumulation of moisture and heat. If you’re ready to go a bit deeper in understanding it, I recommend the Fraunfhofer introduction to WUFI page.
That’s it. That’s what WUFI’s all about. I’ll have to come back to this topic because Joe Lstiburek, who likes to argue with everybody, and Achilles Karagiozis have had an ongoing clash over WUFI. Last year at the Passive House conference in Denver, Joe said that if you need to use WUFI, you’ve done something wrong. Achilles opened the two-day class I just took by saying, “No one can do hygrothermal analysis in their heads.” More on that later.
Now, if you want to go even further back, go watch Guy Pearce play Felicia Jollygoodfellow in the movie, The Adventures of Priscilla Queen of the Desert.
â€ Achilles, by the way, is one of the most interesting and entertaining teachers I’ve had. He’s a dynamo who’s full of information and stories. Although he dashed my hopes with what he said to start the class (see my quote at the end of this article), I loved the class and his teaching style.