If you’re a nerd like me, chances are you’ve heard of WUFI. WUFI is a German acronym that basically translates to heat and moisture movements through a building assembly. This is called hygrothermal analysis, it predicts wetting and drying of an assembly and lets us know if that assembly is risky.
Ideally, we want all of our assemblies to start and remain dry. They almost never do. They can become wet while being built or through bulk water wetting events from the exterior after completion. They can also become wet from interior sources, such as vapor diffusion or moisture moving through a material. This type of wetting of a home is usually not a concern, what’s actually more important with vapor diffusion is how quickly the moisture moves back out of the assembly to promote drying. Air leaks though are a bigger wetting issue. In my very cold climate, an air leak from the interior during the heating season can find a cold surface to condense on. Depending on the materials used in the assembly, drying might be fast or slow, no problem or a big problem. The WUFI software will suggest if the assembly is at risk.
I’ve had the opportunity to be a monthly contributor on a webinar hosted by Rockwool’s Dan Edelman. The webinar also features Travis Brungardt and Joe Cook from Catalyst Built near KC, MO and Chris Laumer-Giddens of LG² in Atlanta. We recently had a discussion about a WUFI analysis of a wall detail for International Falls, Minnesota, the pic below is from one of the webinars.
This is what a WUFI analysis looks like, this one came from Rockwool. Let’s start with this wall assembly. From the exterior, vinyl siding, 1 x 3 fir strips, 1 inch of extruded polystyrene rigid insulation (XPS) R-5, Tyvek, OSB, Dens-packed cellulose insulation, 6 mil polyethylene, and drywall with two coats of latex paint (7 perm). The software was also programmed to have some interior air leaks. There’s a red dashed line at the 80% level, any time the layer surface relative humidity is above the line, the assembly is in risk and anytime there is black on the dashed red line, there’s the potential for mold growth. As you can see, mold may be growing in or on the wall during the summer months. A risky wall assembly. Why is that? The exterior insulation is too low of an R-value to keep the sheeting above the dew point and does not allow outward drying during the winter months. There is also poly in the assembly which prevents any inward drying during the summer. The interior air leaks during the winter allow the OSB sheeting to become wet and cannot dry either during the summer or winter, we call that the dirty diaper effect.
This is a prime example of why the State of Minnesota has amended the energy code to eliminate the requirement for exterior insulation. The wrong choice of exterior insulation can prevent a wall from drying and not having enough R-value on the exterior can allow the sheeting to reach the point where wetting can occur. Risky!
There is one quirk with this assembly, it often works in a very cold climate. Many homes in Canada are built using R-5 exterior insulation and poly on the inside, and without any trouble. The biggest trick is to make the home air-tight. That being said, there is still risk and I wouldn’t personally build this wall.
How could we change this assembly to make it less of a risk?
Modify the wall so that the WUFI analysis looks like this. Everything in the wall is identical except to change the 1 inch of XPS rigid foam to 1.25 inches of Rockwool’s ComfortBoard 80 (R-5.25) and increase the air tightness. The assembly still cannot dry to the interior, but the ComfortBoard is vapor open and allows outward drying. Plus, the home is tighter so that there is less moisture accumulating on the OSB sheeting from interior air leaks. Less wetting and better drying, a better assembly.
One thing we have not talked about with the WUFI illustrations is the brown line. That is the temperature of the OSB wall sheeting at certain times of the year. As you can see, during the winter the temp of the OSB can be below 0°F. If we add more exterior insulation, preferably a type that will allow outward drying, we can warm the sheeting so that the temperature can, for most of the time, stay above the dew point temp. Warming the sheeting will also have the effect of increasing comfort within the home.
Can you have a WUFI analysis done on your building assemblies? Possibly. The bigger manufacturers of building materials have building scientists on staff or contract with independent companies who can perform this service, sometimes at no cost if you are using their products. It’s always better to solve any problem on paper rather than to find the problem after it’s built.