Building designers and researchers have begun to realize that computer modeling programs, including WUFI, sometimes falsely predict that certain common wall assemblies — wall assemblies that have been used successfully for years — should be failing. (WUFI is a so-called “hygrothermal” modeling program — that is, a program that calculates heat and moisture flows through building assemblies. For more information on WUFI, see “WUFI IS Driving Me Crazy.”)
Yet experienced builders know that these wall assemblies aren’t failing. So what’s going on?
Something smells wrong about these WUFI results
Several GBA articles, most notably “Monitoring Moisture Levels in Double-Stud Walls,” have mentioned WUFI’s false predictions of failure.
In that November 2013 article, I described a moisture modeling study performed by two researchers, Lois Arena and Pallavi Mantha. I reported, “Arena and Mantha used WUFI to calculate the predicted moisture content of several components of various wall assemblies, including 12-inch-thick cellulose insulated walls. The researchers wrote, ‘Results from WUFI indicate that condensation potential for the double [stud] cellulose walls is extremely high because the OSB in those wall assemblies is entirely outside of the insulation. … 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.’”
Arena and Mantha eventually concluded that the source of the error was certain moisture values and mold growth assumptions imported into WUFI from ASHRAE Standard 160. My article continued, “For some reason, the ASHRAE 160 values used by Arena and Mantha produced unlikely WUFI results. … The modeling results don’t pass the sniff test. Arena and Mantha clearly recognize that fact, and they accurately deduced that the anomalies stem from their use…