I'm new to WUFI (and building science in general), and while constructing a wall in the Assembly/Monitor screen I noticed that the Total Thermal Performance value is vastly different from the nominal values that manufacturers provide. For example, XPS is generally state to be R-5 per 1 inch. However, WUFI has it at R-5.79 per 1 inch. I know that R-value can change depending on temperature and moisture content, but a wall value I had originally calculate to be R-36 (made up of 3" XPS and R-21 Fiberglass batt at 5.5") is actually R-49 on WUFI when I use those dimensions. Building science professionals, how do you reconcile this? Do you simply figure out what the thickness should be on given your desired R-value? For instance, to get an R-value of R-15 for XPS on WUFI, the thickness should be 2.59 inches. Is that what I should be doing? Any insight would be appreciated.
Thanks for your time.