Hygrothermal simulation programs (HSPs) have the potential to help the way building enclosures are designed if end users are convinced the results are reliable. However, HSPs suffer from a lack of manufacturer-specific material data. HSP users often “create” materials by scouring multiple sources for hygrothermal properties. To the less familiar HSP user, significant error can be introduced into simulations by using properties that are calculated from variable sources. Through a series of case studies, this paper illustrates how material properties can make a difference in hygrothermal calculations and, ultimately, material specification.