Hygrothermal modeling, the analysis of heat and moisture transport through building enclosure assemblies, reveals many of the climate-specific building science secrets that are key to durable design of walls and roofs and other assemblies. Debunking rules of thumb and avoiding answers of “it depends” – data and analysis can reveal exactly how many inches of a certain insulation are required, exactly what perm-rating will optimize an assembly, and what kind of havoc a reservoir cladding system can create. Case studies will be shown that demonstrate when perm rating matters and when it does not, the impact of roof membrane color and the risks of “cool roofing,” and when HT (high-temp) rated membranes are actually required. This presentation’s ulterior motive is to convince the audience that every architecture firm should be doing in-house hygrothermal modeling as an integral component of climate-specific, durable, and resilient design.