On the surface, R-value is a simple thing. In fact, it has become the standard metric of thermal performance precisely because it is easy to explain and understand. Most insulation materials have ‘label R-values’ stamped on their faces (or at least displayed in large print on the packaging), but these values do not tell the whole story of how insulation performs in service. Some complicating factors—such as thermal bridging—have become fairly well-known.