The knowledge base on the topic of hygrothermal performance of building assemblies is as varied as it is vast. Although generally agreeing at the level of fundamental building science principles, this knowledge base can differ substantially in assumptions, analysis methodology, degree of completeness, treatment of material properties and boundary conditions, stringency of performance limit states or criteria, and judgments regarding acceptable or preferred solutions. These uncertainties contribute to confusion in the building design and construction marketplace regarding appropriate methods or best practices, particularly in view of implementing improved energy code requirements for the insulation of building envelopes. The confusion is amplified by gaps in the coordination of water-vapor control strategies in building codes and insulation strategies represented in energy codes. A comprehensive review and state-of-the-art evaluation is needed to better understand and resolve these real or perceived uncertainties while also identifying gaps where continual improvements can be offered or pursued with further research. This paper strives to provide such a review of U.S. and Canadian building code practices together with an assessment of data and findings from various modeling, test hut, and field studies on the hygrothermal performance of modern, light-frame building envelopes. It attempts to reconcile this information on a consistent basis and recommend incremental improvements to better unify current design and construction practices as represented in current U.S. and Canadian building codes. As a result of this effort, an attempt is made to formulate a reasonably comprehensive, consistent, and simple framework for the presentation of prescriptive guidance for design and construction of energy-efficient and durable building envelopes. Conclusions and recommendations for further research also are offered.