A prototype universal building envelope hygrothermal performance standard that is intended to apply to all above- and belowgrade envelope components is described. The standard is expressed as a definition of a water separation plane followed by a set of three requirements. The requirements are applicable to the envelope components on the interior and exterior sides of the water separation plane and on the interior side only when the exterior side components are structurally and biotically inert. The standard is discussed using experimental data to illustrate salient features. Application examples for an above-grade wall system and a commercial roof system are presented and their hygrothermal performance evaluated in terms of the universal standard requirements.