Building Envelope
The building envelope is the complete physical separation between conditioned interior space and the outside environment — including walls, roof, floor (or slab), windows, and doors. Envelope performance is determined by R-values, U-factors, airtightness (ACH50), and continuous detailing at every transition. The envelope is what most building-science investment goes into: a well-insulated, airtight envelope makes everything else (HVAC, ventilation, comfort, embodied carbon) easier and cheaper.