A building envelope (or building envelope assembly) is the physical separator between the conditioned interior and the exterior environment, comprising exterior walls, roofs, floors on grade or over unconditioned space, windows, doors, and associated air, vapor, and moisture barriers ^[1]. An energy-efficient building envelope is designed to minimize unwanted heat transfer, air leakage, and moisture accumulation while maintaining acceptable indoor environmental conditions, by specifying high thermal resistance (low thermal transmittance, U-value), reduced air infiltration, appropriate solar control, and adequate thermal mass ^[2]. Thermal performance is characterized by the thermal transmittance U, expressed in W/(m²·K), calculated layer by layer from design thermal conductivities and inside/outside surface heat-transfer resistances in accordance with ISO 6946 ^[3]. Energy-efficient envelopes are distinguished from conventional envelopes by the deliberate integration of continuous insulation, airtightness detailing, and (in some configurations) ventilated cavities or phase-change materials; the designation is defined by performance targets rather than by a prescribed material or construction method.