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

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-control layers ^[1]. An energy-efficient building envelope is designed to reduce unwanted heat transfer, air leakage, and moisture accumulation while maintaining acceptable indoor environmental conditions. Key performance considerations include thermal resistance, airtightness, moisture control, solar control, and, where appropriate, thermal mass

^[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.

. Thermal transmittance (U-value), expressed in W/(m²·K), can be calculated for applicable building components and elements from the thermal resistances of their constituent layers and the relevant surface resistances in accordance with ISO 6946 ^[4]. ISO 6946 applies principally to relevant building components and elements and excludes windows, doors and other glazed units, curtain walling, building components involving heat transfer to the ground, and components through which air is designed to permeate ^[4]. Energy-efficient envelopes are therefore defined by their ability to achieve appropriate thermal, air, moisture, and solar-control performance while supporting energy-efficient building operation, rather than by a prescribed material or construction method ^[1][2].