The building envelope is the physical barrier between a building's interior and exterior, including walls, roof, foundation, windows, and doors. The envelope protects occupants from weather while controlling heat transfer, air movement, and moisture. Envelope quality significantly affects energy efficiency—well-insulated, airtight envelopes reduce heating and cooling costs, while poor envelopes waste energy and can cause comfort and moisture problems. Building codes increasingly require better envelope performance to reduce energy use and greenhouse gas emissions. Envelope improvements (adding insulation, upgrading windows, sealing air leaks) are common elements of building retrofits and renovation projects.