Air barrier systems create a continuous, air-tight layer in the building envelope preventing infiltration of outside air and exfiltration of conditioned air. A proper air barrier reduces energy consumption and improves occupant comfort by eliminating drafts. The air barrier can be the outer sheathing, a dedicated membrane, or a combination. Common materials include dense foam sheathing, house wrap, or polyethylene film. The key requirement is continuity; all penetrations and joints must be sealed to prevent air leakage. Typical air leakage locations include corners, window and door frames, utility penetrations, and control joints. Sealing is typically done with caulk, sealant, or specialized sealant tape. Tape must adhere to a clean, dry surface for effective sealing. Joints in foam sheathing can be taped with compatible tape. Penetrations are sealed with caulk or foam sealant. Testing with a blower door test pressurizes or depressurizes the building and measures air leakage. Buildings with effective air barriers typically achieve air leakage rates below 5 air changes per hour at 50 Pa pressure difference. High-performance buildings may achieve below 2 air changes per hour. The air barrier must work with the vapor barrier and insulation to control moisture. In cold climates, proper sequencing ensures moisture does not condense inside the wall cavity. Air barriers improve thermal comfort and reduce energy costs significantly. ASHRAE standards address air barrier materials and installation practices.