Measuring uncontrolled air infiltration
Air tightness is quantified as Air Changes per Hour (ACH), the number of times the entire indoor air volume is replaced by outside air through cracks, gaps, and leaks. A single ACH value of 1 means the air in the building is completely exchanged once per hour through uncontrolled pathways. Older, loosely constructed homes typically measure 5-10 ACH, consuming enormous energy to maintain comfort. Building codes and energy standards have progressively tightened this: new construction might aim for 3 ACH, while Passive House buildings achieve 0.6 ACH or better.
Energy cost of air leakage
Every unit of air infiltration carries energy out of the building: cold outside air in winter must be heated, and cool inside air in summer must be reconditioned. Tightening the envelope by sealing cracks around windows, doors, electrical outlets, and ductwork directly reduces heating and cooling energy demand. The investment in careful air sealing, quality weatherstripping, and sealed ductwork typically pays for itself within 2-5 years through utility savings, making it one of the highest return measures in retrofit and new construction. Many builders now conduct blower door tests to quantify air tightness and verify contractual performance.