How do uncontrolled quantities of outdoor air enter buildings?

• A. Gaps in exterior construction
• B. Infiltration through windows
• C. Infiltration through doors
• D. All of the above

Answer: (D)

Uncontrolled outdoor air enters buildings through any leak or opening in the building envelope. Gaps in exterior construction create leakage paths around the structure, so outdoor air can seep in even when systems aren’t supplying air. Windows can leak if weatherstripping is worn or damaged, or if there are cracks around the frame, so air can infiltrate even when a window is closed. Doors can also allow air to enter if they aren’t sealing tightly due to misalignment or damaged weatherstripping, or when they’re opened. These pathways all become active because pressure differences—driven by wind and by the buoyancy of warm air rising (stack effect)—push outdoor air into the building. So, all of these contribute to uncontrolled outdoor air entry.