What is required in dwelling units for use during the non-heating season?

• A. Only natural ventilation
• B. Only mechanical ventilation
• C. Either natural or mechanical ventilation
• D. No ventilation required

Answer: (C)

When the heating system isn’t running, a dwelling must still have a way to exchange indoor air with the outdoors to ensure good indoor air quality. You can meet this requirement with natural ventilation (operable windows or vents that allow outdoor air to flow in and stale air to escape) or with mechanical ventilation (fans or a ducted system that provides a controlled air exchange). Either method is acceptable as long as it delivers the required ventilation rate to keep the indoor air healthy. Relying on no ventilation at all would lead to moisture buildup, odors, and poor air quality, and restricting to only one method when the other would be practical misses the flexibility allowed by the standard.