Section 5.1.1/Natural Ventilation Requirements
ASHRAE 62.1 Section 5.1 covers operable opening area, location, and conditions for naturally ventilated spaces.
Naturally ventilated spaces must have operable openings (windows, doors, louvers) with a total free area of at least 4% of the net occupiable floor area. The openings must be permanently open or openable by the building occupants. At least half of the required opening area must be located in the upper half of the wall to promote effective air circulation. Natural ventilation is only permitted in climate zones and seasons where outdoor air quality and temperature are within acceptable ranges. Spaces that rely solely on natural ventilation must have a means to alert occupants or switch to mechanical ventilation when conditions are unfavorable.
Why this section exists
Natural ventilation can provide adequate outdoor air in moderate climates without the energy cost of mechanical systems. However, natural ventilation is unreliable in extreme temperatures, high humidity, or when outdoor air quality is poor. The 4% opening area ensures sufficient airflow when windows are open. The upper-half requirement promotes stack effect ventilation. The restrictions on when natural ventilation may be used prevent designs that rely on open windows in climates where occupants will keep them closed most of the year.
What plan reviewers look for
Plan reviewers check the window schedule for operable area and calculate the ratio to the floor area served. They verify that operable windows are accessible to occupants (not fixed or sealed). They check the climate zone and whether natural ventilation is appropriate year-round or seasonally. For spaces that use natural ventilation as the primary strategy, they verify a mechanical backup system or occupant notification method is provided.