In some parts of the country, where temperature and humidity levels permit, natural ventilation through operable windows can be an effective and energy-efficient way to supplement HVAC systems to provide outside air ventilation, cooling, and thermal comfort when conditions permit (e.g., temperature, humidity, outdoor air pollution levels, precipitation). Windows that open and close can enhance occupants’ sense of well-being and feeling of control over their environment. They can also provide supplemental exhaust ventilation during renovation activities that may introduce pollutants into the space.

However, sealed buildings with appropriately designed and operated HVAC systems can often provide better indoor air quality than a building with operable windows. Uncontrolled ventilation with outdoor air can allow outdoor air contaminants to bypass filters, potentially disrupt the balance of the mechanical ventilation equipment, and permit the introduction of excess moisture if access is not controlled.

Strategies using natural ventilation include wind driven cross-ventilation and stack ventilation that employs the difference in air densities to provide air movement across a space. Both types of natural ventilation require careful engineering to ensure convective flows. The proper sizing and placement of openings is critical and the flow of air from entry to exit must not be obstructed (e.g., by closed perimeter rooms).

Designers should consider the use of natural ventilation and operable windows to supplement mechanical ventilation.

Consider outdoor sources of pollutants (including building exhausts and vehicle traffic) and noise when determining if and where to provide operable windows.

If operable windows will be used to supplement the HVAC system, ensure that:

• openings for outdoor air are located between 3-6 feet from the floor (head height);
• the windows are adjustable and can close tightly and securely;
• the windows are placed to take maximum advantage of wind direction, with openings on opposite sides of the building to maximize cross-ventilation.