PERFORMANCE: Human Factors
Energy & Cost
Codes & Standards
The feasibility of natural ventilation strategies depends on local air quality, climate conditions, building layout, and other factors. Where natural ventilation is feasible, operable windows can offer fresh air flow without the need for energy input. What's more, occupants often value the psychological benefits of a connection to the outdoors, especially if they can control it by simply opening a window.
A well-designed natural cooling strategy can be effective but its potential is greatly influenced by floor plan depth. Narrow floor plans increase the potential for effective cross-ventilation. Cross-ventilation can move air over deep floor plans, but air temperature increases and air quality drops as it moves across the room.
This overview does not try to answer the question of whether natural ventilation is advisable for particular buildings, but covers the role of operable windows and skylights in cases where natural ventilation is desired, and how this role can be coordinated with mechanical space conditioning.
Window Characteristics that Affect Air Flow
Window placement (location and size of opening) will affect occupant cooling if air is moving fast enough. The average interior air velocity is a function of:
The diagram and table below demonstrate how the size, number, and location of the openings will affect the air flow (Brown and DeKay, 2001).
Average Interior Air Velocity as a Percentage of the Exterior Wind Velocity
Coordinating Natural Ventilation and Mechanical Space Conditioning
Although the temperature of incoming outdoor air is often outside the range of typical thermostat setpoints, it may still provide comfort without the need for offsetting heating or cooling. Field experiments suggest that somewhat warmer or cooler temperatures are acceptable to occupants as long as they are in control of window use. This is acknowledged by ASHRAE Standard 55-2004 Thermal Environmental Conditions for Human Occupancy, which includes an Adaptive Model option that suggests a slightly wider range of acceptable indoor temperature if occupants of naturally ventilated spaces can adapt their clothing to their expectations regarding the seasons and weather. For example, when the outdoor temperature is 86°F, the average preferred temperature in a naturally ventilated building may be 80°F, compared to 77°F in a mechanically ventilated building. These values are examples only, and may not be acceptable if excessive humidity is introduced alongside warm outdoor air. Under favorable climatic conditions and as long as occupants can control the use of windows, however, limited temperature swings due to natural ventilation do not have to necessitate offsetting mechanical space conditioning.
Window operation according to fixed schedules can hold great potential in certain climates. For example, scheduled natural ventilation may provide nighttime cooling and improve indoor comfort and air quality for the next morning.
Design Guidance from ASHRAE's Indoor Air Quality Guide
Air leakage (infiltration/exfiltration) is distinct from natural ventilation for it is the uncontrolled movement of air through joints in the window assembly and surrounding framing.