PERFORMANCE: Human Factors


Casement windows can be used to deflect air into a room (plan view).


ASHRAE Standard 55-2004 Thermal Environmental Conditions for Human Occupancy

ASHRAE Standard 62.1-2007 Ventilation for Acceptable Indoor Air Quality

ASHRAE Indoor Air Quality Guide, The: Best Practices for Design, Construction and Commissioning

Natural Ventilation

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
Windows, skylights and roof windows affect air flow depending on their operator type—projecting versus sliding—and their placement.

  • Casement and projecting windows can typically be fully opened, allowing for greater ventilation. An outward-projecting sash may help direct outdoor air into a room. Under harsher wind conditions, however, inward-projecting sashes may be more feasible.
  • Sliding windows have more limited openings of less than half of the overall window area.
  • Mechanically or manually operable skylights or roof windows allow rising warm air to exit and be replaced by cooler outdoor air entering through open windows at a lower level.

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 exterior wind velocity;
  • the angle at which the wind strikes the opening;
  • and the size of the opening.

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

opening height as a fraction of wall height 1/3
opening width as a fraction of wall width 1/3 2/3 3/3
single opening 12-14% 13-17% 16-23%
two openings in same wall 22% 23%
two openings in adjacent walls 37-45% 37-45% 40-51%
two openings in opposite walls 35-42% 37-51% 47-65%

range = wind 45° perpendicular to opening
Source: Sun, Wind & Light, G.Z. Brown and Mark DeKay, 2001.

Coordinating Natural Ventilation and Mechanical Space Conditioning
To avoid increased energy use, natural ventilation and mechanical system operation should be coordinated so that outdoor air intake does not conflict with mechanical heating and cooling. While limited outdoor air intake concurrent with heating and cooling may be acceptable, prolonged coincidence of natural ventilation with mechanical space conditioning should be avoided through scheduling or a zoned approach. Typically, the operation of windows and skylights relies on occupants and facility managers. To limit potential conflicts between window operation and mechanical space conditioning, automatic controls are available that shut off HVAC operation when windows are left open.

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

  • Cross-ventilation depends on a continuous airflow path. Open-plan layouts are ideal, but where spaces are subdivided, rooms must be designed to allow airflow in and out, between rooms and through occupied areas.
  • Single-sided ventilation—one exterior wall has operable windows or vents— may be less effective, since air speed is typically lower than in a cross-ventilation situations.
  • Locate larger spaces on the windward side of the building. This provides improved air distribution in all linked interior spaces.
  • Use false ceilings above corridors and passageways to create breezeways for airflow between rooms on either side of the passage.
  • Separate the window into upper and lower portions, to independently control daylight, natural ventilation and view.
  • Use operable transom windows in walls to permit a free flow of air and daylight.
  • Use high ceilings to allow heated air to rise out of occupied zones.
  • Caution: energy savings from natural cooling require that supplementary air-conditioning systems are turned off when windows are open.

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.


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