Last updated: 06-03-2008
Almost all historic buildings were ventilated naturally, although many of these have been compromised by the addition of partition walls and mechanical systems. With an increased awareness of the cost and environmental impacts of energy use, natural ventilation has become an increasingly attractive method for reducing energy use and cost and for providing acceptable indoor environmental quality and maintaining a healthy, comfortable, and productive indoor climate rather than the more prevailing approach of using mechanical ventilation. In favorable climates and buildings types, natural ventilation can be used as an alternative to air-conditioning plants, saving 10%-30% of total energy consumption.
Natural ventilation systems rely on pressure differences to move fresh air through buildings. Pressure differences can be caused by wind or the buoyancy effect created by temperature differences or differences in humidity. In either case, the amount of ventilation will depend critically on the size and placement of openings in the building. It is useful to think of a natural ventilation system as a circuit, with equal consideration given to supply and exhaust. Openings between rooms such as transom windows, louvers, grills, or open plans are techniques to complete the airflow circuit through a building. Code requirements regarding smoke and fire transfer present challenges to the designer of a natural ventilation system. For example, historic buildings used the stairway as the exhaust stack, a technique now prevented by code requirements in many cases.
Natural ventilation, unlike fan-forced ventilation, uses the natural forces of wind and buoyancy to deliver fresh air into buildings. Fresh air is required in buildings to alleviate odors, to provide oxygen for respiration, and to increase thermal comfort. At interior air velocities of 160 feet per minute (fpm), the perceived interior temperature can be reduced by as much as 5°F. However, unlike true air-conditioning, natural ventilation is ineffective at reducing the humidity of incoming air. This places a limit on the application of natural ventilation in humid climates.
A. Types of Natural Ventilation Effects
Wind can blow air through openings in the wall on the windward side of the building, and suck air out of openings on the leeward side and the roof. Temperature differences between warm air inside and cool air outside can cause the air in the room to rise and exit at the ceiling or ridge, and enter via lower openings in the wall. Similarly, buoyancy caused by differences in humidity can allow a pressurized column of dense, evaporatively cooled air to supply a space, and lighter, warmer, humid air to exhaust near the top. These three types of natural ventilation effects are further described below.
Wind causes a positive pressure on the windward side and a negative pressure on the leeward side of buildings. To equalize pressure, fresh air will enter any windward opening and be exhausted from any leeward opening. In summer, wind is used to supply as much fresh air as possible while in winter, ventilation is normally reduced to levels sufficient to remove excess moisture and pollutants. An expression for the volume of airflow induced by wind is:
Qwind = K x A x V, where
Qwind = volume of airflow (m³/h)
A = area of smaller opening (m²)
V = outdoor wind speed (m/h)
K = coefficient of effectiveness
The coefficient of effectiveness depends on the angle of the wind and the relative size of entry and exit openings. It ranges from about 0.4 for wind hitting an opening at a 45° angle of incidence to 0.8 for wind hitting directly at a 90° angle.
Sometimes wind flow prevails parallel to a building wall rather than perpendicular to it. In this case it is still possible to induce wind ventilation by architectural features or by the way a casement window opens. For example, if the wind blows from east to west along a north-facing wall, the first window (which opens out) would have hinges on the left-hand side to act as a scoop and direct wind into the room. The second window would hinge on the right-hand side so the opening is down-wind from the open glass pane and the negative pressure draws air out of the room.
It is important to avoid obstructions between the windward inlets and leeward exhaust openings. Avoid partitions in a room oriented perpendicular to the airflow. On the other hand, accepted design avoids inlet and outlet windows directly across from each other (you shouldn't be able to see through the building, in one window and out the other), in order to promote more mixing and improve the effectiveness of the ventilation.
Buoyancy ventilation may be temperature-induced (stack ventilation) or humidity induced (cool tower). The two can be combined by having a cool tower deliver evaporatively cooled air low in a space, and then rely on the increased buoyancy of the humid air as it warms to exhaust air from the space through a stack. The cool air supply to the space is pressurized by weight of the column of cool air above it. Although both cool towers and stacks have been used separately, the author feels that cool towers should only be used in conjunction with stack ventilation of the space in order to ensure stability of the flow. Buoyancy results from the difference in air density. The density of air depends on temperature and humidity (cool air is heavier than warm air at the same humidity and dry air is heavier than humid air at the same temperature). Within the cool tower itself the effect of temperature and humidity are pulling in opposite directions (temperature down, humidity up). Within the room, heat and humidity given off by occupants and other internal sources both tend to make air rise. The stale, heated air escapes from openings in the ceiling or roof and permits fresh air to enter lower openings to replace it. Stack effect ventilation is an especially effective strategy in winter, when indoor/outdoor temperature difference is at a maximum. Stack effect ventilation will not work in summer (wind or humidity drivers would be preferred) because it requires that the indoors be warmer than outdoors, an undesirable situation in summer. A chimney heated by solar energy can be used to drive the stack effect without increasing room temperature, and solar chimneys are very widely used to ventilate composting toilets in parks.
An expression for the airflow induced by the stack effect is:
Qstack = Cd*A*[2gh(Ti-To)/Ti]^1/2, where
Qstack = volume of ventilation rate (m³/s)
Cd = 0.65, a discharge coefficient.
A = free area of inlet opening (m²), which equals area of outlet opening.
g =9.8 (m/s²). the acceleration due to gravity
h = vertical distance between inlet and outlet midpoints (m)
Ti = average temperature of indoor air (K), note that 27°C = 300 K.
To = average temperature of outdoor air (K)
Cool tower ventilation is only effective where outdoor humidity is very low. The following expression for the airflow induced by the column of cold air pressurizing an air supply is based on a form developed by Thompson (1995), with the coefficient from data measured at Zion National Park Visitor Center. This tower is 7.4 m tall, 2.4 m square cross section, and has a 3.1 m² opening.
Qcool tower =0.49 * A* [2gh (Tdb-Twb)/Tdb]1/2, where
Qcool tower = volume of ventilation rate (m³/s)
0.49 is an empirical coefficient calculated with data from Zion Visitor Center, UT, which includes humidity density correction, friction effects, and evaporative pad effectiveness.
A = free area of inlet opening (m²), which equals area of outlet opening.
g =9.8 (m/s²). the acceleration due to gravity
h = vertical distance between inlet and outlet midpoints (m)
Tdb = dry bulb temperature of outdoor air (K), note that 27°C = 300 K.
Twb = wet bulb temperature of outdoor air (K)
The total airflow due to natural ventilation results from the combined pressure effects of wind, buoyancy caused by temperature and humidity, plus any other effects from sources such as fans. The airflow from each source can be combined in a root-square fashion as discussed in ASHRAE (2001). The presence of mechanical devices that use room air for combustion, leaky duct systems, or other external influences can significantly affect the performance of natural ventilation systems.
B. Design Recommendations
The specific approach and design of natural ventilation systems will vary based on building type and local climate. However, the amount of ventilation depends critically on the careful design of internal spaces, and the size and placement of openings in the building.
- Maximize wind-induced ventilation by siting the ridge of a building perpendicular to the summer winds.
- Approximate wind directions are summarized in seasonal "wind rose" diagrams available from the National Oceanographic and Atmospheric Administration (NOAA). However, these roses are usually based on data taken at airports; actual values at a remote building site can differ dramatically.
- Buildings should be sited where summer wind obstructions are minimal. A windbreak of evergreen trees may also be useful to mitigate cold winter winds that tend to come predominantly from the north.
- Naturally ventilated buildings should be narrow.
- It is difficult to distribute fresh air to all portions of a very wide building using natural ventilation. The maximum width that one could expect to ventilate naturally is estimated at 45 ft. Consequently, buildings that rely on natural ventilation often have an articulated floor plan.
- Each room should have two separate supply and exhaust openings. Locate exhaust high above inlet to maximize stack effect. Orient windows across the room and offset from each other to maximize mixing within the room while minimizing the obstructions to airflow within the room.
- Window openings should be operable by the occupants.
- Provide ridge vents.
- A ridge vent is an opening at the highest point in the roof that offers a good outlet for both buoyancy and wind-induced ventilation. The ridge opening should be free of obstructions to allow air to freely flow out of the building.
- Allow for adequate internal airflow.
- In addition to the primary consideration of airflow in and out of the building, airflow between the rooms of the building is important. When possible, interior doors should be designed to be open to encourage whole-building ventilation. If privacy is required, ventilation can be provided through high louvers or transoms.
- Consider the use of clerestories or vented skylights.
- A clerestory or a vented skylight will provide an opening for stale air to escape in a buoyancy ventilation strategy. The light well of the skylight could also act as a solar chimney to augment the flow. Openings lower in the structure, such as basement windows, must be provided to complete the ventilation system.
- Provide attic ventilation.
- In buildings with attics, ventilating the attic space greatly reduces heat transfer to conditioned rooms below. Ventilated attics are about 30°F cooler than unventilated attics.
- Consider the use of fan-assisted cooling strategies.
- Ceiling and whole-building fans can provide up to 9°F effective temperature drop at one tenth the electrical energy consumption of mechanical air-conditioning systems.
- Determine if the building will benefit from an open- or closed-building ventilation approach.
- A closed-building approach works well in hot, dry climates where there is a large variation in temperature from day to night. A massive building is ventilated at night, then, closed in the morning to keep out the hot daytime air. Occupants are then cooled by radiant exchange with the massive walls and floor.
- An open-building approach works well in warm and humid areas, where the temperature does not change much from day to night. In this case, daytime cross-ventilation is encouraged to maintain indoor temperatures close to outdoor temperatures.
- Use mechanical cooling in hot, humid climates.
- Try to allow natural ventilation to cool the mass of the building at night in hot climates.
- Open staircases provide stack effect ventilation, but observe all fire and smoke precautions for enclosed stairways.
Photo of visitor center at Zion National Park showing downdraft cooling tower with evaporative media at the top, and exhaust through high clerestory windows.
(Courtesy of Robb Williamson)
Natural ventilation in most climates will not move interior conditions into the comfort zone 100% of the time. Make sure the building occupants understand that 3% to 5% of the time thermal comfort may not be achieved. This makes natural ventilation most appropriate for buildings where space conditioning is not expected. As a designer it is important to understand the challenge of simultaneously designing for natural ventilation and mechanical cooling—it can be difficult to design structures that are intended to rely on both natural ventilation and artificial cooling. A naturally ventilated structure often includes an articulated plan and large window and door openings, while an artificially conditioned building is sometimes best served by a compact plan with sealed windows. Moreover, interpret wind data carefully. Local topography, vegetation, and surrounding buildings have an effect on the speed of wind hitting a building. Wind data collected at airports may not tell you very much about local microclimate conditions that can be heavily influenced by natural and man-made obstructions. Hints about what type of natural ventilation strategies might be most effective can often be found in a region's historic and vernacular construction practices.
C. Materials and Methods of Construction
Some of the materials and methods used to design proper natural ventilation systems in buildings are solar chimneys, wind towers, and summer ventilation control methods. A solar chimney may be an effective solution where prevailing breezes are not dependable enough to rely on wind-induced ventilation and where keeping indoor temperature sufficiently above outdoor temperature to drive buoyant flow would be unacceptably warm. The chimney is isolated from the occupied space and can be heated as much as possible by the sun or other means. Air is simply exhausted out the top of the chimney creating suction at the bottom which is used to extract stale air.
Wind towers, often topped with fabric sails that direct wind into the building, are a common feature in historic Arabic architecture, and are known as "malqafs." The incoming air is often routed past a fountain to achieve evaporative cooling as well as ventilation. At night, the process is reversed and the wind tower acts as a chimney to vent room air. A modern variation called a "Cool Tower" puts evaporative cooling elements at the top of the tower to pressurize the supply air with cool, dense air.
In the summer, when the outside temperature is below the desired inside temperature, windows should be opened to maximize fresh air intake. Lots of airflow is needed to maintain the inside temperature at no more than 3-5 °F above the outside temperature. During hot, calm days, air exchange rates will be very low and the tendency will be for inside temperatures to rise above the outside temperature. The use of fan-forced ventilation or thermal mass for radiant cooling may be important in controlling these maximum temperatures.
D. Analysis and Design Tools
Handbook methods such as those presented in ASHRAE's Handbook of Fundamentals or Bansal and Minke's Passive Building Design: A Handbook of Natural Climatic Control(ISBN: 044481745X) are very useful in calculating airflow from natural sources for very simple building geometries.
Computational Fluid Dynamics (CFM): In order to predict the details of natural airflow, numerical computational fluid mechanics models can be used. These computer simulations are detailed and labor intensive, but are justified where accurate understanding of airflow is important. They have been used to analyze new buildings including the atrium of a courthouse in Phoenix and the hangar of an air and space museum in the Washington, DC area.
An extensive list of journals, books, and other reference material regarding natural ventilation and other passive technologies is included in the Solstice Archive. For example:
DOE Building Energy Codes Program
EERE Fact Sheet: Cooling Your Home Naturally (PDF 110 KB, 8 pgs)
Software packages for natural ventilation analysis include:
AIRPAK: provides calculation of airflow modeling, contaminant transport, room air distribution, temperature and humidity distribution, and thermal comfort by computational fluid dynamics.
FLOVENT: calculates airflow, heat transfer, and contamination distribution for built environments using computational fluid dynamics.
FLUENT: A computational fluid dynamics program useful in modeling natural ventilation in buildings. It models airflow under specified conditions, so additional analysis is required to estimate annual energy savings.
STAR-CD: STAR-CD uses computational fluid dynamics to help civil engineers, architects and project managers who need better and more detailed understanding of issues involved in heating and ventilation, smoke and pollutant dispersal and fire hazard analysis, and clean room design.
Building models incorporate very limited features for deliberate natural ventilation, but they do include the calculation of natural air infiltration as a function of temperature difference, wind speed, and effective leakage area, or schedules and user-defined functions for infiltration rates.
Designing Low Energy Buildings with Energy-10—An hour-by-hour simulation program designed to inform the earliest phases of the design process. Runs on IBM-compatible platforms. Best operated with Pentium or higher processor and 32 Megs of RAM.
DOE-2: A comprehensive hour-by-hour simulation; daylighting and glare calculations integrate with hourly energy simulation. IBM or compatible Pentium is advisable.
ENERGY PLUS: A building energy simulation program designed for modeling buildings with associated heating, cooling, lighting, ventilating, and other energy flows.