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Natural Ventilation

Human comfort is a function of four primary variables; air temperature, air movement, humidity, and the mean radiant temperature of interior surfaces. Natural ventilation is an energy efficient way to increase human comfort because air movement increases heat transfer from the skin when cooler outside air replaces warm and humid indoor air. Perceived temperature differences of 4-8 degrees F may occur with air movement of 100-350 feet per minute, the upper limit being when papers might flutter off the table.

Contents

1.Definition                                                
2. Use/Application
    a. Established Techniques
    b. Emerging Trends

3. Use an Integrated Approach

4. Resources

5. Associated Strategies

6. Case Studies

Human comfort is a function of four primary variables; air temperature, air movement, humidity, and the mean radiant temperature of interior surfaces. Natural ventilation is an energy efficient way to increase human comfort because air movement increases heat transfer from the skin when cooler outside air replaces warm and humid indoor air. Perceived temperature differences of 4-8 degrees F may occur with air movement of 100-350 feet per minute, the upper limit being when papers might flutter off the table.

Effectiveness of natural ventilation depends upon:

• The temperature and humidity of outside air in evening and early morning, as compared to daytime inside conditions.
• The amount of shading around the building.
• Active participation of building occupants to control window opening and closing at the proper times.
• Active participation of occupants to control ventilation fans at proper times.
• Ambient noise and pollution at the site.

Natural air movement is the product of air flow caused by temperature and/or pressure differences. Wind is produced when a large mass of air moves from a high pressure area in one location to a lower pressure area in another, or from an area of one temperature to a higher or lower temperature – as in on-shore or off-shore breezes one experiences in coastal regions. Natural air movement is influenced by pressure differences as it moves around and through a building; positive on the windward side, negative on the leeward side, and influenced by pressure and air density differences induced by height. For example, it is helpful to have cooling breezes enter a building low and exit high as it releases the heat from the interior.

This is the principle behind the wind chimney, used in ancient cultures in the mid-east where the air was cooled by being directed by passages in the building design to pass over a cooling water source before circulating through the interior spaces. The air carrying the interior heat was then released up and out naturally through the wind chimney.

Soft and hard landscape features or building design elements may be used to channel desirable breezes and shield from objectionable winds and storms. Building elevations can be designed with wing walls, overhangs, and strategically placed openings to enhance otherwise poor wind speed. Abrupt changes in direction, imbalance between windward and leeward openings, and blocking interior walls and doors will impede air flow.

Wind speed does not depend on precise orientation of the building, but principal facades should be oriented to within thirty degrees of the direction of prevailing breezes for maximum benefit. To enhance cross ventilation buildings should be set with long axis to the wind, ideally be long and narrow with rooms not more than two spaces deep, and with strategically placed overhangs and openings to increase air pressure and speed.

Recall older buildings with narrow floor plates, overhangs, high ceilings, operable transom panels over doors, and interior louvered doors - all means of inducing cross ventilation before air conditioning became commonly used. Note also the similarities to long and narrow forms being preferred for daylighting and solar strategies. From a whole building design perspective strive to balance passive solar, daylighting, sun shading, landscaping, and natural ventilation strategies to optimize use of the natural features of the site to maximize energy savings.

Roof shape and cross-section of the building will also affect the air flow. Sloped roofs will tend to shed harsh storm winds better than flat roofs. Window type selection is also important. Casement, awning, and hopper windows reduce effective aperture area by 25-33%, as compared to 50% reduction for double/single hung and horizontal sliding windows. The following air flow diagrams demonstrate several natural ventilation design devices: 

Mixed-mode cooling is a strategy that uses air conditioning and natural ventilation in various combinations. Ideally a building would use natural ventilation whenever suitable for occupant comfort and supplement with mechanical cooling only when needed. Mixed-mode strategies require highly integrated design collaboration between the design professionals to optimize building form, orientation, window size and placement, and appropriate mechanical design. Mixed-mode cooling can be used on swing days or in shoulder seasons in humid climates previously thought not suitable for natural ventilation.

There are several mixed-mode types that are differentiated by their operating strategies:

• Concurrent – uses mechanical cooling and natural ventilation in the same spaces at the same time
• Changeover – switches between mechanical cooling and natural ventilation on a daily or season basis
• Zoned – uses mechanical cooling and natural ventilation in different zones of the building
• Any combination of the above three may be utilized.

Established Techniques

• Set building orientation to receive prevailing breezes.
• Cooling of breezes by vegetative shading and water cooling of air flow to building spaces.
• Balance use of passive solar, daylighting, sun shading, and landscaping strategies to optimize natural ventilation.
• Use of an integrated design process to enhance performance.

Emerging Trends

• Greater emphasis on providing natural ventilation in public and commercial building within temperate climate zones.
• Modern adaptation of traditional architectural devices such as wind chimneys, atria, courtyards, windows, and operable blinds to induce natural air flow.
• Electronic modeling of natural ventilation and building form.

Use an Integrated Approach

A new way of thinking must be adopted to meet the goal of reducing carbon emissions associated with buildings. Your solutions can begin by integrating four possible methods. None works alone, and they are not all relevant in considering every strategy. However, considering the following tactics is necessary:

•  Reduce the overall energy use in your building                                                                                 
  • Appropriate orientation to wind direction, and shielding from harsh winds, can help reduce the overall energy use, so various site and building systems can be reduced in size and cost to make way for further energy saving materials, designs, and technologies.
• Specify energy efficient equipment and technologies
  • Use natural ventilation to reduce size and/or need for cooling equipment and energy use.
  • Specify energy efficient fans and equipment to induce natural ventilation.
  • Investigate use of mixed-mode cooling systems to reduce cooling load and energy use.
• Use renewable strategies and purchase green power
  • Use of natural ventilation is a renewable strategy.
• Educate building owners, operators, and occupants
  • On functional and energy savings advantages of natural ventilation strategies

• Whole Building Design Guide, Resource Pages, Natural Ventilation, Andy Walker, National Renewable Energy Laboratory.
• For Local Climatic Data (LCD) and wind roses, see the website of the National Climatic Data Center, http://www.ncdc.noaa.gov/oa/ncdc.html
• Architect’s Handbook of Energy Practice, now an out-of-print series of publications, 1982, The American Institute of Architects:
  §      Explanation of natural ventilation and air flow is in the volume, Design - Passive Heating and Cooling, in the section entitled Direct Cooling, pages 42-47.
  §      Explanation of a “wind rose” diagram, a graphic representation of wind data, is found in the volume, Predesign – Climate and Site, in the section entitled Wind, pages 24-27.
• For more detailed wind flow diagrams and information, see the Natural Ventilation pages in Architectural Graphic Standards, Eleventh Edition, by Charles Ramsey & Harold Sleeper, John Wiley & Sons, 2007.
• Case Studies of post-occupancy surveys of building having Natural Ventilation, National Renewable Energy Laboratory accessible at, www.eere.energy.gov/buildings/high_performance.
• Mixed-mode Cooling, Gail S. Brager, Ph.D., ASHRAE Journal, Aug 2006.
• Cross-sectional ventilation diagrams were taken from the paper, NISTIR 6781, Natural Ventilation Review and Plan Design and Analysis Tools, by Steven J. Emmerich, W. Stuart Dols, and James W. Axley, National Institute of Standards and Technology, US Department of Commerce, August 2001. Captioning was modified to include reference to architectural features for the purpose of this paper.

Associated Strategies

All 50to50 strategies relate to each other in some way. However, we recommend that you consider investigating these selected 50to50 strategies to assist you in gaining a deeper understanding.

• Building Form
• Building Orientation
• Integrated Project Delivery
• Sun Shading
• Vegetation for Sun Control
• Windows and Openings

Case Studies

  Discovery Center at South Lake Union 
  Lara Swimmer Photography

• Discovery Center at South Lake Union
• Cesar Chavez Library
• Lavin-Bernick Center for University Life
• Hawaii Gateway Energy Center
• Z6 House
• Epicenter, Artists for Humanity
• Global Ecology Research Center
• Government Canyon Visitor Center

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