HVAC & IAQ article summary32m8v0095
Natural ventilation for buildings Article by: Battaglia, Francine Department of Mechanical Engineering, Virginia Polytechnic Institute and State University, Blacksburg, Virginia. Passe, Ulrike Department of Architecture, College of Design, Iowa State University, Ames, Iowa. Last updated: 2016 DOI: https://doiorg.ezproxy2.library.drexel.edu/10.1036/10978542.YB150704 (https://doiorg.ezproxy2.library.drexel.edu/10.1036/1097 8542.YB150704)
What is natural ventilation, and why use it? Ventilation principles Spatial composition Building design strategies Tools and numerical models Benefits
Ventilation is a way to introduce fresh air into a room and simultaneously extract warm, stale, polluted, or odorous air. Natural ventilation has become an important consideration as we seek ways to improve indoor air quality (clean air) in buildings. The energy required to cool and ventilate buildings continues to increase, along with associated costs and the building carbon footprint. Natural ventilation can be utilized to decrease the amount of energy needed to condition buildings, and delving into the past few centuries, we find buildings designed specifically to exploit natural ventilation. Historical examples include the Palladian villas (Italy), the corbelled domed roofs of the Harran houses (Turkey), and the wind catchers of Yazd (Iran). Three extraordinary and famous twentiethcentury houses that utilize natural ventilation strategies are the How House (Los Angeles, CA), the Affleck House (Bloomfield, MI), and the Esherick House (Chestnut Hill, PA). To reduce energy requirements and costs to ventilate buildings and provide thermal comfort (how humans perceive comfort), these historical buildings can serve as a basis for future designs.
What is natural ventilation, and why use it? Ventilation is dependent on the motion of air, which can also accompany heat transfer. Convection is a form of energy transfer due to both molecular motion and bulk motion (advection). The occurrence of a temperature gradient in addition to fluid motion is convective heat transfer. When the flow is a result of an external source, such as a fan or atmospheric wind, the heat transfer is classified as forced convection. Natural (or free) convection occurs when temperature differences in the fluid cause density differences, creating a buoyancydriven flow. If the buoyancy force is dominant, a stronger upward flow is produced as hot air rises and cold air descends, forming convection currents. Therefore, natural ventilation is a means to passively cool or heat a room by circulating air without requiring mechanical systems. The resulting effect moves the air in a region and assists with removing heat and humidity, and exchanging air. See also: Buoyancy (/content/buoyancy/099500); Convection (heat) (/content/convectionheat/160000); Heat transfer (/content/heat transfer/311100)
Natural ventilation is effective in improving air quality, providing thermal comfort, and reducing energy consumption when utilized properly. In the United States, rising prices of natural gas and electricity, and concerns about the depletion of valuable natural resources, motivate research to explore how society uses energy. More than 70% of the energy used in
the United States is consumed by homes, businesses, schools, and industries. One approach to addressing energy concerns is the development of alternative fuels, which still requires resources. However, if attention is turned to architectural building design, strategies can be developed to utilize the building structure to provide thermal comfort. Ideally, if natural ventilation could achieve the same environment as mechanical heating, ventilation, and air conditioning (HVAC) systems, then building designs using natural ventilation would not consume natural gas and electricity for energy needs to ventilate and condition the indoor environment. A major benefit of natural ventilation is that it is a free resource that we can use without harming the environment. See also: Air conditioning (/content/airconditioning/017000); Heating system (/content/heatingsystem/757298); Ventilation (/content/ventilation/729900)
The use of natural driving forces is an underutilized design strategy to control the indoor environment. Air movement is achieved when there are pressure differences or temperature differences. For instance, opening a window creates a pressure difference between the interior and exterior of a building. Even if the air temperature is the same between the two environments, air can still move, a phenomenon known as singlesided ventilation. Opening two windows can enhance air movement, and the case of opposing open windows is known as cross ventilation. In cross ventilation, the pressure difference is often created by wind. A higher pressure on the windward side of the building and a lower pressure on the leeward side drive the airflow from windward to leeward. A temperature difference between the indoor and outdoor environment creates a pressure difference, and air ventilates through an opening, where such buoyancydriven flow is called the stack effect. Most flows are a combination of buoyancydriven and winddriven effects (Fig. 1). See also: Pressure (/content/pressure/543500); Temperature (/content/temperature/683500)
Table 1 Indoor thermal comfort conditions
Outdoor air temperature 10°C (50°F) 33.5°C (92.3°F)
Lower limit Upper limit Lower limit Upper limit
90% acceptable 17.5°C (63.5°F) 23.5°C (74.3°F) 23°C (73.4°F) 30.5°C (86.9°F)
80% acceptable 18.5°C (65.3°F) 24.5°C (76.1°F) 26°C (78.8°F) 31°C (87.8°F)
Used with permission from ASHRAE Standard 552013, copyright © ASHRAE (www.ashrae.org), 2013.
Fig. 1 Sketch of cross ventilation combined with a stack atrium to increase circulation. (From U. Passe and F. Battaglia, Designing Spaces for Natural Ventilation: An Architect's Guide, Routledge, Taylor & Francis, 2015)
One aspect of natural ventilation is that it can couple the building design and environmental factors (such as wind) to exchange stale building air with fresh air. Indoor air quality (also referred to as indoor environmental quality) and thermal comfort are pressing issues because, on average, most Americans spend as much as 90% of their time indoors, according to the U.S. Environmental Protection Agency's (EPA's) report on indoor pollution to the U.S. Congress in 1989, which is still a benchmark study almost 30 years later.
There are two goals when ventilating buildings: providing a cooler environment for occupants, and exchanging air to maintain a healthy indoor environment. Cool air can be achieved by reducing the air temperature or by increasing the air movement to induce evaporation. The exchange of air helps to eliminate pollutants and volatile organic compounds, and to replenish oxygen. There has been great concern for health issues, with the focus shifting from outdoor to indoor pollution. It has been shown that ventilation rates lower than 36 m /h (1271 ft /h) can adversely affect occupants, especially children and the elderly. See also: Indoor air pollution (/content/indoorairpollution/757411)
A parameter to indicate quality ventilation is the air change rate (ACH), defined as the volume flow rate into a room divided by the room volume. ACH simply means the movement of a volume of air in a 1h period. To maintain minimum air quality, the recommended ACH is 1 to 2, and to remove heat or pollutants, ACH should range from 2 to 15. The American Society of Heating, Refrigerating, and AirConditioning Engineers (ASHRAE) has set forth recommendations for thermal comfort (Table 1), providing lower and upper limits as a function of outdoor temperature. Thermal comfort is determined from the conditions in which 80% of the occupants perceive that the environment is acceptable. If the indoor temperatures are lower, 90% of the occupants are satisfied. Airflow velocities are recommended based on perceived comfort levels with temperature changes (Table 2).
Table 2 Acceptable temperature increase due to moving air
Air speed, m/s (ft/s) Temperature increase, °C (°F)
0.6 (2.0) 1.2 (2.16)
0.9 (3.0) 1.8 (3.24)
1.2 (3.9) 2.2 (3.96)
Used with permission from ASHRAE Standard 552013, copyright © ASHRAE (www.ashrae.org), 2013.
Spatial composition To utilize natural ventilation effectively, the design of a building must allow air to flow freely. Spatial layout considers how areas and rooms are interconnected in a building. The utilization of building space must be conducive to develop pressure differences from one end of the building to the other to facilitate airflow. The amount of flow can be controlled by the size and orientation of the spaces. Thus, the principles of natural ventilation must be considered in the design of the building and not simply be an afterthought.
Three spatial concepts that enhance natural ventilation are the stack effect, the wind catcher, and cross ventilation. A tall space can act as either a wind catcher or a stack chimney, depending on the pressure difference. When the ambient pressure is higher than the interior building pressure, the vertical space behaves like a wind catcher, drawing air downward into the building. Otherwise, when ambient pressure is lower than the interior, air moves up the stack. These spatial
attributes are found in the three previously mentioned “breathing” buildings: the How House designed by Rudolph M. Schindler (1925), the Affleck House designed by Frank Lloyd Wright (1940), and the Esherick House designed by Louis I. Kahn (1961).
Building design strategies Surveys have shown that occupants have a preference for open windows and natural light. Currently, society readily uses HVAC systems to condition interior environments, whereby the building topology has been decoupled from the HVAC requirements in the building design. Countless beautiful buildings with excessive HVAC requirements are wasting energy because it is not easy or efficient to condition the interior thermally.
Natural ventilation must be incorporated into the design of the building and should be an important consideration alongside the building envelope, energy use, occupancy habits, human health, and comfort. Proper use of natural ventilation will provide energy at no cost and with no adverse environmental effects. However, buildings that utilize natural ventilation must be designed differently, keeping in mind that their control and operation are different from that of those that use traditional HVAC systems. The caveat is knowing when and how often to open windows, and the appropriate sizes of the openings. A challenge is that natural ventilation relies on environmental factors such as atmospheric wind and temperature, which change daily. Thus, control of natural ventilation becomes the responsibility of the occupants.
In the design of buildings that utilize natural ventilation, architects employ certain rules of thumb based on recommendations by the Chartered Institution of Buildings Services Engineers. The rules of thumb include recommendations for cross ventilation and singlesided ventilation. Cross ventilation is expected to be effective if the space depthtoheight ratio is 5:1 with an unrestricted flow path. For singlesided ventilation with only one opening, the depthto height ratio is 2:1, and for two openings, the ratio is 2.5:1. With increasing floortoceiling height, the room temperatures are more stratified and pollutants rise above the occupied space.
Intuition and experience have been the deciding factors in building designs, but software tools are being developed and used to assist architects and engineers. These tools can be coupled with weather data to predict temperature distributions and airflow patterns within a building. Specific scenarios can be considered so that the designer can try to optimize the best spatial composition and spatial principles to create a wellfunctioning, naturally ventilating building.
Tools and numerical models The U.S. Environmental Protection Agency has created tools to assist designers and provide guidance for indoor air quality with the construction of new buildings. One tool is the Indoor Air Quality Building Education and Assessment Model (I BEAM), and another popular tool is Indoor airPLUS. There are also tools that help determine thermal comfort of the building occupants. Software tools also exist that evaluate energy requirements to maintain a comfortable building environment. The U.S. Department of Energy developed EnergyPlus, which provides information related to energy consumption of a building, and the National Institute of Standards and Technology (NIST) tool is CONTAM, which provides air quality, including the transport of contaminants. Another tool developed by NIST is LoopDA, which can determine the sizes of natural ventilation openings. These tools are very useful to provide guidelines for designing buildings as well as analyze existing buildings so that improvements can be made to reduce energy costs.
Ventilation can be assessed using experimental methods such as wind tunnel and field measurements, empirical and analytical methods, multizone and zonal methods, and computational fluid dynamics (CFD). Experiments can be costly due to equipment and resource requirements. Empirical models use experimental data to develop simple relationships. Analytical models are developed by simplifying equations that govern the motion of mass, momentum, and energy. A more inclusive and powerful tool is CFD, which can predict pressure, temperature, and fluid motion, as well as provide a way to
visualize airflow. Examples of CFD computer program tools include ANSYS Fluent, CFX, and OpenFOAM. See also: Computational fluid dynamics (/content/computationalfluiddynamics/757259); Wind tunnel (/content/wind tunnel/746800)
A model of the Esherick House was simulated using Fluent. Unique features of the house are the windows and wooden shutters, whereby the shutters help control airflow (Fig. 2). The front of the house has small windows, while the rear side has larger windows. Different scenarios were simulated using CFD for cooling (Fig. 3), assuming the front shutters and balcony doors were open. Simulations not only predict temperature changes through the house, but how wind is entrained and moves through the open floor layout. CFD can be used to predict and visualize many scenarios using a three dimensional blueprint of a building to determine effective ventilation.
Fig. 2 Front view of the Esherick House with shutters closed. [Photograph by Smallbones (public domain) via Wikimedia Commons]
Benefits Ventilating buildings using natural driving forces is a complex but worthwhile endeavor. The push for green design strategies to save energy and natural resources supports the use of natural ventilation in future architectural designs. It is worth noting that even HVAC systems have their drawbacks. Although thermostats can be programmed to control interior heating and cooling, occupants are not necessarily satisfied with the thermal comfort, and they may feel that the environment is drafty or stuffy, too cold or too hot. Natural ventilation is an economical way to provide desirable fresh (cool) air, albeit controlled by the occupants. Bearing in mind the pros and cons, natural ventilation has great potential to be a standard incorporated into new construction to create a new generation of healthy, energyefficient buildings.
Francine Battaglia Ulrike Passe
Fig. 3 CFD simulations of the Esherick House, showing temperature distribution (color) and airflow patterns (white lines). T = temperature. T(°F) = 1.8 T(°C) + 32. (Image courtesy of P. J. Stoakes, Simulation of Airflow and Heat Transfer in Buildings, M.S. thesis, Virginia Tech, Blacksburg, VA, 2009)
American Society of Heating, Refrigerating, and AirConditioning Engineers (ASHRAE), ASHRAE Standard 552013: Thermal Environmental Conditions for Human Occupancy, Atlanta, GA, 2013
D. Etheridge, Natural Ventilation of Buildings: Theory, Measurement and Design, Wiley, 2012
D. Etheridge and M. Sandberg, Building Ventilation: Theory and Measurement, Wiley, 1996
U. Passe and F. Battaglia, Designing Spaces for Natural Ventilation: An Architect's Guide, Routledge, Taylor & Francis, 2015
P. J. Stoakes, Simulation of Airflow and Heat Transfer in Buildings, M.S. thesis, Virginia Tech, Blacksburg, VA, 2009
Breathing Buildings (http://www.breathingbuildings.com/home)
Massachusetts Institute of Technology: CoolVent (http://coolvent.mit.edu)
University of California–Berkeley, Center for the Built Environment: CBE Thermal Comfort Tool (http://www.cbe.berkeley.edu/research/thermaltool.htm)
University of Sidney, Professor Richard de Dear: Human Heat Balance (calculator) (http://web.arch.usyd.edu.au/~rdedear)
U.S. EPA: An Office Building Occupants Guide to Indoor Air Quality: Why is Indoor Air Quality Important? (http://www.epa.gov/indoorairqualityiaq/officebuildingoccupantsguideindoorairquality#whyindoor)