Passive ventilation

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The ventilation system of a regular earthship Earthship-ventilation-cooling-tube-schematic.svg
The ventilation system of a regular earthship
Dogtrot houses are designed to maximise natural ventilation. John Looney House.jpg
Dogtrot houses are designed to maximise natural ventilation.
A roof turbine ventilator, colloquially known as a 'Whirly Bird' is an application of wind driven ventilation. Roof Turbine Ventilator.jpg
A roof turbine ventilator, colloquially known as a 'Whirly Bird' is an application of wind driven ventilation.

Passive ventilation is the process of supplying air to and removing air from an indoor space without using mechanical systems. It refers to the flow of external air to an indoor space as a result of pressure differences arising from natural forces.

Contents

There are two types of natural ventilation occurring in buildings: wind driven ventilation and buoyancy-driven ventilation. Wind driven ventilation arises from the different pressures created by wind around a building or structure, and openings being formed on the perimeter which then permit flow through the building. Buoyancy-driven ventilation occurs as a result of the directional buoyancy force that results from temperature differences between the interior and exterior. [1]

Since the internal heat gains which create temperature differences between the interior and exterior are created by natural processes, including the heat from people, and wind effects are variable, naturally ventilated buildings are sometimes called "breathing buildings".

Process

The static pressure of air is the pressure in a free-flowing air stream and is depicted by isobars in weather maps. Differences in static pressure arise from global and microclimate thermal phenomena and create the air flow we call wind. Dynamic pressure is the pressure exerted when the wind comes into contact with an object such as a hill or a building and it is described by the following equation: [2]

where (using SI units):

= dynamic pressure in pascals,
= fluid density in kg/m3 (e.g. density of air),
= fluid velocity in m/s.

The impact of wind on a building affects the ventilation and infiltration rates through it and the associated heat losses or heat gains. Wind speed increases with height and is lower towards the ground due to frictional drag. In practical terms wind pressure will vary considerably creating complex air flows and turbulence by its interaction with elements of the natural environment (trees, hills) and urban context (buildings, structures). Vernacular and traditional buildings in different climatic regions rely heavily upon natural ventilation for maintaining thermal comfort conditions in the enclosed spaces. [3]

Design

Design guidelines are offered in building regulations and other related literature and include a variety of recommendations on many specific areas such as:

The following design guidelines are selected from the Whole Building Design Guide, a program of the National Institute of Building Sciences: [4]

Wind driven ventilation

Wind driven ventilation can be classified as cross ventilation and single-sided ventilation. Wind driven ventilation depends on wind behavior, on the interactions with the building envelope and on openings or other air exchange devices such as inlets or windcatchers.

The knowledge of the urban climatology i.e. the wind around the buildings is crucial when evaluating the air quality and thermal comfort inside buildings as air and heat exchange depends on the wind pressure on facades. As observed in the equation (1), the air exchange depends linearly on the wind speed in the urban place where the architectural project will be built. CFD (Computational Fluid Dynamics) tools and zonal modelings are usually used to design naturally ventilated buildings. Windcatchers are able to aid wind driven ventilation by directing air in and out of buildings.

Buoyancy-driven ventilation

Buoyancy driven ventilation arise due to differences in density of interior and exterior air, which in large part arises from differences in temperature. When there is a temperature difference between two adjoining volumes of air the warmer air will have lower density and be more buoyant thus will rise above the cold air creating an upward air stream. Forced upflow buoyancy driven ventilation in a building takes place in a traditional fireplace. Passive stack ventilators are common in most bathrooms and other type of spaces without direct access to the outdoors.

In order for a building to be ventilated adequately via buoyancy driven ventilation, the inside and outside temperatures must be different. When the interior is warmer than the exterior, indoor air rises and escapes the building at higher apertures. If there are lower apertures then colder, denser air from the exterior enters the building through them, thereby creating upflow displacement ventilation. However, if there are no lower apertures present, then both in- and out-flow will occur through the high level opening. This is called mixing ventilation. This latter strategy still results in fresh air reaching to low level, since although the incoming cold air will mix with the interior air, it will always be more dense than the bulk interior air and hence fall to the floor. Buoyancy-driven ventilation increases with greater temperature difference, and increased height between the higher and lower apertures in the case of displacement ventilation. When both high and low level openings are present, the neutral plane in a building occurs at the location between the high and low openings at which the internal pressure will be the same as the external pressure (in the absence of wind). Above the neutral plane, the internal air pressure will be positive and air will flow out of any intermediate level apertures created. Below the neutral plane the internal air pressure will be negative and external air will be drawn into the space through any intermediate level apertures. Buoyancy-driven ventilation has several significant benefits: {See Linden, P Annu Rev Fluid Mech, 1999}

Limitations of buoyancy-driven ventilation:

Natural ventilation in buildings can rely mostly on wind pressure differences in windy conditions, but buoyancy effects can a) augment this type of ventilation and b) ensure air flow rates during still days. Buoyancy-driven ventilation can be implemented in ways that air inflow in the building does not rely solely on wind direction. In this respect, it may provide improved air quality in some types of polluted environments such as cities. For example, air can be drawn through the backside or courtyards of buildings avoiding the direct pollution and noise of the street facade. Wind can augment the buoyancy effect, but can also reduce its effect depending on its speed, direction and the design of air inlets and outlets. Therefore, prevailing winds must be taken into account when designing for stack effect ventilation.

Estimating buoyancy-driven ventilation

The natural ventilation flow rate for buoyancy-driven natural ventilation with vents at two different heights can be estimated with this equation: [5]

English units:
where: 
QS= Buoyancy-driven ventilation airflow rate, ft3/s
A= cross-sectional area of opening, ft² (assumes equal area for inlet and outlet)
Cd= Discharge coefficient for opening (typical value is 0.65)
g= gravitational acceleration, around 32.2 ft/s² on Earth
Hd= Height from midpoint of lower opening to midpoint of upper opening, ft
TI= Average indoor temperature between the inlet and outlet, °R
TO= Outdoor temperature, °R
SI units:
where: 
QS= Buoyancy-driven ventilation airflow rate, m3/s
A= cross-sectional area of opening, m2 (assumes equal area for inlet and outlet)
Cd= Discharge coefficient for opening (typical value is 0,62)
g= gravitational acceleration, around 9.81 m/s² on Earth
Hd= Height from midpoint of lower opening to midpoint of upper opening, m
TI= Average indoor temperature between the inlet and outlet, K
TO= Outdoor temperature, K

Assessing performance

One way to measure the performance of a naturally ventilated space is to measure the air changes per hour in an interior space. In order for ventilation to be effective, there must be exchange between outdoor air and room air. A common method for measuring ventilation effectiveness is to use a tracer gas. [6] The first step is to close all windows, doors, and openings in the space. Then a tracer gas is added to the air. The reference, American Society for Testing and Materials (ASTM) Standard E741: Standard Test Method for Determining Air Change in a Single Zone by Means of a Tracer Gas Dilution, describes which tracer gases can be used for this kind of testing and provides information about the chemical properties, health impacts, and ease of detection. [7] Once the tracer gas has been added, mixing fans can be used to distribute the tracer gas as uniformly as possible throughout the space. To do a decay test, the concentration of the tracer gas is first measured when the concentration of the tracer gas is constant. Windows and doors are then opened and the concentration of the tracer gas in the space is measured at regular time intervals to determine the decay rate of the tracer gas. The airflow can be deduced by looking at the change in concentration of the tracer gas over time. For further details on this test method, refer to ASTM Standard E741. [7]

While natural ventilation eliminates electrical energy consumed by fans, overall energy consumption of natural ventilation systems is often higher than that of modern mechanical ventilation systems featuring heat recovery. Typical modern mechanical ventilation systems use as little as 2000 J/m3 for fan operation, and in cold weather they can recover much more energy than this in the form of heat transferred from waste exhaust air to fresh supply air using recuperators.

Ventilation heat loss can be calculated as:

Where:

The temperature differential needed between indoor and outdoor air for mechanical ventilation with heat recovery to outperform natural ventilation in terms of overall energy efficiency can therefore be calculated as:

Where:

SFP is specific fan power in Pa, J/m3, or W/(m3/s)

Under typical comfort ventilation conditions with a heat recovery efficiency of 80% and a SFP of 2000 J/m3 we get:

In climates where the mean absolute difference between inside and outside temperatures exceeds ~10K the energy conservation argument for choosing natural over mechanical ventilation might therefore be questioned. It should however be noted that heating energy might be cheaper and more environmentally friendly than electricity. This is especially the case in areas where district heating is available.

To develop natural ventilation systems with heat recovery two inherent challenges must first be solved:

  1. Providing efficient heat recovery at very low driving pressures.
  2. Physically or thermally connecting supply and exhaust air streams. (Stack ventilation typically relies on supply and exhaust being placed low and high respectively, while wind driven natural ventilation normally relies on openings being placed on opposing sides of a building for efficient cross ventilation.)

Research aiming at the development of natural ventilation systems featuring heat recovery have been made as early as 1993 where Shultz et al. [8] proposed and tested a chimney type design relying on stack effect while recovering heat using a large counterflow recuperator constructed from corrugated galvanized iron. Both supply and exhaust happened through an unconditioned attic space, with exhaust air being extracted at ceiling height and air being supplied at floor level through a vertical duct.

The device was found to provide sufficient ventilation air flow for a single family home and heat recovery with an efficiency around 40%. The device was however found to be too large and heavy to be practical, and the heat recovery efficiency too low to be competitive with mechanical systems of the time. [8]

Later attempts have primarily focused on wind as the main driving force due to its higher pressure potential. This however introduces an issue of there being large fluctuations in driving pressure.

With the use of wind towers placed on the roof of ventilated spaces, supply and exhaust can be placed close to each other on opposing sides of the small towers. [9] These systems often feature finned heat pipes although this limits the theoretical maximum heat recovery efficiency. [10]

Liquid coupled run around loops have also been tested to achieve indirect thermal connection between exhaust and supply air. While these tests have been somewhat successful, liquid coupling introduces mechanical pumps that consume energy to circulate the working fluid. [11] [12]

While some commercially available solutions have been available for years, [13] [14] the claimed performance by manufacturers has yet to be verified by independent scientific studies. This might explain the apparent lack of market impact of these commercially available products claiming to deliver natural ventilation and high heat recovery efficiencies.

A radically new approach to natural ventilation with heat recovery is currently being developed at Aarhus University, where heat exchange tubes are integrated into structural concrete slabs between building floors. [15]

Standards

For standards relating to ventilation rates, in the United States refer to ASHRAE Standard 62.1-2010: Ventilation for Acceptable Indoor Air Quality. [16] These requirements are for "all spaces intended for human occupancy except those within single-family houses, multifamily structures of three stories or fewer above grade, vehicles, and aircraft." [16] In the revision to the standard in 2010, Section 6.4 was modified to specify that most buildings designed to have systems to naturally condition spaces must also "include a mechanical ventilation system designed to meet the Ventilation Rate or IAQ procedures [in ASHRAE 62.1-2010]. The mechanical system is to be used when windows are closed due to extreme outdoor temperatures noise and security concerns". [16] The standard states that two exceptions in which naturally conditioned buildings do not require mechanical systems are when:

Also, an authority having jurisdiction may allow for the design of conditioning system that does not have a mechanical system but relies only on natural systems. [16] In reference for how controls of conditioning systems should be designed, the standard states that they must take into consideration measures to "properly coordinate operation of the natural and mechanical ventilation systems." [16]

Another reference is ASHRAE Standard 62.2-2010: Ventilation and Acceptable Indoor Air Quality in low-rise Residential Buildings. [17] These requirements are for "single-family houses and multifamily structures of three stories or fewer above grade, including manufactured and modular houses," but is not applicable "to transient housing such as hotels, motels, nursing homes, dormitories, or jails." [17]

For standards relating to ventilation rates, in the United States refer to ASHRAE Standard 55-2010: Thermal Environmental Conditions for Human Occupancy. [18] Throughout its revisions, its scope has been consistent with its currently articulated purpose, “to specify the combinations of indoor thermal environmental factors and personal factors that will produce thermal environmental conditions acceptable to a majority of the occupants within the space.” [18] The standard was revised in 2004 after field study results from the ASHRAE research project, RP-884: developing an adaptive model of thermal comfort and preference, indicated that there are differences between naturally and mechanically conditioned spaces with regards to occupant thermal response, change in clothing, availability of control, and shifts in occupant expectations. [19] The addition to the standard, 5.3: Optional Method For Determining Acceptable Thermal Conditions in Naturally Ventilated Spaces, uses an adaptive thermal comfort approach for naturally conditioned buildings by specifying acceptable operative temperature ranges for naturally conditioned spaces. [18] As a result, the design of natural ventilation systems became more feasible, which was acknowledged by ASHRAE as a way to further sustainable, energy efficient, and occupant-friendly design. [18]

See also

Related Research Articles

<span class="mw-page-title-main">Heating, ventilation, and air conditioning</span> Technology of indoor and vehicular environmental comfort

Heating, ventilation, and air conditioning (HVAC) is the use of various technologies to control the temperature, humidity, and purity of the air in an enclosed space. Its goal is to provide thermal comfort and acceptable indoor air quality. HVAC system design is a subdiscipline of mechanical engineering, based on the principles of thermodynamics, fluid mechanics, and heat transfer. "Refrigeration" is sometimes added to the field's abbreviation as HVAC&R or HVACR, or "ventilation" is dropped, as in HACR.

<span class="mw-page-title-main">Ventilation (architecture)</span> Intentional introduction of outside air into a space

Ventilation is the intentional introduction of outdoor air into a space. Ventilation is mainly used to control indoor air quality by diluting and displacing indoor pollutants; it can also be used to control indoor temperature, humidity, and air motion to benefit thermal comfort, satisfaction with other aspects of the indoor environment, or other objectives.

A solar chimney – often referred to as a thermal chimney – is a way of improving the natural ventilation of buildings by using convection of air heated by passive solar energy. A simple description of a solar chimney is that of a vertical shaft utilizing solar energy to enhance the natural stack ventilation through a building.

<span class="mw-page-title-main">Heat recovery ventilation</span> Method of reusing thermal energy in a building

Heat recovery ventilation (HRV), also known as mechanical ventilation heat recovery (MVHR) or energy recovery ventilation (ERV), is a ventilation system that recovers energy by operating between two air sources at different temperatures. It is used to reduce the heating and cooling demands of buildings.

Displacement ventilation (DV) is a room air distribution strategy where conditioned outdoor air is supplied at a low velocity from air supply diffusers located near floor level and extracted above the occupied zone, usually at ceiling height.

The stack effect or chimney effect is the movement of air into and out of buildings through unsealed openings, chimneys, flue-gas stacks, or other containers, resulting from air buoyancy. Buoyancy occurs due to a difference in indoor-to-outdoor air density resulting from temperature and moisture differences. The result is either a positive or negative buoyancy force. The greater the thermal difference and the height of the structure, the greater the buoyancy force, and thus the stack effect. The stack effect helps drive natural ventilation, air infiltration, and fires.

<span class="mw-page-title-main">Underfloor heating</span> Form of central heating and cooling

Underfloor heating and cooling is a form of central heating and cooling that achieves indoor climate control for thermal comfort using hydronic or electrical heating elements embedded in a floor. Heating is achieved by conduction, radiation and convection. Use of underfloor heating dates back to the Neoglacial and Neolithic periods.

<span class="mw-page-title-main">Passive cooling</span> Building design approach

Passive cooling is a building design approach that focuses on heat gain control and heat dissipation in a building in order to improve the indoor thermal comfort with low or no energy consumption. This approach works either by preventing heat from entering the interior or by removing heat from the building.

<span class="mw-page-title-main">Air conditioning</span> Cooling of air in an enclosed space

Air conditioning, often abbreviated as A/C (US) or air con (UK), is the process of removing heat from an enclosed space to achieve a more comfortable interior temperature and in some cases also strictly controlling the humidity of internal air. Air conditioning can be achieved using a mechanical 'air conditioner' or by other methods, including passive cooling and ventilative cooling. Air conditioning is a member of a family of systems and techniques that provide heating, ventilation, and air conditioning (HVAC). Heat pumps are similar in many ways to air conditioners, but use a reversing valve to allow them both to heat and to cool an enclosed space.

Thermal comfort is the condition of mind that expresses subjective satisfaction with the thermal environment. The human body can be viewed as a heat engine where food is the input energy. The human body will release excess heat into the environment, so the body can continue to operate. The heat transfer is proportional to temperature difference. In cold environments, the body loses more heat to the environment and in hot environments the body does not release enough heat. Both the hot and cold scenarios lead to discomfort. Maintaining this standard of thermal comfort for occupants of buildings or other enclosures is one of the important goals of HVAC design engineers.

Infiltration is the unintentional or accidental introduction of outside air into a building, typically through cracks in the building envelope and through use of doors for passage. Infiltration is sometimes called air leakage. The leakage of room air out of a building, intentionally or not, is called exfiltration. Infiltration is caused by wind, negative pressurization of the building, and by air buoyancy forces known commonly as the stack effect.

Air changes per hour, abbreviated ACPH or ACH, or air change rate is the number of times that the total air volume in a room or space is completely removed and replaced in an hour. If the air in the space is either uniform or perfectly mixed, air changes per hour is a measure of how many times the air within a defined space is replaced each hour. Perfectly mixed air refers to a theoretical condition where supply air is instantly and uniformly mixed with the air already present in a space, so that conditions such as age of air and concentration of pollutants are spatially uniform.

HVAC is a major sub discipline of mechanical engineering. The goal of HVAC design is to balance indoor environmental comfort with other factors such as installation cost, ease of maintenance, and energy efficiency. The discipline of HVAC includes a large number of specialized terms and acronyms, many of which are summarized in this glossary.

<span class="mw-page-title-main">Underfloor air distribution</span>

Underfloor air distribution (UFAD) is an air distribution strategy for providing ventilation and space conditioning in buildings as part of the design of a HVAC system. UFAD systems use an underfloor supply plenum located between the structural concrete slab and a raised floor system to supply conditioned air to supply outlets, located at or near floor level within the occupied space. Air returns from the room at ceiling level or the maximum allowable height above the occupied zone.

<span class="mw-page-title-main">Dedicated outdoor air system</span>

A dedicated outdoor air system (DOAS) is a type of heating, ventilation and air-conditioning (HVAC) system that consists of two parallel systems: a dedicated system for delivering outdoor air ventilation that handles both the latent and sensible loads of conditioning the ventilation air, and a parallel system to handle the loads generated by indoor/process sources and those that pass through the building enclosure.

Dynamic insulation is a form of insulation where cool outside air flowing through the thermal insulation in the envelope of a building will pick up heat from the insulation fibres. Buildings can be designed to exploit this to reduce the transmission heat loss (U-value) and to provide pre-warmed, draft free air to interior spaces. This is known as dynamic insulation since the U-value is no longer constant for a given wall or roof construction but varies with the speed of the air flowing through the insulation. Dynamic insulation is different from breathing walls. The positive aspects of dynamic insulation need to be weighed against the more conventional approach to building design which is to create an airtight envelope and provide appropriate ventilation using either natural ventilation or mechanical ventilation with heat recovery. The air-tight approach to building envelope design, unlike dynamic insulation, results in a building envelope that provides a consistent performance in terms of heat loss and risk of interstitial condensation that is independent of wind speed and direction. Under certain wind conditions a dynamically insulated building can have a higher heat transmission loss than an air-tight building with the same thickness of insulation. Often the air enters at about 15 °C.

Airflow, or air flow, is the movement of air. The primary cause of airflow is the existence of air. Air behaves in a fluid manner, meaning particles naturally flow from areas of higher pressure to those where the pressure is lower. Atmospheric air pressure is directly related to altitude, temperature, and composition.

CFD stands for computational fluid dynamics. As per this technique, the governing differential equations of a flow system or thermal system are known in the form of Navier–Stokes equations, thermal energy equation and species equation with an appropriate equation of state. In the past few years, CFD has been playing an increasingly important role in building design, following its continuing development for over a quarter of a century. The information provided by CFD can be used to analyse the impact of building exhausts to the environment, to predict smoke and fire risks in buildings, to quantify indoor environment quality, and to design natural ventilation systems.

<span class="mw-page-title-main">Cross ventilation</span> Movement of air through a building

Cross ventilation is a natural phenomenon where wind, fresh air or a breeze enters upon an opening, such as a window, and flows directly through the space and exits through an opening on the opposite side of the building. This produces a cool stream of air and as well as a current across the room from the exposed area to the sheltered area.

<span class="mw-page-title-main">Ventilative cooling</span>

Ventilative cooling is the use of natural or mechanical ventilation to cool indoor spaces. The use of outside air reduces the cooling load and the energy consumption of these systems, while maintaining high quality indoor conditions; passive ventilative cooling may eliminate energy consumption. Ventilative cooling strategies are applied in a wide range of buildings and may even be critical to realize renovated or new high efficient buildings and zero-energy buildings (ZEBs). Ventilation is present in buildings mainly for air quality reasons. It can be used additionally to remove both excess heat gains, as well as increase the velocity of the air and thereby widen the thermal comfort range. Ventilative cooling is assessed by long-term evaluation indices. Ventilative cooling is dependent on the availability of appropriate external conditions and on the thermal physical characteristics of the building.

References

  1. Linden, P. F. (1999). "The Fluid Mechanics of Natural Ventilation". Annual Review of Fluid Mechanics. 31: 201–238. Bibcode:1999AnRFM..31..201L. doi:10.1146/annurev.fluid.31.1.201.
  2. Clancy, L.J. (1975). Aerodynamics. John Wiley & Sons.
  3. "Lessons from Sustainable and Vernacular Passive Cooling Strategies Used in Traditional Iranian Houses". ResearchGate.
  4. Walker, Andy. "Natural Ventilation". National Institute of Building Sciences.
  5. ASHRAE Handbook. Atlanta, GA: American Society of Heating, Refrigerating and Air Conditioning Engineers. 2009.
  6. McWilliams, Jennifer (2002). "Review of air flow measurement techniques. LBNL Paper LBNL-49747". Lawrence Berkeley National Lab.
  7. 1 2 "ASTM Standard E741-11: Standard Test Method for Determining Air Change in a Single Zone by Means of a Tracer Gas Dilution". West Conshohocken, PA: ASTM International. 2006.{{cite journal}}: Cite journal requires |journal= (help)
  8. 1 2 Schultz, J. M., 1993. Naturlig ventilation med varmegenvinding, Lyngby: Laboratoriet for Varmeisolering, DTH. (Danish)
  9. Calautit, J. K., O'Connor, D. & Hughes, B. R., 2015. A natural ventilation wind tower with heat pipe heat recovery for cold climates. Renewable Energy, I(87), pp. 1088-1104.
  10. Gan, G. & Riffat, S., 1999. A study of heat-pipe heat recovery for natural ventilation. AIVC, 477(12), pp. 57-62.
  11. Hviid, C. A. & Svendsen, S., 2008. Passive ventilation systems with heat recovery and night cooling. Kyoto, Advanced building ventilation and environmental technology for addressing climate change issues.
  12. Hviid, C. A. & Svendsen, S., 2012. Wind- and stack-assisted mechanical, Lyngby: DTU Byg.
  13. Autodesk, 2012. Passive Heat Recovering Ventilation System. [Online] Available at: sustainabilityworkshop.autodesk.com/project-gallery/passive-heat-recovering-ventilationsystem
  14. "Ventive". ventive.co.uk. Retrieved 2018-07-28.
  15. "How it works". www.stackhr.com. Retrieved 2018-07-28.
  16. 1 2 3 4 5 "ANSI/ASHRAE Standard 62.1-2010: Ventilation for Acceptable Indoor Air Quality". Atlanta, GA: American Society of Heating, Refrigerating and Air-Conditioning Engineers. 2010.{{cite journal}}: Cite journal requires |journal= (help)
  17. 1 2 "ANSI/ASHRAE Standard 62.2-2010: Ventilation and Acceptable Indoor Air Quality in Low-Rise Residential Buildings". Atlanta, GA: American Society of Heating, Refrigerating and Air-Conditioning Engineers. 2010.{{cite journal}}: Cite journal requires |journal= (help)
  18. 1 2 3 4 "ANSI/ASHRAE Standard 55-2010: Thermal Environmental Conditions for Human Occupancy". Atlanta, GA: American Society of Heating, Refrigerating and Air-Conditioning Engineers. 2010.{{cite journal}}: Cite journal requires |journal= (help)
  19. de Dear, Richard J.; Gail S. Brager (2002). "Thermal Comfort in Naturally Ventilated Buildings: Revisions to ASHRAE Standard 55". Energy and Buildings. 34 (6): 549–561. Bibcode:2002EneBu..34..549D. doi:10.1016/S0378-7788(02)00005-1. S2CID   110575467.

University-based research centers that currently conduct natural ventilation research:

  1. The Center for the Built Environment (CBE), University of California, Berkeley. http://www.cbe.berkeley.edu/
  2. Lawrence Berkeley National Laboratory, Berkeley, California. http://www.lbl.gov/
  3. Department of Architecture, Massachusetts Institute of Technology. http://architecture.mit.edu/building-technology/program/research-topics
  4. Faculty of Architecture, Design and Planning, University of Sydney, Australia. https://web.archive.org/web/20111107120122/http://sydney.edu.au/architecture/research/research_archdessci.shtml

Natural Ventilation Guidelines:

  1. Whole Building Design Guide, National Institute of Building Sciences http://www.wbdg.org/resources/naturalventilation.php
  2. "Natural Ventilation for Infection Control in Health-Care Settings," a report (including design guidelines) by World Health Organization for naturally ventilated health-care facilities.http://whqlibdoc.who.int/publications/2009/9789241547857_eng.pdf