Natural ventilation is simply the supply and removal of air from the indoor environment only by natural means.
This is accomplished by wind, pressure and temperature differences between the inside and outside of the building caused in part by the
orientation of fenestrations to natural wind patterns without the aid mechanical fans or other electric equipment. There are many types of natural ventilation including cross-ventilation (wind driven), single sided ventilation (buoyancy driven) and the stack effect (buoyancy driven). Natural ventilation has many benefits over mechanical systems and can be easy to incorporate into a building as long as a concentrated effort early in the design process is made to account for some of the disadvantages of the system.
There are many advantages of a proper functioning natural ventilation system. Natural ventilation should not cost anything above normal design and construction costs, it is purely a matter of design knowledge. There are no pollutants or contaminants like CO2 or refrigerants to cause harm to the environment. There are no places for contaminants to become trapped and dispersed to other areas of the home. Condensation is not an issue in Hawai‘i because the natural temperature
differences from outside to inside are not great enough to cause sweating on the walls and ceilings.
Natural ventilation also removes polluted indoor air with fresh outside air, flushing out potential VOCs, chemicals and other harmful pollutants. When functioning properly, naturally ventilated spaces can
Figure 7.4: Methods of Natural Ventilation
Source: Own work
reduce indoor air temperature and humidity to comfortable levels. It is not to say that natural ventilation is perfect and there are some issues that should be addressed.
Perhaps the greatest issue with natural ventilation is the expectation and conformity of architects, builders and developers to the typical residential planning and architecture typology.
Natural ventilation can only take place if there is an opportunity for air to be drawn into the indoor space. If we take a typical house like those in the case study and site it on a lot which has been planned to be part of a close proximity, cul de sac style neighborhood it would be much more difficult to design an effective natural ventilation system. If we expound upon this, we should understand that the typical type of house has an interior plan which does not lend itself to natural ventilation. Interior rooms and walls often cause interruptions to the airflow and standardized windows and doors make it difficult to create either the necessary pressure differences or open air space to bring air into the home.
Some of the other issues with natural ventilation include the intrusion of dust and other airborne particulates, mold spores, automobile exhaust, noise pollution and other outdoor VOCs from the built and natural environment. These potential pollutants can easily infiltrate the average naturally ventilated home if there is not an additional designed element to help alleviate some of these concerns. Of course, these elements would vary depending on the location of the home. For example, on most areas of Hawai‘i island there would be little concern of automobile exhaust or say, industrial pollution from a steel mill. However, there may be some concerns with mold spores, which are in abundance in the tropical climate, dust from large plots of undeveloped land and Vog, which is the mixture of sulfur dioxide and other volcanic gases emitted from Kīlauea volcano. Vog is a big concern for residents of this island, per the United States
Figure 7.5: Comparison of Ventilation Types
Source: Own work
Geological Service, the volcano emits anywhere from 2,000 to 4,000 tons of SO2 per day160 and may cause many health issues such as headache, respiratory issues, watery eyes, sore throat161 and one study finds it may raise blood pressure or lead to a deterioration of existing health problems.162
Another issue with natural ventilation has to do with the variability of the climate. In cross-ventilated systems especially, wind driven forces are the determining factor on the potential rate of natural ventilation through a space. If there is no wind than it is difficult to reach the minimum differences in pressure to draw the air from the windward openings (positive pressure side) to the leeward openings (negative pressure side). Conversely, in strong wind events like storms, hurricanes or unusually strong trade or Kona (leeward) winds, naturally ventilated spaces can become too windy causing issues with thermal comfort, dust infiltration and other safety issues.
While not usually a problem in most areas of the state, naturally ventilated homes require windows to be open leaving residents susceptible to noise pollution from the outside. In many of the issues related to natural ventilation, designed mitigation efforts can help alleviate the problems to create a successful ventilation system year around.
HYBRIDYZING
One method for improving natural ventilation systems is to incorporate some means of auxiliary ventilation through ceiling fans, motorized vents and operable louvers and screens. Ceiling fans can help systems which use the stack effect to draw hot air up and out of the space and in other systems where airflow rates are low because of too little or too much wind (and having to shut the windows) they can provide additional cooling and air circulation. Motorized vents can also be used in areas where excess heat or humidity are created like the kitchen, restrooms and laundry areas. Operable louvers can also be used either through a system of human mechanization or electrical control to control wind or airflow in higher wind situations.
160 “Frequently Asked Questions about Air Quality in Hawaii,” Hawaii Volcano Observatory, United States Geological Survey, accessed November 14, 2016. https://hvo.wr.usgs.gov/hazards/FAQ_SO2-Vog-Ash/P1.html
161“Frequently Asked Questions and Answers on Vog and Volcanic Emissions from Kilauea,“ Hawaii State Department of Health, accessed November 22, 2016,http://health.hawaii.gov/cab/files/2013/05/kilauea_vog_qa_1.pdf.
162 Bernadette M. Longo, “Adverse Health Effects Associated with Increased Activity at Kīlauea Volcano: A Repeated Population-Based Survey,”ISRN Public Health, vol. 2013 ID 475962 (2013).