It is a common misconception that natural ventilation is the sole cause of deteriorated indoor air quality (IAQ). This concept has caused many individuals and organizations to believe that closing windows and using mechanical ventilation to improve indoor air quality is counter-productive. This is not true. In fact, there are numerous ventilation/IAQ issues that overlook natural ventilation entirely.
The Building Science ventilation paradigm
The prevailing assumption is that indoor air quality is affected most by three factors: temperature, relative humidity and air movement. Natural ventilation is thought to be the primary influencer of air movement, and the other two variables are regulated by mechanical ventilation systems.
As a result of this paradigm, the prevailing ventilation design philosophy is that mechanical systems always be operated at the highest efficiency possible. The consequence of this thinking is that natural ventilation is often compromised in favour of an artificially balanced environmental condition.
The biological paradigm
A biological paradigm of ventilation is concerned primarily with how we live, how we eat and how we breathe. It recognizes the fact that in order to obtain the benefits of fresh air we must open our windows and take the fresh air in, rather than putting it all over the house and hope that it is distributed evenly. This is the principle of air exchange, applied to residences and schools.
Natural ventilation and heat
If natural ventilation is the primary source of heating and cooling air, then it follows that closed windows, without natural ventilation, would result excessive indoor heat. Although this is in fact true, it is rarely the case. This is primarily because rooms are designed to have certain properties, namely mass and thermal resistance (thermal mass), and these properties are compromised by artificial heating and cooling.
The heating and cooling systems in most buildings are not capable of bringing the temperature to the desirable point in any predictable timeframe. This is especially true in schools and offices, where temperature is the most important influence on air movement. For example, a classroom may be heated only enough to meet the design requirement of 20°C at two o’clock in the afternoon, but the daytime temperature might reach above 28°C, resulting in natural air movement from the room regardless of the windows are open or closed.
Natural ventilation and illness/disease
In general, the presence of natural ventilation usually results in less illness or disease. Historically, latrines were re-circulated with outdoor air, and often resulted in disease epidemics. The introduction of mechanical ventilation systems reduced this threat. This brings us to another point — often the presence of a particular type of disease is linked to a single contaminant that has been identified through the process of trace analysis. Often, other contaminants have been found, but they have been ignored because they were not in the original contaminant analysis.
In homes and schools, ventilation systems are often re-circulated with indoor air, and although the air temperature may be balanced, the air quality is not. Contaminants resulting from occupancy (such as mould, dust and dander), pets and even the occupants themselves, are re-circulated with the heating and cooling air, thereby exposing building occupants to the contaminants.
Natural ventilation and energy
Natural ventilation should be optimized as a source of thermal insulation. This is important to consider, particularly in climate zones 5a and 5b, where the cost of heating and cooling is significant. Dependant upon the design of the residential or school structure, the position of the windows and the interior partitions, a room can have a positive or negative effect on the HVAC system.
The emerging paradigm
The concept of the paradigm, with it’s emphasis on the biological process of ventilation, is now gaining credibility. Although this is not always evident, the ventilation system is always limited, and near the limit of it’s ability to provide adequate indoor air changes. The components of the design that govern air change are:
The air exchange rate.
The problem with the emerging ventilation paradigm is that it can be derived exclusively from scientific literature. In order to implement it, a comprehensive understanding of indoor environments and how the design of buildings affects them, is needed. As a result, it is often misunderstood and neglected. Even main stream code organizations such as BOMA are unaware of the biological aspects of ventilation and IAQ, and many engineers lack the prerequisite knowledge and experience to design buildings in accordance with the biological paradigm.
Where has ventilation gone wrong?
On one hand, the paradigm of the past — the ventilation comfort model (VCM) — was not entirely correct either. This concept was developed to describe the indoor climate conditions experienced by healthy individuals in good physical condition, who need to be referenced for the purposes of establishing standards and codes for ventilation. In actual fact, convenience prevailed and the VCM became the de facto standard for determining the design of residential and schools ventilation systems. The VCM is still being taught at technical schools, and is widely applied in the of buildings.
In hindsight, it is easy to identify the shortcomings of the VCM:
· the model was technically dubious;
· it did not account for the multitude of respiratory illnesses; and
· it did not take into account the effects of people who are not in good physical condition, or who have certain respiratory illnesses.
Conceptually, a paradigm shift is long overdue. This will only happen when the real problems of buildings are identified, understood and appreciated by designers and building owners alike.
Natural ventilation should not be compromised
It is easy to identify buildings that have compromised natural ventilation, with the consequent detrimental effects on IAQ and occupant comfort. These buildings operate in an artificial thermodynamic equilibrium, where the ventilation supply is re-circulated with the indoor air, resulting in a constant concentration of contaminants re-circulated with the air flow. Indoor air quality is variable and often sub-standard, making occupants more susceptible to disease. In buildings with good IAQ, occupants exhibit less illness and accident rates are lower.
Natural ventilation is the most cost effective approach to reducing the energy consumption of a building. Compromising natural ventilation for energy issues, is a lose – lose proposition. It is counterproductive to design buildings this way.
How do we learn about our buildings?
Physicians, homeowners, building owners and the public need to understand that the existing design is flawed. Today, buildings are designed from the perspective of how buildings work, rather than how we work and live in them. It is believed that buildings fail, rather than occupants or occupants fail buildings. The same symptoms are always being sought and treated, rather than the true cause.
Natural ventilation to improve air quality must be designed into buildings rather than left to chance. An individual’s comfort is often compromised by the residence itself, and a building’s operating costs are frequently un-necessarily high. Buildings should be designed to optimize requirements, not to satisfy codes and standards. Codes and standards should serve the needs of buildings — they should not determine them.
As engineers and designers, we should be required to know how buildings work, rather than design to satisfy Codes and Standards that are too often based on the VCM, which is highly inappropriate for schools and residences.
· Ventilation, a necessary evil by Charles G. Straube
· Ventilation and Indoor Air Quality, Field Guide by David J. Billings, Ph.D and