Dr John Gallagher, Trinity College Dublin, discusses how the concept of passive design can influence a new way of thinking in natural ventilation pathways with the goal of reducing outdoor air pollutants entering buildings.

Groups within the research community are constantly helping us improve our understanding of how air flows in the built environment, and how it distributes pollution around our cities.

On the other hand, other research groups focus solely on designing ventilation systems for the indoor environment, ensuring suitable air exchange rates are adhered to and thermal comfort is provided.

But is there enough work being undertaken at the interface, investigating the interactions via pathways between the indoor and outdoor environment? And what can we learn from one, that helps us improve how we design the other?

Personal exposure to air pollutants is clearly highlighted as damaging to population health in both settings.

Therefore, how outdoor air is entrained to the indoor environment and what indoor air is emitted to the outdoor environment, can generate its own challenges as pollutants are transported in these air flows.

Mechanical ventilation

Reflecting upon the global mechanical ventilation sector, the industry is expanding like never before, with a six to seven per cent growth rate per year at present, and the sector is estimated to be valued at $250 billion in the next five years.

To put this into more quantifiable figures, part of this growing market is currently represented by more than 117 million air conditioning units among other forms of heating, ventilation and air conditioning (HVAC) systems available on the market.

One thing is definite, designers turn to mechanical ventilation as a fundamental component of modern buildings, to provide the necessary indoor environmental requirements of a modern structure, offering clean air, humidity control and overall thermal comfort.

The question to ask is whether we have turned our back on passive or natural ventilation solutions? Rather than working in partnership with its artificial substitute, there is a clear conflict when it comes to these systems working effectively in parallel.

Thinking passive ventilation

The humble window as we know it is approximately 700 to 800 years old. It was initially created to serve as an opening to allow light to penetrate indoors during the day, and as a form of protection from the outdoor environment.

Needless to say, little has changed of its function in the 21st century, with hinges and runners offering the flexibility to open and close sections and allow you some control over the amount of ‘passive’ or natural ventilation you permit into your home or workplace.

However, controlling the air flow rate or the temperature is not feasible in the natural environment, and it can be a contest between these factors. We all have our personal criteria for what encompasses adequate indoor environmental quality.

What typically occurs is that we rely on mechanical ventilation as the primary mechanism to control air flow rates and temperature management at the simple touch of a button. Whether everyone in the building is comfortable with the one-for-all setting is a discussion for another day.

Rethinking building ventilation

There is no doubt that mechanical ventilation systems play an important role in providing the inhabitants of a building with a continuous exchange of air while maintaining a tolerable temperature for those living or working in the building.

However, for this option to work effectively it is perhaps naive to completely reject natural ventilation.

Thus, it is in this area that there is an opportunity for innovation to deliver a ‘natural’ solution that can compete with mechanical alternatives.

However, first we need to understand the environment around a building both before and after its construction, to determine how it fits within its surroundings.

What affects natural ventilation

A natural ventilation opening, a door or window, is positioned on a building based on the decision and approval of the architect. And the opening on the window will be a default decision after window frame material, its energy performance properties and even its colour.

The entrainment and quality of air entering a building by natural pathways can be affected by several factors: the aspect ratio or shape of the street, localised wind conditions, and the sources and types of pollutants.

A designer will ensure that south-facing windows are considered to maximise natural light, therefore why is air flow patterns around a building not considered?

It may not be as predictable as the movement of the sun, but there are some typical patterns that could only benefit our decisions for positioning a window opening.

Innovation is needed in the area of natural ventilation, and sharing knowledge highlighted in research with designers and contractors can change how we view the humble window.

Green infrastructure can play its role

When focusing upon delivering solutions to address the challenge of air pollution in particular, green infrastructure in the form of trees and hedgerows for the outdoor environment, or shrubs and plants for the indoor environment, are considered as a natural solution.

This is now well supported by scientific evidence and informed technical guidance documentation, as green infrastructure has been shown to improve air quality and reduce the urban heat island effect in our cities.

As this passive solution has been shown to be effective to outdoor air quality, with respect to air pollution and thermal comfort among other factors, the next steps are to consider how these learnings can impact upon building ventilation design for the indoor environment.

Next steps for passive building ventilation

In terms of research, much of the work is done but its implementation from research to practice is a bridge that needs to be built.

We understand both indoor and outdoor pollution sources, and how it moves within its local environment.

We understand that there are pathways between the indoor and outdoor environment, as well as the mechanisms that allow the building to breathe.

Perhaps the part we don’t understand, or perhaps we simply don’t consider, is if the right people also know this information, that is, the designers and the contractors. All this information that research has generated is worthless without it being shared and applied.

A passive approach may benefit ventilation, but a passive approach to sharing this wealth of knowledge will not.

Author: Dr John Gallagher, assistant professor in the Department of Civil, Structural & Environmental Engineering, Trinity College Dublin.