Your lawyers since 1722

Air quality in green buildings: Re-imagining the balance between ventilation & carbon emissions

Over the last month, WorldGBC has been examining the tensions and compatibilities between reducing carbon emissions and improving health and wellbeing in buildings in a series of blogs. The series continues with experts from individual fields examining some of the features we have covered in more depth. In this piece, we hear from Jenn McArthur, Assistant Professor at Ryerson University in Toronto, Canada, who has been investigating higher energy use associated with increased ventilation and considering solutions to mitigate this.

Have you ever stepped outside a building – perhaps a conference centre or your office – and realised how much better you feel in the fresh air? Studies in schools and offices have consistently found that people’s performance and productivity decreases with inadequate ventilation and increases above baseline levels when the provision of outdoor air rises.

Historically, a number of green building rating systems have encouraged modest outdoor air provision above standard levels (for example, ANSI/ASHRAE 62.1). This balances the indoor environmental quality (IEQ) improvements with the potential increase in energy costs and carbon emissions caused by the increased ventilation. But, if there’s an opportunity to get even more productive students or workers with more ventilation, shouldn’t we take it? Is there a way to ‘have your cake and eat it’ too?

Productivity studies have noted an exponential relationship between outdoor air and increased cognitive and task performance, estimated in the range of 1.5-2.5 per cent per doubling of outdoor air rates. Considering that the ratio of energy costs to salaries is typically 1:100 , this small productivity gain has a significant economic benefit when compared with the incremental energy and carbon cost.

Making ventilation carbon-neutral

A study at Ryerson University in Toronto, Canada, is investigating the net economic impact of providing additional ventilation in buildings, but offsetting the associated carbon emissions, making it carbon neutral. We are looking at increasing mechanical ventilation to boost productivity, where all carbon emissions associated with conditioning the increased outdoor air are offset, as an enhancement to the green building rating system requirements.

Medium-sized offices, in each of the eight ASHRAE climate zones across North America, were modeled using the US Department of Energy 2.2 engine (eQuest), with a dedicated outdoor air system and local fan coil units served by a central high-efficiency boiler and chiller.  Outdoor air rates ranging from the ASHRAE 62.1 baseline (“1X”) to ten times this amount (“10X”) were tested. While seemingly extreme, the 10X increase represents only a two-to three-fold increase on other global air quality standards that prescribe much higher minimum outdoor air rates, for example EN13779:2007.

Impact of increased ventilation on energy use

Heating energy use obviously increases when ventilation is increased, but the relative increase depends on the climate. We found that generally, with 10X ventilation rates, heating energy use increased more in milder climates compared to colder climates. This is because in mild climates, very little energy is required to treat outdoor air that is already close to a comfortable temperature. Whereas, in colder climates, much more energy is required to bring colder outdoor air to a comfortable temperature.

In all climates, electricity consumption from non-heating loads contributed most to the carbon emissions for the lower ventilation rates (1.25X-9X). In the coldest climates, the carbon emissions from natural gas dominated for the higher ventilation rate (at 10X).

The net cost of carbon-neutral increased ventilation

The total cost of the increased ventilation had three components: incremental electricity cost, incremental natural gas cost and carbon offset cost, based on a nationally-available provider. These costs ranged from an average of US$0.72/m2/year (range: $0.17-$2.01) for 2X ventilation to US$9.07/m2/year (range $4.62-$24.76) for 10X ventilation.

The net benefit per square metre of office space of carbon-neutral 2X increased ventilation across different climate zones ranged from US$46.46 to US$48.78 and from US$139.07 to US$159.21) for the 10X ventilation rates.

The benefit/cost ratios ranged from approximately 10 in the coldest climate to approximately 275 in more temperate climates for the 2X ventilation. For the 10X ventilation, the benefit/cost ratios were reduced due to the increased energy costs, but still ranged between approximately 6 and 35.

The research shows that even with offsetting the carbon emissions of the additional ventilation, there was a clear business case to be made – there was a obvious net benefit due to the dramatic increase in productivity caused by the enhanced ventilation.

Are carbon offset requirements the answer? 

This research gives rise to two questions for consideration in future versions of energy codes and green building rating systems.

Firstly, given the net benefit of increased ventilation when the energy and carbon impacts are minimised, should green building rating systems encourage higher outdoor air rates for enhanced ventilation?

Secondly, does the net economic benefit of enhanced ventilation in fact justify a wholly new approach to balancing energy performance and indoor environmental quality? For example, could requiring the purchase of carbon offsets for incremental emissions associated with ventilation enable the win-win of improving occupant health and productivity and decreasing building-related emissions?

Wider implications for rating systems

But this study throws up even larger questions for consideration. Should the positive health and economic benefits to building owners and occupants be used to finance energy or carbon reductions? Does this holistic consideration of productivity, wellness, and energy consumption present new opportunities to building designers, code officials, and developers of green building rating systems as we work to reduce the environmental impact of the built environment?

Join the discussion

We would love to hear your views on Jenn McArthur’s ideas and the wider topic. Do you have your own experiences of tensions between the two goals of reducing carbon emissions and improving health and wellbeing, particularly when it comes to indoor air quality? How have you addressed them? Please send comments and case studies to Colin at

Read the firstsecondthird and fourth pieces in this blog series

Find out more about our Better Places for People project.