How architects can find the balance between fresh air and energy consumption to help reduce the transmission of Covid
Covid-19 made designers acutely aware of the need for effective building ventilation. Its continued presence means they need to consider its presence as part of a building’s wider ventilation strategy.
Chris Iddon, a research fellow at the University of Nottingham, is a ventilation expert looking into the risk of Covid transmission in the built environment and co-author of CIBSE's COVID Ventilation Guidance. He talks of Covid and ventilation in terms of relative exposure risk – see graph below, showing the relationship between aerosol concentration and ventilation rate.
Ventilation is the process of introducing outside air into indoor spaces while removing stale air.
The graph shows that the more outside air you put into a space the lower the risk of occupants inhaling the virus, which is the advice we were all given during the pandemic.
What is significant about this graph, however, is the shape of the curve. ‘It shows that removal mechanisms are inversely proportional to the inhaled amount of virus,’ explains Iddon. In essence, there is a very sharp drop in relative risk as the amount of ventilation in a space goes from zero to a small amount. Then its impact on reducing risk becomes less and less. Improving the air in a poorly ventilated space has a greater effect on risk reduction than increasing the ventilation by the same amount in an adequately ventilated space.
This law of diminishing returns is particularly important when finding an appropriate balance between ventilation rates and energy consumption. Every additional litre of fresh air supplied to a building will have an energy penalty, both from tempering the air and from the fan energy needed to push it through the building. Of course, every extra litre of air will also reduce the risk of inhaling the virus. The challenge for designers is in finding the appropriate balance between ventilation and energy use.
Iddon says the typical Part F Building Regulations outside air requirement of 10 litres/second/person, or 1 litre/second/m2 of floor area is ‘a good rule of thumb to adopt for all spaces’ when considering the risk of Covid transmission in both new and existing buildings. His message is: ‘Target what you need to hit, and if you can improve on that there will be certain benefits.’
It is not just about avoiding virus transmission. Health and wellbeing benefits from improved fresh air supply rates also include better sleep, fewer sick days and an improved ability to concentrate. But seasonal conditions bring challenges. ‘The focus of guidance I’ve written is to increase ventilation as much as is reasonably possible,’ says Iddon, ‘but what is reasonable in winter is different to what is reasonable in summer.’
The emerging consensus among engineers is that the most effective means of improving ventilation rates in both summer and winter in new-build office and commercial buildings, without incurring a huge energy penalty is to adopt a mixed-mode or hybrid ventilation system. This is a combination of mechanical and natural ventilation systems, which exploits the best features of each to improve the indoor environment and reduce energy demand.
Designing for best ventilation
- For cross ventilation the floor plate should be no wider than five times the height of the space.
- Provide for natural ventilation at the outset of a design; it’s hard to retrofit.
- Use air quality information (as required by the planning process) to help decide whether to use mixed‑mode ventilation. It could allow you to provide opening windows even in the city.
- Plan for improving air quality. As electric cars replace diesel and petrol ones, opening windows could become an option if they have been planned for. Ensure this provision does not get lost in value engineering.
- Consider air-cleaning technologies as a temporary measure if it is impossible to increase the ventilation rate sufficiently in the near term
In a mixed-mode system, mechanical ventilation often with heat recovery is used to ensure minimum fresh air supply rates can be maintained in winter when the outside air is cold. Ideally air is not recirculated between different occupied zones so as to keep transmission risk down. In spring and autumn, openable windows boost the ventilation rate by allowing a space to be naturally ventilated. The natural ventilation mode can also be used in summer if conditions are favourable, however, there is also an option of reverting to mechanical ventilation with comfort cooling.
A major benefit of mixed-mode ventilation is that it gives occupants control over the type of airflow in their environment. Post-occupancy evaluations frequently show that people are prepared to tolerate warmer internal temperatures when they have some level of control.
'Mixed-mode is definitely starting to happen in buildings more and more often,' says Edwin Wealend, head of research and innovation at consultant Cundall and chair of the CIBSE Air Quality Task Group. 'If you look at the London Energy Transformation Initiative (LETI) guidance on transitioning to net zero operational carbon [which is supported by RIBA], to get energy consumption down to the level needed, you have to look at using natural ventilation in offices, which is not something we've been used to designing for.'
To function effectively, a mixed-mode ventilation system will need to be integral to a building’s design from the outset. ‘If you look at passive design best practice,’ says Wealand, ‘you want openable windows and a shallow floor plate no more than about 12m deep, or a floor plate that opens out on to an atrium to enable stack ventilation.’
Mixed-mode is equally applicable to the ventilation of residential schemes. The concept of mechanical ventilation with heat recovery (MVHR) in winter and openable windows in summer has been employed in some, though not all, Passivhaus homes. MVHR is now the norm in most new apartment blocks, says Hoare Lea’s Ashley Bateson. However, a mixed-mode solution will enable heat to be purged in summer, preventing overheating and helping compliance with the new Part O of the Building Regulations.
While mixed mode is set to become an increasingly popular option to improve the ventilation of both new residential and commercial buildings, a major challenge will be in improving the ventilation of existing buildings, to which Iddon’s relative exposure risk graph applies every bit as much.
‘What the pandemic has done is shone a light on the rooms and spaces that are not properly ventilated,’ says Shaun Fitzgerald, a ventilation expert who supported Foster + Partners with the ventilation strategy for Bloomberg’s London HQ and is a member of the government’s Environment Modelling Group, part of SAGE.
‘First base for me,’ he says, ‘is ensuring that ventilation in all workplaces where there are multiple occupants is brought up to modern Building Regulation standards, since we know tackling really poorly ventilated spaces provides the most benefit. We can then debate whether the current Building Regulations are fit for purpose for risk mitigation in managing Covid, and indeed future airborne virus pandemics and ensuring occupant wellbeing.’
Fitzgerald and Iddon are both team members of the government-funded Airborne Infection Reduction through Building Operation and Design for SARS-CoV-2, or Airbods as it is more commonly known. It is set to publish its findings later this year, including ventilation guidance for architects and consultants for use at the early design stage. The findings may even lead to changes in the Building Regulations.
Case Study: Mixed-mode, WELL and BREEAM – the future of building ventilation
- EDGE London Bridge is a 27-floor commercial office, aiming for BREEAM Outstanding and WELL Platinum certification.
- The office floors incorporate an underfloor air supply and high-level extract, delivering outside air at 2 litres/s/m2 to comply with the WELL standard.
- In addition to mechanical ventilation, the scheme incorporates perimeter openings to allow the office floors to be naturally ventilated.
- EDGE London Bridge has an ambitious targeted regulated energy consumption of less than 23kWh/m2, which aligns with the RIBA 2030 Climate Challenge operational intensity targets for new-build offices.
CIBSE Air Quality Task Group
Building Regulation Part F
Building Regulation Part O
LETI Net Zero:
Airborne Infection Reduction through Building Operation and Design for SARS-CoV-2 (Airbods)
Environmental Modelling Group