Net zero and Covid resilience

Improving the building fabric will allow for higher ventilation rates to minimise Covid-19 risk even in cold weather, says Dr Shaun Fitzgerald

Dr Shaun Fitzgerald

Covid-19 has wreaked havoc on the world in so many devastating ways over the past 18 months. One of the major learnings about the SARS-CoV-2 virus is that ventilation within buildings can be a really effective way of reducing the risk of its transmission.

Sir Patrick Vallance, the government’s chief scientific adviser, commissioned a report from the Royal Academy of Engineering on how to reduce the risk of infection indoors. In the study the experts say there is an ‘urgent need’ to improve ventilation as people return to offices after 19 July.

The recommendations are that multiple-occupancy spaces should be well ventilated to minimise the buildup of virus should there be infectious people present. It is relatively easy to deliver plentiful amounts of outdoor air when it is late spring or early summer. If a building is ventilated with opening windows, throwing these open to flood the space with outdoor air can create a very natural and pleasant space. There are questions, however, about the appropriate amount of ventilation in colder weather, especially as we seek to make a serious dent in the energy consumption associated with the heating and ventilation of buildings in our race to zero emissions. How does increased ventilation in cold weather square with reducing heating energy?

If we look at much of the current building stock, some of the main challenges we face in terms of reducing energy lie in improving the insulation levels, reducing uncontrolled air leakage, and reducing heat losses via radiation.

Improving insulation levels helps cut the heat lost by conduction through the building fabric. The second action, of sealing  gaps in the fabric, means a building that is empty at night will not leak warm air from the previous day. This will help keep the building warm overnight and reduce the need for the heating system to come on in the early morning, to pre-heat the building. Finally, installing coatings on windows, or foil-backed insulation in the attic space, can prevent heat loss by radiation.

By opening high-level windows a small amount you can get good levels of ventilation, but without cold draughts

Once the building fabric has been upgraded to a good level, the space can be well ventilated when the building is occupied, even in cold weather, without excessive energy use. This is because natural heat gains within the space are often sufficient to maintain the interior at a comfortable 21°C and provide ventilation of around 10 L.s-1 per person without additional heating or a heat-recovery scheme, even when the external temperature falls down to 5°C.

The combined heat gains from IT equipment, lighting, solar and the occupants are considerable (often 200W/person). As long as the ventilation system is designed and operating to exploit these heat gains, by pre-mixing the incoming cold air with warm room air, adequate levels of ventilation can be achieved without cold draughts.

Unfortunately, many existing buildings have poorly maintained ventilation systems. If a building has opening windows, for example, not all of the windows may work. Many at low level, and easily accessed by occupants, are better maintained than higher-level windows. Consider sash windows, for example – the bottom sashes are much easier to open than the top ones, so these get used. Top windows can get painted shut, and are then, basically, out of action.

While losing a few opening windows may not seem problematic in normal times, it really is a problem in winter. This is because, by opening all the high-level windows a small amount, you can get rather good levels of ventilation, but without cold draughts. The incoming cold fresh air can mix with the air in the space and be warmed before it hits the nearest occupant.

Without high-level windows, however, people will find it intolerable to be provided with the same level of ventilation in cold weather via low-level windows. Their first instinct will be to close them, and this is what we need to avoid from a health perspective. If there are radiators under the windows, these will be used to ameliorate the cold draughts, even though the heating isn’t needed from an energy perspective – and this is what we need to avoid from a climate perspective.

Well-designed and maintained ventilation systems are the answer to the challenges of Covid-19 resilience and net-zero buildings. It is not a choice between Covid-19 and net zero; it is a case of improving buildings to meet both goals.

Dr Shaun Fitzgerald FCIBSE is director at the Centre for Climate Repair at Cambridge and co-author of the CIBSE Covid-19: Ventilation guide