Staying cool in a heatwave

Central East London and South East North East North West South West West Midlands Yorkshire and Humber

A heat wave is set to strike this week, on top of the already unusually warm spring we have experienced this year. The Met Office has shown that climate change made the record-breaking UK summer that we saw in 2018 30 times more likely, and that by 2050, heat waves are likely to happen every other year.

This means that keeping cool, both in the workplace and at home, is becoming more important and will be even more so in future. But before you start searching for air conditioners, it’s worth giving the matter further thought.

Air-conditioning is the least energy efficient way of cooling people, because it refrigerates all the air in an enclosed space in order to cool the occupants. The larger the space and the fewer the people within it, the more energy it will take to cool each occupant. This article will discuss just some of the more efficient options for keeping cool.

The first thing to consider is that thermal comfort (which defined by a person’s state of mind in terms of whether they feel too hot or too cold) is not solely determined by the temperature of the building they are occupying. The main factors that influence thermal comfort relating to the environment are:

  • Air temperature
  • Radiant temperature (i.e. the average temperature of surfaces around a person, which may include things like a sunny window)
  • Air velocity (speed of air flowing)
  • Humidity


Personal factors provide more variables to consider, such as: clothing and metabolic heat (a person’s internal heat levels, which will be affected by things like whether they are sitting or moving around, their gender, size and fitness levels).

Therefore, different individuals can find the same environments more or less comfortable, dependent on these factors. In multiple-occupancy buildings, such as workplaces, is not always possible to please everyone, but thermal comfort should not be measured by room temperature, but by the number of people complaining of discomfort.

Reducing heat gains (e.g. from sunny windows and other heat sources), increasing air movement and enabling personal cooling methods are relatively straightforward ways to cool buildings and occupants, because they can be retrofit measures that do not require significant investment and disruption.

Reducing heat gains

Solar gain

It is often common knowledge within a building that certain windows/glazed areas contribute uncomfortable amounts of heat into the space, particularly if they are south-facing, so this is usually easily diagnosed. There are a variety of mechanisms to manage this, such as:

  • Internal shading
    • Horizontal slats angled upwards are a good choice, because a proportion of the light/heat will be reflected away from your window, the rest will be reflected onto the ceiling and back into the room, and occupants will still be able to have a view of the outside
    • Thermal blinds are perforated and still allow visibility outside, while reflecting heat away, and can be switched around in the winter to reflect heat back inside
  • External shading
    • Window coatings such as films can reflect heat away while still allowing some ingress of light and a view to the outside
    • Slats can be built outside glazed areas, and can be angled to allow ingress of winter sun (which comes in at a lower angle), but block summer sun (that comes in at a higher angle.) On ground floors this could be a pergola with climbing plants surrounding it, which will provide further shade in the summer, yet die back and allow solar gain in the winter
    • Awnings – these can be retractable (electronic or manual) to enable their use during hotter periods, but stowed away during cooler and windier periods
    • Fabric shades – reflective fabric is temporarily hooked into place, covering or semi-covering glazed areas


Reducing other sources of heat gain

Other sources of heat come from roofs, walls, heating systems and hot pipes (for hot water that will be kept on over the summer), lighting and other electrical appliances.

If you have access to a thermal camera or infrared thermometer, this can help identifying heat sources, especially “hidden” sources such as heat pipes hidden away beneath floorboards. To reduce these heat sources, ensure roofs, walls and hot pipes are insulated. This will have the additional benefit of keeping heat in during the winter.

Heating, appliances, and lighting should be as efficient as possible and switched off when not in use.

Air velocity

Opening windows to allow flow of air is a simple and free mechanism to increase air velocity and should be encouraged if air temperatures are lower outside than inside. However, once air-temperatures have increased beyond those inside, windows should be closed and other mechanisms deployed.

The humble rotating fan is highly effective, providing more comfort and requiring much less energy than air-conditioning, because moving air around requires much less energy than refrigerating it. LowTech Magazine states that cooling people by increasing local airflow is at least 10 times more energy efficient than refrigerating the air in a given space.

Personal factors

In addition to these considerations, individuals should be encouraged to manage their personal comfort in a way that suits them. Different companies will have different policies and considerations to make, particularly if staff are customer-facing, but as far as possible, staff should be encouraged to wear cooler clothes that are looser and lighter-weight fabrics. It may be worth having a discussion in a team meeting about keeping cool, which could cover dress code (what is and isn’t allowed), opening windows, use of fans/air conditioning/spray bottles, and drinking cool drinks.

Getting started

A good place to start is to walk around your building and try to identify sources of heat and discuss with staff what they think are problem areas and times. You may wish to discuss the issue with all or some members of staff, e.g. your Green Team. Once you have identified problems, you can consider possible solutions and prioritise those accordingly.

It may be that specialist support and investment is required. iiE members who are interested in keeping their workspaces cool can call their iiE advisor if they would like further advice.