Keeping buildings cool in a heatwave
Heat waves are generally short-lived in the UK so it is not surprising that we are not very good at dealing with them.
Here is the main problem. … Sun shines in through windows and heats any surface it falls on, and this heats the room.
In hot countries, external shutters are the norm, and fitted to most windows. These are the most effective way of stopping the sun’s heat entering a building.
The best that most people can do in the UK is to close the blinds or curtains
during the day. However, this only has a limited effect because the heat has already got through the glass. That said, if kept behind the curtain, some of the heat has a chance of escaping to outside.
Shading is always a good way to reduce overheating in the summer. Modern architecture tries to build-in some form of shading. Traditionally, shops have had pull-out canopies, in-part, needed because shops tend to have big Windows. Canopies can reduce overheating dramatically.
What can we do?
DOORS & WINDOWS
Generally, keep doors and windows closed during the day if the air inside the house air is cooler than the air outside. You are in effect keeping the “cool” in (If there is any left to preserve). Use a fan indoors.
Open windows at night, and keep them open early morning when it’s likely to be cooler outside than in.
SHADING
When you go out, close curtains, or better, arrange some shading outside. Even a parasol positioned above or across a south-facing window can have dramatic effect.
Don’t expect instant results. It takes some time (days!) for the internal fabric (walls, floors, contents) of your house to cool down once it has overheated.
In emergencies, e.g. to help the elderly person, it could be worth temporarily pinning an old sheet on the outside of a window in extreme heat.
FANS
We keep cool though the evaporation of sweat. This is very effective and more so if there is a draught or breeze. Cooling fans simply move the air…..they cause a draught. Generally, the larger diameter the fan, the more effective, and the quieter it can be (small fans generally spin faster and make more noise).
I am no expert on available fans, but I note cheap fans are probably false economy… they can become noisy and may not last long.
Check the power and controllability. You will want a fan with a wide range of powers from
high breeze to low very quiet.
CEILING FANS
These are as common as lightbulbs in very hot countries, and
could be a worthwhile fitment for a house that is prone to overheating, e.g.
with excessive south-facing windows.
John Ewbank shares some useful experience from his own
ceiling fan https://johnewbank.co.uk/ceiling_fans_in_the_uk/
AIR-CONDITIONING
My personal opinion is to try any or all of the above before investing in air-conditioning. Indeed, many architects try to make good modern buildings that can keep cool without need of air conditioning.
An air-conditioner uses exactly the same mechanism as a fridge and is far more expensive to run than the simple cooling fans mentioned above.
Interesting fact. Almost 1Kilowatt of heat is delivered with every square metre of direct sunshine.
Portable electric heaters are typically up to 2 Kilowatts. Some rooms will get sunshine the equivalent of one electric heater on during mid summer’s day…
not exactly what is wanted!! This illustrates the importance of shading.
Why is is not intuitive to keep the “cool” in?
When we experience a breeze, even if the air is the same temperature as us (warm), it feels cool to us because our skin is damp, and we cool through the evaporation of sweat. So, on a hot day (say 5° hotter than normal room temp), a breeze through our open doors and windows may feel nice, but actually, it is warming the walls and contents of the room. Having a fan indoors, and closing winows to keep the “cold” in, is not as intuitive as closing them to keep the heat in, in summer.
John Cantor (2026)