By Jeremy Climas

31 July 2020

A lot of homes aren’t designed to keep cool in the increasingly common heatwaves that the UK now sees as a result of climate change. Here’s some advice on how to make the best of it.

I’m going to go straight into some science and then tell you about the practical steps to make the science work for you. Thermal comfort indoors is down to the solution of two heat transfer problems - 'skin to room' and 'ambient to room'.

Skin to room

I’ll start off by saying that a rough approximation of ‘thermally comfortable’ is a condition of your body where it is able to reject all the heat you’re generating with your skin being at about 30°C and not overly sweaty. If your body is able to manage that you’re probably thermally comfortable. The higher the skin temperature gets or the more sweat on the skin, the less comfortable you’ll feel.

That skin condition depends on a few factors, those being: air temperature, radiant temperatures, humidity, air movement around you and clothing.

Ambient to room

Most of those comfort factors in the ‘skin to room’ section depend on how the building is behaving and what conditions you’ve managed to get the room to be at. This all depends on the ‘ambient to room’ heat transfer problem. In hot weather you need to know the tricks to keep your desk spot as comfortable as possible.

The rules are:

Cool the place down when that’s possible – open it up as much a possible when it’s cooler outside than inside. Open up two sides to get cross flow if possible as this will massively boost ventilation rates and cool it down as quick as possible – run extract fans overnight. Overnight is the best time if you can (there’s lots of reasons people can’t).

Minimise heat gains – any energy of any sort entering your home will decay to heat.Turn off everything you can: lights, TV, oven and don’t overboil the kettle. Block out sun, ideally externally; curtains and blinds might cut out 10% of the heat, but an external shade can cut out 90%. Anything to stop sun getting in is good. Keep people out of the house as much as possible, eat meals outside, go for a walk in the evening. You’re a 150W heater – do you want a 150W heater in your already hot home?!

Shut the place up when it’s warmer outside than in –shut all the windows and doors to stop that warm air getting in and making your home hotter. Often this might be quite early in the day (it was 9.30 in my house this morning!).

Cool yourself – to help your body reject that heat here’s what you can do at the ‘skin to room’ heat transfer: Wear minimal clothes, as little as you can get away with. Keep the sun off – make sure no sun is directly on you. Get a good fan, one that’s quiet (unpaid tip – I got a Honeywell Turbo Fan that does a great job). Point it straight at yourself, don’t just stir up air in unoccupied areas. Using a fan will actually mean the air temperature is slightly higher (your fan’s electrical consumption decays to heat just the same as anything else), but it will make your skin cooler by helping move heat away which is what this is all about.

As I said at the start, many homes aren’t really up to the job of being comfortable in hot weather, mostly due to bad design and insufficient regulation. Even though this is luckily changing now, it doesn’t help all the people in late 20^th century homes - so this blog gives you tips to get it as good as possible.