A few times of year in the UK, we get a solid patch of high temperatures, leading to hundreds of headlines about the heatwave, with copy full of the usual cliches (‘mercury rising’, and so on). And, the demand for fans goes through the roof, as people look for ways to stay cool.
While choosing one of our best fans can help in hot weather, how and when they’re used is as important as the details of the fan itself – something that’s regularly overlooked in reviews and round-ups.
In particular, the effect of humidity is something rarely mentioned during a heatwave, yet this factor has a big part to play in comfort levels.
(Welcome to Smarter Life, our new weekly column from smart home tech expert David Ludlow. Each week, we will delve into the smart home world, past and present, analysing the biggest products and how they affect our lives.)
Relative humidity explained
When we talk about humidity, we’re talking about the amount of water vapour in the air. There are two humidity measurements: absolute humidity (AH) and relative humidity (RH). Absolute humidity is expressed as the weight of water vapour per unit volume of air, which is usually expressed as grams per metre cubed (g/m³).
However, relative humidity is normally used when we talk about humidity, and relative humidity readings are found on smart devices such as thermostats and fans. Relative humidity is expressed as a percentage: it’s the percentage of the water vapour in the air compared to the maximum amount of vapour that the air can hold.
The hotter the air, the more vapour can he held; as the air temperature cools down, the amount of water reduces. For example, if it’s 28°C with a relative humidity of 55% during the day and cooled to 23°C overnight, the relative humidity would increase to 73%.
The opposite is true: if it were 28°C and 55% relative humidity during the morning, but temperatures soared to 32°C in the afternoon, then humidity would drop to 44%.
In these cases, the total amount of moisture vapour in the air is constant, but changing temperatures affect the RH.
Harder to sweat
As humidity increases, and the air becomes saturated with moisture, it becomes hard for water to evaporate. That’s important because of the way humans keep cool: sweating. When you sweat, heat should evaporate the moisture, keeping you cool.
If the relative humidity is too high, sweat can’t evaporate, and you start to feel hot and sweaty; that muggy feeling we all get. The fact that relative humidity drops at night as temperatures fall explains why nights are often so uncomfortable, even if the actual temperature is lower.
Typically, indoor humidity should be between 40% and 60%; above that, the environment starts to feel uncomfortable. At 70% RH, it becomes harder for sweat to evaporate, and at 90% sweat evaporation stops entirely according to the Canadian Centre for Occupational Health and Safety (CCOHS).
It’s the interplay between humidity and temperature that’s important: 90% humidity at 21°C isn’t such a problem, as it’s a comfortable temperature; 65% humidity at 34°C is dangerous. The CCOHS uses Humidex readings. A Canadian innovation, Humidex combines temperature and humidity into a single score that defines how hot and humid weather feels to reflect the perceived temperature. The Humidex table is a useful tool to see that interplay between temperature and humidity.
The problem with fans
As we’ve pointed out before, fans do not cool a room. They just move air around and don’t affect air temperature at all. The cooling effect is because the air flow aids in evaporation, so a breeze (from a fan or otherwise) feels cool because of the sweat evaporating. If it’s too humid, then sweat can’t evaporate, so fans don’t have a cooling effect.
The same is also true of evaporative coolers. These use a water tank, with moisture pulled up in front of a fan. Air blowing causes the moisture to evaporate, blowing colder-than-ambient-temperature air. Evaporative coolers only work when the RH is low enough, and they increase the RH of a room by adding moisture to the air.
That’s not to say that fans and evaporative coolers are useless; they work best in places with low relative humidity. For example, humidity is generally quite low in London; if you live by the coast, such as in Cornwall, you’ll have much higher humidity levels.
Keeping cool
It’s worth keeping an eye on relative humidity levels in your home with a hygrometer. Some smart thermostats, such as Tado X, monitor relative humidity so you can see the actual levels in your home. It’s best to aim for humidity between 40% and 60%.
Getting humidity to the right level can be done in several ways. Opening windows to let cooler night air in can refresh the air and lower humidity. Richard Stanton’s excellent humidity calculator lets you type in the external and internal temperatures, and the outdoor humidity level, showing you what the indoor humidity should be as the outdoor air comes inside.
As our guide on how to stay cool in a heatwave pointed out, a fan can be useful in speeding up air exchange.
There are times when it’s just so humid that natural air exchange won’t reduce humidity levels enough. If your home is very humid, you need to find ways to remove moisture from the air. A dehumidifier, such as the MeacoDry Arete Two 20L, can help. Once humidity is lower, a fan is more effective at keeping you cool.
Remember, from the Humidex table we linked to above, comfort is based on a combination of temperature and humidity, so lowering the temperature of a room can help you feel more comfortable. Dehumidifiers and fans don’t actually reduce a room’s temperature, but air conditioning does, while also lowering humidity.
Sure, AC is more expensive to run than a fan, but that’s because the units do something different. You can invest in air conditioning, either a properly-installed split air conditioning, or a portable unit, such as the De’Longhi Pinguino 100 Silent, which will cater for one room. By lowering temperatures and humidity, AC creates a more comfortable environment.
In all cases, one of the best things you can do is to stop heat from entering your home in the first place. In hot countries, it’s usual to see external blinds and shutters, designed to prevent the sun from warming a house through its windows. Here, in the UK, our homes are typically built for colder weather, so we can suffer under hot temperatures and strong sunlight, but with temperatures rising, we should think more about passive ways to keep our homes cool.
As an extreme example, my south-facing garden office has UPV French doors across the front. In extreme heat, the internal temperature has reached 48°C, even with internal blinds shut and heat-reducing film applied to the inside. This is because the basic glass I have allows the shortwave radiation from the sun to pass through, heating up objects inside, which release longwave radiation that gets trapped inside by the double glazing. This solar gain, can heat a room even with all of the internal blinds shut.
When I fitted external blinds, I found that the office still gets hot, but rarely gets above 32°C. More importantly, with the external blinds down, I can use a portable AC unit to make the environment comfortable; before the blinds, the AC was overwhelmed.
Finding ways to stop the sun from penetrating a room is important, as it keeps rooms cooler, which in turn can make fans and AC units work more efficiently. If your house gets very hot via sunlight through the windows, looking at options to prevent this can help, which can include Low-E glass replacements, external shutters or blinds, or even just using a parasol or awning to shade windows.
Using a combination of tactics is the only true way to keep you and your house comfortable, just don’t expect a fan by itself to miraculously cool you.
