Heatwave myths: Which?'s expert scientists reveal what really works (and what doesn't)

When temperatures rise, so does the number of supposed heatwave 'hacks' online, promising to keep you and your home cool.

But while some tips are grounded in science, others offer only brief relief – and a few can actually make things worse.

Our science team has examined some of the most common claims, from putting ice in front of a fan and freezing your bedsheets to using blackout blinds and taking a cold shower before bed.

Here, they explain which tricks genuinely help, which depend on the conditions, and which you can safely ignore.

Jump straight to each hack to find out what our science team have to say:

Putting foil on the outside of your windows | Putting ice in front of a fan | Taking a cold shower | Using blackout blinds | Keeping your windows open | Sleeping downstairs | Using houseplants to cool a room | Pointing a fan in different directions | Putting bedsheets in the freezer | Modifying your air conditioning unit

Does foil on the outside of your windows keep heat out?

One of the more eye-catching heatwave hacks involves covering your windows with aluminium foil to keep your home cool.

While it might not do much for your home's kerb appeal, there's genuine science behind why it can help reduce indoor temperatures.

A less conspicuous alternative that's also gaining popularity online is heat-reflective window film, which works on the same principle by reflecting some of the sun's heat away from your home.

'Window glass allows solar energy to flood right into your room where it becomes trapped, creating a mini greenhouse effect and warming up the house.

'Aluminium foil is incredibly effective at reflecting heat and light, and its low emissivity means it doesn't radiate the energy it does absorb. This helps stop the heat entering the home.

'Applying it to the outside of the window is best because it blocks the heat before it enters and prevents thermal stress from damaging the glass, which can potentially cause it to crack.

'But there is a price to this hack. It completely blocks out all natural light, and the glare might impact your neighbours – you might say it radiates "jacket potato couture".

'Heat-reflective window film is a less conspicuous alternative. Most products are designed to be fitted to the inside of the window.

'Homeowners should check that any film they buy is appropriate for the type of glass they're applying it to.'

Dr Neal Rimmer, Which? senior scientific adviser

Should you put ice in front of a fan?

A popular DIY cooling trick is to place a bowl or tray of ice in front of an electric fan. The idea is that the fan blows air over the ice, creating a cooler breeze than the fan would produce on its own. But does it work?

'Placing ice in front of an electric fan reduces the air temperature of the immediate surroundings.

'The air from the fan melts the ice faster and continuously cools the air passing over it.

'The colder air stream can help cool you down by pushing away the insulating warm layer of air against your skin and replacing it with comparatively cooler air.

'The ice-cooled air creates a greater temperature difference between the air and your warm skin, allowing more body heat to escape into the surroundings.

'This hack is particularly useful if the air temperature is greater than skin temperature (32°C to 35°C), as it reduces the air temperature enough to allow body heat to dissipate again.

'However, as the ice melts, it also increases humidity. In humid conditions, sweat evaporates less easily, making it harder for your body to cool itself naturally. So while the hack can work, it's less effective when the air is already humid.'

Olivia Pratt, Which? scientific adviser

Be sure the ice is in a bowl on a stable surface, to avoid spills, and avoid draping wet cloths over a fan, as moisture near electrical appliances can be a safety risk. For more on this, see our piece on how heatwave hacks could affect your home insurance

Is a cold shower before bed the best way to cool down?

After a hot, sticky day, jumping into a cold shower before bed can seem like the quickest way to cool down. But while it might feel refreshing at first, our experts say it's not the best way to lower your body temperature.

'During a heatwave, a cold shower may sound like a great idea and feel even better, but it will actually backfire.

'Paradoxically, shocking your body with cold water triggers a survival reflex where blood vessels near the surface of the skin constrict to try to conserve heat, so you end up feeling warmer.

'A better option is a lukewarm shower, which can open up your blood vessels, making it easier for the heat to escape.

'For added impact, leave your skin damp when you get out. The remaining water draws heat from your skin as it evaporates, giving you an additional cooling effect.'

Monika Gil, Which? senior scientific adviser

Can blackout blinds help cool your home?

Blackout blinds are often used to keep bedrooms dark, but they can also help keep your home cooler during a heatwave. Closing them before the sun reaches your windows can reduce the amount of heat that builds up indoors.

'Blackout blinds do more than just manage light exposure – they're also highly effective tools for preventing heat build-up during summer.

'Sunlight entering through glass creates a greenhouse effect, trapping thermal energy inside and drastically raising the temperature indoors.

'By keeping blackout blinds fully closed on sun-facing windows during peak daylight hours, you physically block this solar radiation from penetrating the room. Once the sun sets and the outside air cools, simply open the blinds to allow any trapped heat to dissipate.'

Jacob Compton, Which? scientific adviser

Should you keep windows open in a heatwave?

When your home feels unbearably hot, it can be tempting to throw open every window in the hope of letting cooler air in. But whether this helps or hinders depends entirely on the temperature outside.

'It's a common misconception that opening your windows will instantly cool down a stifling house during a heatwave.

'If the outside air is hotter than the air inside, opening windows simply draws that heat indoors and accelerates warming. Instead, you should keep windows firmly closed during the hottest parts of the day to maintain a cooler indoor environment.

'Only open them in the late evening or early morning when external temperatures drop, ideally creating a through-draught to flush out accumulated warm air.'

Jacob Compton, Which? scientific adviser

Is sleeping downstairs cooler?

Many people decamp to the sofa during a heatwave in the hope of a cooler night's sleep.

As it happens, there's a good scientific reason why the downstairs of your home may feel less stifling than upper floors.

'This is true, due to a process known as convection currents.

'During this process, cooler air – which is more dense and therefore heavier – sinks. 

'Meanwhile, warmer air – which is less dense – rises.

'The warm air eventually cools down and sinks again, only to be replaced by more warm air. This creates a constant flow, so the air upstairs in your home will always be warmer than the air downstairs.'

Kamisha Darroux, Which? principal scientific adviser

Do houseplants cool a room?

Houseplants are often credited with naturally cooling indoor spaces. But can a few leafy companions really help you beat the heat? 

'Houseplants can create a slightly cooler microclimate through a process called transpiration, where they release moisture from their leaves into the air.

'It sounds promising, but the cooling effect is tiny and limited to the area immediately around the leaf – so don’t expect a few monsteras to lower the temperature of your living room.

'Interestingly, some research suggests that a large system of indoor plants can improve ‘thermal comfort’ – in other words, people feel more comfortable and temperature tolerant in a room full of greenery, even when the temperature remains the same as in a foliage-free room.

'So, while houseplants won’t help defeat the heat, they can help your space feel a bit more pleasant – you just need enough of them to resemble a small indoor jungle.'

Ruby Gordon, Which? scientific adviser

Should a fan face you or a window?

If you've ever wondered whether your fan should point at you or out of the window, the answer is that both can be correct.

It all depends on the temperature outside your home and whether you're trying to cool yourself or the room.

'If you want to cool yourself down, the fan should face you. This enhances the evaporation of sweat from your skin, which makes you feel cooler.

'A fan facing an open window will draw air from inside the house and push it outside.

'When it’s cooler outside than inside, for example in the early morning, evening and at night, this will help blow the hot air out of your home. This will create negative pressure, so there needs to be an open window on the cooler side of your house to draw fresh air in.

'You can enhance this effect by having another fan stationed at the window on the cooler side, facing inwards, which will help pull cooler air into the house.'

Dr Steph Kipling, Which? science team manager

Beat the heat with the best fans for 2026

Does putting bedsheets in the freezer work?

Putting your bedsheets in the freezer is a popular hot-weather hack that's widely shared online. But does it actually help you sleep in the heat?

'Freezing your bedsheets – in a sealed plastic bag – will temporarily cool you down for a few minutes.

'However, the cold air in the fabric pores will quickly dissipate and be replaced with warmer air from your body and the environment.

'This is because heat moves from a hotter area to a colder area. The fibre type (cotton/linen/polyester), the fabric weave (loose/tight/how fibres are spun), and the fabric density can slightly influence for how long the bedsheet stays cold.

'Spraying the sheets lightly with water before placing in the freezer will result in the sheets staying cool for longer, as water transfers heat better than air. The moisture will also increase evaporative cooling. However, this will increase the air humidity and carries a risk of the frozen water melting and dripping into the mattress. 

'An easier and quicker solution that works using the same principles and doesn’t take up space or time in the freezer, is to dampen a tea towel or hand towel, wring it out enough that it doesn’t drip, and lay it across your body (legs/torso).

'This should cool you down long enough to get to sleep. This works particularly well in combination with an electric fan creating a breeze, which increases the rate of evaporative cooling from the damp towel.'

Olivia Pratt, Which? scientific adviser

Struggling to sleep in hot weather? Find 5 cooling mattresses for hot sleepers – hand-picked by our mattress expert

 Should you modify your air conditioning unit?

DIY air conditioner hacks are becoming increasingly popular on social media, with some claiming you can make a portable air conditioner more effective by turning a single-hose model into a dual-hose one by adding an extra duct – typically made from PVC or flexible aluminium.

But does the science stack up – and is it worth the risk? 

'Portable air conditioners consist of two key components: an evaporator that cools the indoor air and a condenser that expels the extracted heat.

'A single-duct unit uses freshly cooled indoor air to carry the heat away from the condenser before blasting it outside. This creates negative air pressure and a suction effect whereby hot air is pulled in from outside.

'Adding a second duct means that outside air is used to dissipate heat from the condenser, eliminating the suction effect. This trick should work as long as your appliance has two separate vents on the back.

'However, we wouldn't recommend trying this, as there are a few ways it can go wrong – for example, if the additional hose gets blocked, the appliance could overheat. And modifications like this are likely to void your warranty.'

Monika Gil, senior scientific adviser

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