Evaporative cooling vs air conditioning: does it work in the UK?

Evaporative cooling cools air by evaporating water into it, and it can only ever cool that air down towards what's called the wet-bulb temperature. In the UK that limit bites hard. On a genuinely hot, dry afternoon a decent evaporative cooler takes about 5 to 9°C off the air coming out of it. On an ordinary summer afternoon it's more like 3 to 4°C, and at night, when British air is often 80 to 90% humidity, it barely manages 1°C. Air conditioning removes heat and moisture and sends both outside, so it works whatever the weather is doing.

So the honest verdict on evaporative cooling vs air conditioning is this: evaporative cooling cools air, not rooms. It's worth having in a garage, workshop, or conservatory with the doors open, or blowing straight at you on a dry heatwave afternoon. It's the wrong buy for a closed bedroom on a sticky night.

Shopping for one of those £50 to £250 tower "air coolers"? Our air cooler vs air conditioner guide has the product picks. This page is about how the technology works and why British weather limits it.

Evaporative cooling vs air conditioning at a glance

Prices correct as of October 2026.

What is evaporative cooling and how does it work?

Evaporative cooling is the same trick your body uses when you sweat. Water needs energy to turn into vapour, and it takes that energy from the air around it as heat. The air ends up cooler, but also wetter.

A direct evaporative cooler (the portable tower kind, sometimes called a swamp cooler in the US) uses a fan to pull air through wet pads. Water evaporates off the pads and the air leaves a few degrees cooler and a lot more humid. Engineers measure how good a cooler is by its "saturation effectiveness": how much of the possible temperature drop it actually achieves. According to ASHRAE, the US engineering body whose handbook is the industry reference, a clean fibre-pad cooler manages about 80%, a poor pad 50% or less. Domestic towers have small pads and short air paths, and one UK lab test of a popular tower put it at around 45 to 70%.

Indirect evaporative cooling uses a heat exchanger so the wet, cooled air chills the supply air without mixing with it, which means no added moisture. It's less effective (about 40 to 80%), and two-stage systems combine indirect then direct cooling to add less humidity overall. In the UK these are commercial and industrial products, and they're rarely sold for homes.

ASHRAE is blunt that direct evaporative cooling is unsuitable for indoor comfort once the outdoor wet-bulb temperature passes about 21°C, and that direct coolers "should not recirculate indoor air". A portable cooler in a closed room does exactly that.

How air conditioning is different

Air conditioning doesn't turn heat into humidity. A refrigerant circuit soaks up heat from the room at the indoor coil and dumps it outside, either through the outdoor unit of a split system or through the exhaust hose of a portable. Because the indoor coil is cold, moisture condenses on it and drains away, so the room gets drier as it gets cooler.

That's why air con works in any humidity, day or night. If you want the full mechanics, our guide to how air conditioning works walks through the refrigerant cycle.

Does evaporative cooling work in the UK?

Sometimes. The answer depends less on how hot it is and more on how much room the air has left to absorb water, and that's what wet-bulb temperature measures.

The wet-bulb temperature is the lowest temperature you can reach by evaporating water into air. The gap between the normal air temperature and the wet bulb (the "wet-bulb depression") is the most cooling an evaporative cooler could ever give you. In dry air that gap is big. In humid air it shrinks to almost nothing.

Relative humidity on its own can mislead you here. As the Met Office explains, relative humidity changes through the day even when the actual moisture in the air hardly changes, because warm air can hold more water than cool air. On a hot afternoon relative humidity falls; overnight, as the air cools, it climbs back towards 100%. So an evaporative cooler has the most to work with at 3pm, and the least in the evening and at night, which is exactly when most of us want a cool bedroom.

The Met Office also notes that, as a group of islands, the UK is more humid than many inland areas. That said, WeatherSpark's analysis rates London summers as rarely "muggy". Our real issue is fairly high relative humidity at mild temperatures, which leaves a small wet-bulb gap.

The numbers bear this out. We looked at ERA5 reanalysis data via Open-Meteo for every July and August day from 2015 to 2025. At Heathrow, the average 3pm is about 22°C at 56% relative humidity, and the average 3am is about 15°C at 87%. On hot days, when it reaches 28°C or more, the afternoon averages about 30°C at 39%, which gives a wet-bulb gap of around 10°C. That's genuinely useful. The night after, though, averages 18°C at 82%, and the gap shrinks to about 2°C. We checked these against the Met Office's own Heathrow weather station for summer 2025, and the afternoon figures match within a point or two; the station reads a few points drier at night, which doesn't change the picture.

Manchester is damper on a normal day, with afternoons averaging 65% humidity. Its hot days are rarer, but when they come the afternoon drops to about 41%, much like London's. And our record heat can be very dry: on 19 July 2022, when Coningsby hit 40.3°C, relative humidity across central and eastern England dropped below 20%.

So here's the honest picture. On a genuinely hot UK afternoon, an evaporative cooler can knock a few degrees off the air coming out of it (the outlet air, not the whole room). At night, and on ordinary humid days, there's very little to work with.

What that means in real UK conditions

We worked these through using the standard ASHRAE psychrometric equations, so they're our calculations, rounded, not measurements. The two Heathrow rows use the ERA5 averages above. The two outlet columns show a budget cooler (50% effective) and a good one (70%).

The real-world tests line up. Trusted Reviews put the Igenix IG9703 through its paces: 32°C air at 50% humidity came out at 26.2°C, almost exactly our "good cooler" figure. In drier 35.5°C air it knocked off about 8°C. In 28.7°C air at 51% humidity, it managed only about 3.4°C.

Notice the humidity column too. In typical UK conditions the air coming out sits somewhere around 75 to 95% relative humidity. Which leads to the bigger problem.

Why an evaporative cooler struggles in a closed room

An evaporative cooler doesn't remove any heat from your room. It converts some of that heat into water vapour, and the vapour stays put.

Here's a worked example. The Honeywell TC09PM moves about 300 m³ of air an hour. On a 30°C, 40% humidity afternoon, running at 70% effectiveness, it cools that air by roughly 7°C. That works out at about 0.67 kW of cooling, and to do it, the cooler evaporates about a litre of water an hour. On a muggy 25°C, 70% day it manages only about 0.27 kW.

Now put it in a 4 m by 4 m bedroom with a 2.4 m ceiling and shut the door and window. That room holds about 45 kg of air. On paper, a litre of water an hour would push it from 40% humidity towards saturation in well under an hour. In reality your furniture soaks some of it up and some air leaks out, so it takes a bit longer, but within an hour or two the room is noticeably more humid. As the humidity climbs, the wet-bulb gap shrinks, the cooler's effect fades, and the room just feels clammy.

That's why Honeywell and the retailers selling these units say they need "a flow of air like an open window and/or door", and why Australian whole-house systems are designed to swap all the air in the house roughly every two minutes, with the windows open.

Compare that with a 9,000 BTU portable air conditioner, rated at about 2.6 kW of cooling, or a 2.5 kW split, both of which send the heat and moisture outside.

Does evaporative cooling make a room more humid?

Yes. Indoor humidity is usually best kept around 40 to 60%, and air coming out of a cooler at 80 to 90% sits well above that. Prolonged high humidity is good news for dust mites and bad news for condensation on cold surfaces. We cover the health side, including dust mites and humidity, in our guide to whether air conditioning is bad for you.

If humidity is your main gripe rather than heat, our guide to dehumidifiers vs air conditioners explains why a dehumidifier is effectively the opposite of an evaporative cooler.

Running costs compared

Evaporative coolers are cheap to run, there's no getting around it. Here's what each one costs at the Ofgem price cap for 1 October to 31 December 2026: 26.32p per kWh (GB average, direct debit). There's no VAT on domestic electricity from 1 October 2026 to 31 March 2027, so this rate isn't directly comparable with the previous quarter's 26.11p, which included 5% VAT.

The sum is simple: power in kW × hours × 26.32p.

The air conditioner figures assume full rated power, which overstates real use because the compressor slows down or cycles off once the room is cool. Assuming it runs at 60% of full power for eight hours, a 1.0 kW portable costs about £1.26 a day (1.0 × 0.6 × 8 × 26.32p), and a 2.5 kW split averaging 0.47 kW costs about 99p a day.

Prices and rates correct as of October 2026. The energy price cap changes quarterly, so check current rates before budgeting.

Pence per hour flatters evaporative coolers, though. On a dry afternoon a good portable cooler delivers roughly 0.3 to 0.7 kW of cooling to the air passing through it, and none of that is heat leaving the room. A portable air conditioner removes about 2.6 kW of actual heat. For more on air con bills, see are air conditioners expensive to run?

Water use and keeping a cooler clean

A domestic cooler evaporates roughly 0.4 to 1 litre an hour depending on the weather, so a 9 litre tank lasts around a working day when the air is dry.

Hygiene deserves sensible care rather than panic. The Health and Safety Executive lists evaporative cooling systems among those most likely to cause legionella, but its rules are aimed at employers and landlords, and the systems it means are cooling towers and evaporative condensers, not plug-in home coolers. We couldn't find any UK guidance specific to domestic portable coolers. Western Australia's health department, where home evaporative cooling is common, says portable units should be completely drained and cleaned at least once during hot weather and kept dry when not in use.

So empty the tank, clean and dry the pads, and don't leave water standing for days in warm weather. Landlords supplying one in a let property would be sensible to include it in their legionella risk assessment (our reading of HSE's general approach, not specific HSE guidance).

When evaporative cooling is the right choice

Evaporative cooling earns its place where fresh air is flowing through, and on the dry afternoons of a proper heatwave:

• a garage or workshop with the door up

• a conservatory with the doors and vents open (our conservatory air conditioning guide covers the long-term options)

• a patio or outdoor seating area

• as a spot cooler blowing straight at you while you work

It's also why evaporative cooling is sold in the UK for warehouses, factories, and marquees: they swap their air so fast that humidity never builds up, which is how industrial suppliers can truthfully say it works here.

When air conditioning is the right choice

Choose air conditioning for any closed room you want genuinely cooler: bedrooms at night, home offices with the door shut, top-floor rooms that bake all evening. It's also the safer bet for anyone vulnerable to heat, because it keeps working on the humid nights when evaporative cooling gives up. Our bedroom air conditioning guide goes into the quiet-at-night side of things.

If you can't run air con at all, start with the free and low-cost tricks in how to keep your house cool.

Whole-house evaporative cooling: why Australia uses it and the UK doesn't

In much of Australia, a roof-mounted unit pulling outside air through wet pads and ducting it to ceiling vents is a normal way to cool a house. Choice, the Australian consumer group, puts a typical unit at about AUD 2,500 (roughly double once installed) drawing around 200 to 400 W. It needs the windows open, several hundred litres of water a day, and a dry climate; Choice says it isn't suited to humid areas.

Britain's mild, fairly damp summers don't fit that model, and we couldn't find a UK supplier of domestic whole-house systems.