Keeping a Small Flat Cool in a Heatwave Without Air Con

Our top floor flat hit 31 degrees in the bedroom one July night, and neither of us slept past 4am. A small flat traps heat brilliantly in winter, which is lovely in January, and the same thing turns it into an oven the moment the weather turns warm.

Keeping a small flat cool without air conditioning took us two summers of trial and error to get right. We now keep the bedroom around 24 to 25 degrees on most hot nights, spending less than 60 pounds on kit. This is what actually worked, in the order we would do it again.

Keeping a small flat cool starts with why it overheats

A small flat has a lot of heat sources packed into very little air. Two people, a fridge, a laptop running all day and a hob in the same room as the sofa all add up. In a 45 square metre flat there is simply less volume to absorb that heat, so the temperature climbs faster than in a house.

Then there is the building. Top floor flats take heat through the roof all afternoon. Flats facing south or west soak up sun through the windows until evening. Ours has both problems, plus a neighbour below whose warmth rises through the floor. Knowing where your heat comes from tells you which fixes matter most.

We bought a cheap digital thermometer for about 8 pounds and put it in the bedroom. Writing down the reading at 7am, 3pm and 10pm for a week showed us that the flat was coolest around 5am and hottest around 6pm. Everything else came from that pattern.

Keeping the sun out is the biggest single win

Sunlight coming through glass turns into heat inside the room, and once it is in, it is hard to get out. Stopping it before it reaches the glass, or right at the glass, matters more than any fan.

Close blinds and curtains on sunny windows by mid morning. Do not wait until the room feels warm. By then the walls and furniture have already absorbed the heat.
Use light coloured or reflective backing. Our dark blue curtains were making things worse. We clipped a white blackout lining behind them for 18 pounds and the bedroom ran about two degrees cooler in the afternoon.
Consider reflective window film on the worst window. A renter friendly static cling film cost us 15 pounds for our west facing living room window and peels off cleanly when we leave.

If you rent, everything above is removable, which matters when the deposit is on the line.

Moving air the right way

A fan does not cool air. It moves it, which cools your skin and, used cleverly, pushes hot air out and pulls cool air in. Where you put it changes everything.

Cross ventilation at night

Once the outside temperature drops below the inside temperature, usually after 9 or 10pm, open windows on opposite sides of the flat if you can. Air will flow through on its own. If your windows are all on one side, open one high and one low, since warm air leaves at the top.

Fan facing out, not in

Put a box fan in an open window facing outwards in the evening. It pushes the hottest air out of the room, and cooler air gets drawn in through another window or the door. We found this dropped the bedroom by about three degrees in an hour, compared with one degree with the fan blowing into the room.

Close up in the morning

As soon as it is warmer outside than in, usually around 9 or 10am, shut the windows and close the blinds. This step felt wrong to us at first. Surely fresh air is good? But opening windows in the afternoon just lets hot air in.

Cutting the heat we make ourselves

In a small flat the kitchen is a big contributor. On hot days we change how we cook:

We use the oven only early in the morning or not at all. A full oven at 200 degrees can lift a small open plan room by several degrees.
We lean on the microwave, cold suppers and the slow cooker running on the balcony.
Extractor fan on, window open, lid on every pan, as we described in our post on getting rid of cooking smells in a small flat. The same airflow carries heat out too.

Beyond cooking, we unplug chargers and switch the television and second monitor fully off. Each one only adds a little warmth, but in a room this size, a little adds up. Swapping our last two old halogen bulbs for LEDs helped too.

The wet towel trick we stopped doing

Almost every heatwave tip list suggests hanging a damp sheet in front of a fan, or putting a bowl of ice in front of it, for homemade air conditioning. We tried both. Neither did much, and in our flat the damp sheet made things worse.

Evaporative cooling works well in dry climates. In a humid British summer, the air is already holding a lot of moisture, so the sheet barely cools anything and the extra humidity makes the room feel stickier. A bowl of ice melts in about 40 minutes and cools perhaps a square metre in front of the fan. In a flat that already struggles with damp, adding water to the air is the last thing we wanted. Ventilation and shading beat both easily.

What did work was cooling ourselves rather than the room: a lukewarm shower before bed, not cold, since a cold one makes your body work to warm up again, and a hot water bottle filled with cold water from the fridge.

Making the bedroom bearable

Sleep is where heat hurts most. Our bedroom routine on hot nights:

Blinds shut all day, windows opened at about 9.30pm
Fan in the window facing out until bedtime, then turned to blow across the bed on its lowest setting
Duvet swapped for a single cotton sheet each, which also helped with our different sleep schedules since the early riser no longer drags the covers
Linen pillowcases, about 20 pounds for a pair, which really do feel cooler than cotton

In winter we work hard to hold heat in, as we covered in keeping a small flat warm through winter. Summer is the same thinking turned around: control when air comes in and goes out.

Frequently asked questions

How can I cool down a small flat without air conditioning?

Block sunlight with light coloured curtains or blinds early in the day, keep windows shut while it is hotter outside, and ventilate hard at night. A fan facing out of an open window pushes hot air out faster than one blowing in.

Should I keep windows open or closed in a heatwave?

Closed during the day when outside air is hotter, and open at night once the outside temperature drops below the indoor one. A cheap thermometer makes it easy to know when to switch.

Is a portable air conditioner worth it for a small flat?

It can be for a top floor flat that regularly passes 28 degrees, but a single hose unit costs 250 pounds or more and uses a lot of electricity. Try shading and night ventilation first, since they often get you most of the way for a fraction of the cost.