How to keep a flat cool in a UK heatwave (before you buy AC)

UK flats are built to trap heat, and in a heatwave they do it magnificently. But before any machine gets involved, the temperature a flat reaches is mostly decided by three controllable things: how much sun gets through the glass, when you let outside air in, and how much heat you generate indoors. Run those three well and many flats stay livable through a hot week; run them badly and no fan will save you. Here is the routine, in priority order.

Sunlight through glass is usually the biggest single heat source in a flat — a sun-struck window works like a radiator running all afternoon. The physics favours stopping the sun before it enters: external shading (shutters, awnings) is far more effective than internal, because a curtain absorbs the heat inside the room. External shading is rare on UK buildings, so the practical version is:

• Close curtains and blinds on sun-facing windows before the sun reaches them — east glass by breakfast, south by late morning, west by early afternoon. Once the room is hot you are managing damage, not preventing it.
• Light-coloured or thermally lined curtains and blinds reflect meaningfully more heat back out than dark ones; blackout linings help both heat and sleep.
• Reflective window film intercepts solar gain before it enters and helps most on relentlessly sunny elevations — but check suitability for your glazing first, as some films can cause certain double-glazed units to overheat, and film also cuts winter solar warmth.
• If any window has outside shading you can improvise — a balcony parasol, an externally clipped shade — outside beats inside every time.

The most common heatwave mistake is having windows open at the wrong time. Air exchange helps only when the outside air is cooler than the inside air. On a 33 °C afternoon, an open window imports heat; at 11pm, it is your best cooling appliance.

• Close windows (and keep shading down) from mid-morning while outside is hotter than inside.
• After the outside temperature drops below the indoor temperature — evening or later in a serious heatwave — open everything that is safe to open and purge the day's heat all night.
• Create a cross-breeze: air needs an entry and an exit on different sides of the flat. A fan blowing hot air out of one window while cooler air enters another moves far more heat than a fan pointing at you.
• Close everything again in the morning before the day warms up, trapping the overnight coolness. Repeat daily.
• Ground-floor and easily reached windows need judgment overnight — window restrictors and lockable stays let you purge without leaving the flat open, and insect mesh makes all-night opening much more pleasant.

Everything that uses electricity in the flat ends up as heat in the flat. During a heatwave the aim is to be a smaller radiator:

• The oven is the worst offender — a single roast dinner dumps a huge slug of heat that the flat then holds all evening. Cold meals, the microwave, an air fryer, or outdoor cooking all beat it.
• Tumble dryers and dishwashers heat and humidify; run them after dark during the purge, or not at all. Line-dry, but preferably not indoors — drying laundry humidifies the room, and humid heat feels worse.
• Gaming PCs, consoles and large TVs are genuine space heaters — hundreds of watts for hours. Shorter sessions, or moving them out of the room you sleep in, is a real difference.
• Anything incandescent or halogen still lurking in light fittings is mostly a heater; LEDs run near-cold.
• Standby loads are minor for heat but free to eliminate while you are at it.

Top-floor flats fight two extra battles: heat from every flat below rises into them, and the roof above them soaks up sun all day and re-radiates it well into the night. That is why a top-floor flat often peaks in the evening and refuses to cool down when the outside air already has.

• If you have loft access, check the insulation. Loft insulation is a summer defence too — it slows the roof's heat driving down into the ceiling.
• Keep any loft hatch closed and draught-sealed in a heatwave... unless the loft has real outside ventilation and is cooler than the flat at night, in which case brief evening venting through it can help. Feel the air and judge.
• Sleep low if the flat allows it — the temperature difference between an upstairs sleeping mezzanine and the floor below can be the difference between sleep and none.
• Accept that the purge starts later for you: your walls and ceiling keep radiating stored heat after sunset, so the crossover point where outside air is cooler arrives later in the evening than the weather app suggests.

• A fan works by evaporating your sweat faster — it cools people, not rooms. Aim it at people, and give it cooler night air to work with.
• Cool water on wrists, forearms and the back of the neck; a lukewarm (not ice-cold) shower before bed lowers you without triggering a warming rebound.
• Cotton or linen bedding, and a thin sheet instead of a duvet; some people freeze a hot-water bottle for the foot of the bed.
• Drink more water than feels necessary, and check on neighbours who are elderly or unwell — heat is a genuine health risk, not just discomfort, and NHS heat-health advice is worth reading at the start of every hot spell.

The routine above has a ceiling. In a sustained heatwave — especially in a top-floor or heavily glazed flat — there comes a point where the flat no longer cools overnight, the bedroom holds 27–28 °C at midnight, and every day starts hotter than the last. If that is your flat, or someone in it is vulnerable to heat, mechanical cooling is the next step, and the honest options are narrower than the marketing suggests: a refrigerant portable air conditioner is the only plug-in device that actually lowers room temperature, as the comparison guide explains.

Two pieces of forward planning make that step much cheaper and calmer: know your size band before you need it, and know that units vanish from stores in the first days of a heat forecast. Checking confirmed store-level availability near your postcode — rather than ringing round shops — is the fast way to find a unit when the rush is on, and the buying guide covers the full strategy, including where restock alerts fit when everywhere near you is sold out.