British houses are designed to keep heat in — which is wonderful in January and miserable when it’s 35°C outside. When people ask how can I keep my house cool during a heatwave, they’re usually suffering through their third sleepless night, and the good news is that passive cooling techniques can drop indoor temperatures by 3–5°C without air conditioning.

I’ve survived the 2022 40°C scorcher and every heatwave since in a south-facing Victorian terrace — basically a brick oven. Here’s what actually works, the physics behind it, and the common mistakes that make things worse.

How can I keep my house cool during a heatwave? Night purging

The single most effective technique has an unglamorous name: night purging. British heatwaves almost always bring cool nights (18–20°C even when days hit 35°C) — open every window wide once the outside air drops below the indoor temperature, typically from about 10pm to 7am, and let the cool air flush the day’s heat from the walls and floors. Thermal mass (brick, concrete, tile) absorbs coolth overnight and releases it slowly the next day, damping the temperature swing. Then — critically — close everything up in the morning: windows, curtains, blinds, the lot.

The morning close-down is where most people fail. A house opened up all day in a heatwave just becomes an oven with windows; a house sealed against the hot day with cool night air banked in its walls stays 4–5°C cooler inside than out. Think of it as the opposite of winter: in winter you seal against the cold day and night; in a heatwave you ventilate at night and seal by day. Security caveat: only open windows you can secure — ground-floor windows need restrictors or locks, and never leave accessible windows open overnight in urban areas.

  • Open everything wide 10pm–7am when outside is cooler than inside
  • Close windows, curtains and blinds every morning
  • Bank coolth in brick/concrete thermal mass overnight
  • Never leave the house open all day in a heatwave
  • Secure ground-floor windows — use restrictors

Shading: stop the heat getting in

Once heat is inside, you’re fighting it; shading stops it at the glass. External shading is dramatically more effective than internal — an external blind or shutter blocks up to 90% of solar gain, while internal blinds manage 30–40% (the heat’s already inside, warming the room). The cheap wins: reflective window film (£15–£25 per window, DIY) on south and west-facing glass, light-coloured curtains closed during the day, and — the classic — hanging a light sheet or emergency foil blanket outside the worst windows. It looks odd; it works brilliantly.

Longer-term, south-facing rooms benefit enormously from external solutions: awnings, pergolas with deciduous planting (shade in summer, sun in winter — nature’s smart glass), and shutters. Inside, close curtains on sunny sides and open them on shaded sides; rooms are often heated by one or two windows, so targeted shading beats whole-house curtain-drawing. And turn off the heat sources you forget: ovens, tumble dryers, halogen lights, even incandescent bulbs — every watt of waste heat matters when you’re fighting for degrees.

Fans, water and sleeping

Fans don’t cool air — they cool people, by evaporating sweat — so point them at humans, not empty rooms. The best trick: a bowl of ice or frozen water bottles in front of a fan creates a genuinely cooler breeze (the poor man’s air con, and it works). A damp sheet hung in front of an open window at night cools incoming air evaporatively. Ceiling fans should run anticlockwise in summer (downdraft); portable fans are best placed to pull cool night air in, not push hot day air around.

For sleeping — the real heatwave battleground: cool the bedroom aggressively in the evening (windows open, fan on), then sleep with minimal covering (a damp cotton sheet is the classic), keep a spray bottle of water by the bed, and put pillowcases in the freezer for 30 minutes before bed (glorious). Lukewarm showers cool you better than cold ones (cold triggers your body to conserve heat). And stay hydrated — heat exhaustion creeps up, especially in the elderly. If the house won’t drop below 28°C at night, consider it a health issue, not a comfort one, and cool the person actively.

When passive cooling isn’t enough: the AC question

British heatwaves are getting hotter and longer, and there’s a point where passive measures hit their limit — particularly for top-floor flats, south-facing new builds with huge glazing, and anyone with health conditions affected by heat. The honest assessment: a portable air conditioner (£300–600) cools one room effectively and is the right answer for the 2–3 weeks a year Britain is genuinely hot. The downsides are real — they’re noisy, the exhaust hose needs a window kit (and leaks heat back in), and they use 1–1.5kW, roughly £30–50p an hour.

The better long-term answer for whole-house cooling is a split-system air-to-air heat pump (£1,500–2,500 per room fitted) — which is also a highly efficient heater in winter, making it a year-round investment rather than a heatwave gadget. Running costs are a third of portable units for the same cooling. The planning reality: most domestic installations are permitted development, but check in conservation areas and flats (leasehold permission needed). And the environmental note: cooling a well-shaded, night-purged house needs a fraction of the energy of cooling a heat-trap — do the passive measures first, then size the active cooling for what’s left. That’s the efficient order.

  • Portable AC (£300–600): right for 2–3 hot weeks, one room
  • Split-system heat pump (£1.5k–2.5k/room): efficient, heats in winter too
  • Passive measures first — then size AC for the remainder
  • Check leasehold/conservation rules before installing splits
  • Portables: noisy, need window kits, ~£40p/hour to run

Beating the heat, passively

Night purge, morning seal, shade the sunny glass externally, fans on people not rooms, and cool the sleeper not just the bedroom. These techniques kept my brick-oven terrace at 26°C through a 38°C day — no air con, no real expense, just physics applied properly.

For year-round thermal sense, best way to insulate a draughty victorian terrace covers insulating old houses and reduce my heating bills this winter is the winter heating flip-side. For heatwave health guidance, the Energy Saving Trust’s advice has excellent official advice.

The cooling hierarchy: shade and night-purge first (free, effective to ~35°C), fans and evaporative tricks second, portable AC for the worst weeks, split systems for the long-term reality of hotter summers. Britain’s climate is changing; the houses need to change with it — starting with the free physics, ending with the honest technology.