How to Keep Your House Cool in a Heatwave (Without Air Conditioning)

Most UK homes were designed for one job: holding heat in. Cavity walls, thick loft insulation and sealed double glazing are brilliant in January and a liability at 34°C in August. Once a heatwave settles in, a typical semi behaves like a storage heater — it soaks up warmth all day and releases it into your bedroom at 11pm.

The fix is a timing game, not a spending game. Nearly everything on this checklist is free or under £20, and the upgrades that cost real money are priced below with labour and materials split, so you can judge a quote. Work through it in order — the sequence matters more than any single task.

Solar gain through glass is the biggest heat source in most homes — a south-facing bay window can push a room up 2–3°C by midday. The window for stopping it closes early: once the sun is on the glass, the heat is already indoors.

• •Close curtains and blinds on east- and south-facing windows by 9am (5 minutes, free). Pale or lined fabric works best; dark unlined curtains re-radiate heat into the room.
• •Shut every window once the outside temperature passes the inside temperature — typically 10–11am in a heatwave. Opening a window at 2pm 'for a breeze' pumps 32°C air into a 25°C room.
• •Move west-facing shading into place by 1pm; those rooms take their hit from mid-afternoon onwards.
• •If your curtains are thin, clip a £3 foil emergency blanket into the window recess, shiny side out — ugly, but effective.

Everything with a plug is a small radiator; a busy kitchen's appliances can add 1–2°C on their own.

Humidity is half the misery: 26°C at 70% humidity feels worse than 29°C in dry air. If your home runs damp, a dehumidifier set to 50–55% makes the same temperature noticeably more bearable — £120–£220 for a decent compressor model, and 5p–8p an hour to run. It's also a sensible week to clean your curtains and blinds; dusty fabric holds heat and odour in warm weather.

• •Skip the oven. Thirty minutes at 200°C dumps roughly 1.5kWh of heat into the kitchen — cold meals, the microwave or the barbecue outside instead.
• •Run the dishwasher and washing machine after 9pm on delay start and line-dry everything; a tumble dryer cycle releases 2–4kWh of warm, humid air.
• •Swap any remaining halogen bulbs for LEDs (£2–£4 each). A 50W halogen runs at over 200°C; the LED equivalent barely warms.
• •Shut down gaming PCs and consoles when idle; under load a rig is a 300–500W fan heater.

The purge is where you claw the temperature back. The moment outside air drops below inside — usually 8:30–9:30pm in a UK heatwave; check a thermometer rather than guessing — the house becomes coolable until roughly 7am.

If you have a ceiling fan, make sure it is spinning the right way for summer — anticlockwise, pushing air down. It won't lower the actual temperature, but the perceived 3–4°C difference costs about 1p an hour.

• •Open windows on opposite sides of the house to set up cross-flow; a single open window barely moves air.
• •On sash windows, open the top sash 15cm and the bottom sash 15cm — hot air exits at the top while cooler air feeds in below.
• •Point a pedestal fan out of an upstairs window to exhaust hot air. It shifts more heat than pointing it at yourself.
• •Open internal doors and the loft hatch so rising warm air has somewhere to go.
• •Only leave accessible windows open where sash stops or restrictors (£5–£10) make them secure; trickle vents can stay open everywhere.

External shading stops heat before it reaches the glass and blocks 80–90% of solar gain; internal curtains and blinds manage 30–45% because the heat is already through the window. That gap is why southern Europe fits shutters, and it's the upgrade most UK homes never consider.

• •Reflective window film: £6–£12 per square metre, DIY, about 30 minutes per window. The cheapest real fix for one hot south-facing room.
• •Blackout or thermal-lined curtains: £25–£70 a pair from the usual high-street retailers; hanging them is a ten-minute DIY job.
• •Manual awning over a patio door: £150–£450 in materials plus £100–£250 fitting. Materials should be roughly two-thirds of a fair quote — if labour exceeds half the total for a two-hour bracket-and-bolt job, ask why.
• •External roller shutters: £350–£900 per window supplied and fitted, of which labour is typically £120–£180. Any installer who refuses to split materials from labour on the quote is the one to walk away from.

Five items cover almost every task above; buy before the forecast turns, because fans sell out by day two of any hot spell.

• •Digital indoor/outdoor thermometer (£8–£12) — the entire strategy depends on knowing which side of the glass is hotter.
• •Two pedestal or box fans (£15–£30 each) — one to exhaust air out, one to drive cross-flow.
• •Reflective film or a foil emergency blanket (£3–£12) for the worst-offending window.
• •Window restrictors or sash stops (£5–£10) so you can ventilate securely overnight.
• •A hot water bottle — filled with cold water and fridge-chilled for the pillow end.

Loft insulation works both ways. The same 270mm of mineral wool that holds warmth in through January keeps the 45–50°C roof space out of your bedrooms in August. Topping up from 100mm costs £150–£250 in materials for a typical three-bed semi and £400–£600 professionally installed, so the labour share is £250–£350 — fair for a day in a hot loft, though it's a job you can do yourself if you can stand the itching. The same applies to draught-proofing: gaps that leak heat out in winter leak hot air in during a heatwave, and £40 of brush strip and sealant closes most of them.

Be honest about portable air conditioning. A decent unit costs £250–£400 and draws about 1kW — 25p–30p an hour — but a cheap one venting its hose through a half-open window feeds hot air straight back in. If a supplier quotes more than £150 to fit a through-wall vent kit, get a second price; it is under two hours' work. The whole-house items on a home energy efficiency checklist do double duty in both seasons.

If the heatwave has already arrived and you have ten minutes, this is the order of priority.

• •Tonight: purge from 9pm with cross-flow and a fan exhausting out of an upstairs window, then close everything at 7am.
• •Tomorrow, 9am: curtains and blinds shut on the sunny side, every window closed before the outside air overtakes the inside.
• •This weekend: film the worst window and fit the thermometer, so from Monday you're working from data rather than guesswork.
• •One boundary: if bedrooms stay above 26°C overnight for days and someone elderly or unwell lives with you, stop optimising and spend — one properly vented portable AC in that bedroom is a health measure, not a luxury.