How to Keep Your House Cool in a Heatwave (Without Air Conditioning)
UK homes overheat in a heatwave because of how they are built, not because of anything you are doing wrong. This guide covers the steps that reduce indoor temperature, not just the ones that make the heat more bearable. A tool below tells you when to open your windows based on today’s numbers.
British houses are built to retain heat, not release it. Thick walls, small windows and insulation that work well in January work against the occupant in July. Walls, floors and furniture absorb solar energy through the day and release it slowly, so a house can be warmer indoors at 9pm than it was outside at 3pm.
Only around one in twenty UK homes has any form of air conditioning, compared with roughly nine in ten in the US. Most households are working with fans, blinds and timing rather than a machine that lowers the temperature directly. The timing matters more than most advice suggests, and it is the part most commonly done backwards.
Should you open your windows right now
Open or Close: The Window Checker
Enter today’s outdoor and indoor temperatures and the time of day for a direct answer.
What reduces indoor temperature in a heatwave
1. Block the sun before it reaches the glass
Solar gain through unshaded windows is the fastest source of heat gain in a room, ahead of body heat, cooking or electronics. A light-coloured or reflective blind fitted close to the glass blocks more heat than curtains hung loosely in front of it. Dark, loose fabric absorbs heat and re-radiates it into the room over several hours. External shutters or awnings are more effective again, since they stop heat before it reaches the window at all.
2. Seal the house during the day, ventilate it at night
Once outdoor air is warmer than indoor air, an open window is a heat inlet, not a source of relief. Close windows, blinds and interior doors to unused rooms during the hottest part of the day, typically late morning through early evening. When outdoor temperature drops below indoor temperature, usually somewhere between 9pm and 7am in a heatwave, open windows on opposite sides of the house to create a through-draught that clears the heat stored in walls and furniture during the day.
3. Use fans to move air, not to lower temperature
A fan does not reduce room temperature. It increases evaporation from the skin, which is what produces the cooling sensation. That still has a real effect on comfort, and running one costs a few pence an hour. Positioning a fan to blow out of an open window in the evening helps push warm indoor air out. A ceiling fan set to spin anticlockwise in summer pushes air downward, which increases the cooling effect in the room below.
4. Reduce heat generated inside the house
Ovens, tumble dryers, dishwashers, desktop computers and games consoles all add heat to a room. Cooking, drying and laundry are best scheduled after dark during the worst days, or replaced with a microwave, an outdoor barbecue or cold food. A kitchen that has run an oven for an hour can stay measurably warmer than the rest of the house for the remainder of the evening.
5. Use one cool room rather than trying to cool the whole house
Cooling a single room is more achievable than cooling the entire property. The room with the least direct sun exposure, often north-facing, on a lower floor, with fewer or smaller windows, is the best candidate. Keep its door shut, blinds down and electronics off, and use it during peak heat. Research from Loughborough University on UK overheating found that external shading alone can reduce indoor temperature by more than 6°C in a shaded room, which indicates that shading one room properly has more effect than spreading effort across the whole house.
6. Use the loft carefully
Heat rises and collects in the loft, usually the hottest space in the house. Opening the loft hatch overnight creates a stack effect that draws warm air upward and pulls cooler air into the rooms below. This should only be done once the roof itself has cooled. Opening the hatch while the roof is still hot pushes that heat down into the living space instead of venting it.
What does not work
- Opening windows through the day for fresh air. If outdoor temperature is higher than indoor temperature, this adds heat to the house.
- A bowl of ice in front of a fan. This cools a very small area for a short time and does not replace proper airflow or shading.
- Running a portable air conditioning unit with windows open. The unit works against the incoming hot air, runs continuously, and adds cost with limited benefit.
Health during a heatwave
Cooling the building is one part of managing a heatwave. The other is monitoring the health of anyone vulnerable in the household. Heat-health alerts are colour-coded yellow, amber or red and can indicate risk before conditions become extreme, particularly for older people, babies, young children, pregnant women, and anyone with a heart, lung or circulatory condition. Fluid intake should be maintained through the day rather than in response to thirst. Loose, natural fabrics and reduced activity during peak heat lower the risk further. Heat exhaustion symptoms include heavy sweating, dizziness, headache and nausea, and can progress to heatstroke if untreated.
Heatwave home cooling: quick answers
Why is my house hotter than it is outside?
Walls, floors and furniture absorb heat through the day. Over several consecutive hot days, this stored heat builds up faster than the house can release it overnight, so by day three or four indoor temperature can exceed outdoor temperature at the same time of day.
Does opening windows actually cool a house down?
Only when outdoor air is cooler than indoor air, typically overnight and in the early morning during a heatwave. During the hottest part of the day, opening windows generally increases indoor temperature.
Are fans worth using in a heatwave?
Yes, for comfort rather than temperature reduction. A fan increases evaporation from the skin, which can make a room feel several degrees cooler without changing the actual temperature reading. Running cost is low compared with air conditioning.
Is it worth buying a portable air conditioner for occasional UK heatwaves?
This depends on how often the home overheats and which room needs cooling. A portable unit is effective in a single closed room but performs poorly if doors and windows to the rest of the house remain open. Upfront and running costs should be weighed against the number of nights per year it would realistically be used.
What temperature counts as a heatwave in the UK?
The Met Office defines a heatwave as three consecutive days at or above a regional threshold, ranging from 25°C in parts of northern England and Scotland to 28°C in parts of London and the South East. Northern England can reach official heatwave conditions at a temperature that would not qualify further south.
Sources: Met Office regional heatwave thresholds. UK Health Security Agency hot weather guidance. House of Commons Library overheating research. Loughborough University home-shading study, July 2026.