How do I keep a house cool in summer?

How to keep heat out rather than trying to remove it afterwards — shading, ventilation timing, and what actually helps at night.

Difficulty: beginner
Time: 1 hr
Read: 3 min

Short answer

Keep the heat out during the day: close curtains and blinds on sunny sides, keep windows shut while it's hotter outside than in, and then ventilate hard once it's cooler outside in the evening. Blocking sunlight before it enters is far more effective than cooling a room afterwards.

Most heat entering a house in summer comes through windows as direct sunlight. Once it's inside, it's absorbed by floors, walls and furniture and released slowly all night. That's why shading during the day matters far more than any amount of fanning in the evening.

What you'll need

Blackout or thermal blinds (optional)
Fans (optional)
Thermometer (optional)

Step by step

Close curtains and blinds on any window getting direct sun.External shutters or awnings are more effective than internal blinds, because they stop the heat before it's through the glass.
Keep windows closed while it's hotter outside than inside.Open windows on a hot day let heat in. Check with a thermometer rather than guessing.
Ventilate hard once the outside temperature drops below the inside.Usually late evening. Open windows on opposite sides of the house for cross-ventilation.
Close everything again in the morning before it heats up.You're trapping the cool night air in. This daily cycle is the core technique.
Use fans to move air across you rather than to cool the room.A fan doesn't lower the temperature — it makes you feel cooler by aiding evaporation. Point it at people, not at the room.
Turn off unnecessary heat sources.Ovens, tumble dryers, incandescent bulbs, computers and games consoles all add measurable heat.
Create a through-draught at night with a fan in a window pointing out.Extracting hot air on one side while another window is open draws cool air in more effectively than a fan alone.
Consider longer-term measures: reflective window film, external shading, loft insulation.Loft insulation works both ways — it keeps summer heat out of the rooms below as well as winter heat in.

Tips

A damp towel or bowl of water in front of a fan does add some evaporative cooling, though it also raises humidity, which is counterproductive in already-humid weather.
Sleeping downstairs during a heatwave is an obvious but effective option — heat rises and upstairs rooms are consistently warmer.
Light-coloured external blinds reflect more heat than dark ones.
In a heatwave, check on elderly neighbours and anyone with health conditions. Heat is a genuine health risk, not just discomfort.

Common mistakes

Opening windows on a hot day — You let hot air in. Keep them shut until the outside temperature falls below the inside.
Leaving a fan running in an empty room — Fans cool people, not rooms. In an empty room the motor is adding a small amount of heat.
Leaving curtains open on sunny windows — Direct sunlight through glass is the largest single heat input.
Using the oven at the hottest part of the day — It adds a substantial amount of heat to the house at exactly the wrong time.

Questions people ask

Do fans actually cool a room?

No — they slightly warm it, since the motor uses energy. They cool people by increasing evaporation from skin, which is why pointing them at people rather than around the room is the right approach.

When should I open the windows?

Once the outside temperature is below the inside temperature, usually in the evening. A cheap indoor/outdoor thermometer makes this decision straightforward.