The short answer
A room overheats when heat gets in or builds up faster than it can escape. The usual causes are sunshine through large or west-facing windows, a roof directly above, heat from people and equipment, and too little ventilation.
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One room can be noticeably warmer than the rest of the house. Sometimes the cause is obvious, like a wall of glass facing the afternoon sun, and sometimes it's several smaller things adding up. Working out which applies tells you what will help, and how much cooling the room would need.
Sunshine through windows
On a sunny day, sunlight through glass is often the biggest single source of heat in a room. It passes through the glass and warms the floor, walls and furniture, and that heat can't get back out through the glass as easily as the light came in.
Which way the windows face decides when a room heats up:
| Windows face | Direct summer sun | What to expect |
|---|---|---|
| East | Morning | Warms early, often cooler by afternoon |
| South | Middle of the day | Strong sun, but high in the sky and easier to shade |
| West | Afternoon and evening | Low sun at the warmest time of day; often the hottest rooms |
| North | Little, apart from early and late in midsummer | Usually the coolest |
Size matters too. Patio doors, bifold doors and floor-to-ceiling glazing let in far more heat than a small window. Glass in a sloping roof, such as a rooflight or conservatory roof, faces the sky and catches the sun for much of the day. Solar control glass lets in less of the sun's heat than ordinary glass, but a large area of it can still warm a room.
A roof above the room
Rooms directly under the roof take heat from above as well as through their windows. Roof coverings get very hot in strong sunshine, and that heat passes down into the rooms below, especially where there's little insulation in between. Warm air also rises through the house, so upstairs rooms tend to be warmer than downstairs ones. Loft rooms and top floors covers these rooms in more detail.
Heat from people and equipment
Almost all the electricity used in a room ends up as heat in it. A room full of equipment or people gets warmer however well it's shaded.
- A person sitting still gives off roughly 100 watts of heat, so a room with several people warms up quickly.
- Computers, games consoles and televisions all add heat, and a powerful computer working hard can add several hundred watts.
- Halogen and old-style filament bulbs give off far more heat than LED lighting.
- Ovens, hobs and tumble dryers add heat, and often moisture too.
- Hot water cylinders and hot pipes can add heat even when the heating is off.
That's why home offices, kitchens and busy living rooms can overheat without much sun. Keeping a home office comfortable looks at one common example.
Why the heat stays
Some rooms don't gain heat any faster than others but struggle to lose it.
- Ventilation. A room with windows on only one side, windows that open only a little, or windows kept shut for security or noise gets little air moving through it.
- Heavy materials. Brick, stone and concrete absorb heat slowly and release it slowly. Over a run of hot days they warm through and keep a room warm at night.
- Lightweight construction. Timber-framed extensions, garden rooms and many loft conversions heat up quickly in sunshine, although they also cool quickly once air can move through them.
- Insulation and airtightness. Good insulation slows heat getting in through walls and roofs, but it also holds in heat that arrives through the windows or is made indoors. A well-insulated room with a lot of glass and little ventilation can overheat easily.
What helps
The fixes follow the causes:
- Shade windows before the sun reaches them, ideally from the outside.
- Ventilate when the air outside is cooler than inside, usually overnight and early in the morning, and keep windows closed while it's warmer outside.
- Switch off equipment and lighting you don't need.
- Check the loft insulation above top-floor rooms.
Keeping cool without air conditioning goes through each of these in turn.
Where a room still overheats, any cooling needs to match the heat the room actually gains, not just its floor area. A room with a large west-facing window or a desk full of equipment usually needs more capacity than a shaded room of the same size, which is why a proper assessment beats a rule of thumb. What size air conditioner you need explains how that's worked out.
Related guides
- Loft rooms and top floorsLoft rooms and top floors run hotter than the rest of the house because the roof heats up in the sun, warm air rises, and roof windows catch sunshine for much of the day. Shading, ventilation, insulation and well-planned cooling help.
- Keeping cool without air conditioningKeep the sun off your windows during the day, open up when the air outside is cooler than inside, usually from late evening to early morning, and cut the heat produced indoors. Fans make you feel cooler, though they don't cool the room.
- What size air conditioner do you need?Most bedrooms and smaller living rooms suit a unit of about 2.0 to 3.5 kW, and large open-plan spaces often need 5.0 kW or more. Sun, glazing, insulation and ceiling height can matter as much as floor area.
- Which rooms benefit mostBedrooms usually benefit most, because heat makes sleeping hard, followed by top-floor and loft rooms, home offices and sunny living spaces. The best candidates are rooms you use a lot that regularly overheat, rather than every room in the house.