Why is my house so hot and humid in summer?
Most UK homes get hot in summer because sun pours in through glass and roofs, and brick and concrete store that heat and give it back at night. They feel humid because warm air holds more moisture, and cooking, showers and drying washing add more. Top floors, single-aspect flats and some new, airtight homes get the worst of it.
- of homes in England overheat (CCC)
- About 20%
- overheating rules for new homes began
- June 2022
- UKHSA: fans help below this
- 35°C
- on all four brands we fit
- Dry mode
Sun through glass and roofs
The biggest source of summer heat is the sun itself. Sunshine through a window heats the floor, walls and furniture it lands on, and that heat stays in the room. Large east, west or south-facing windows with no shading are on UKHSA's list of features that make homes overheat.
Roofs take the sun all day. A loft room or top-floor flat sits right under hot tiles or a flat roof, often with roof windows that let sun straight in. East-facing rooms heat up in the morning; west-facing rooms catch the low, strong evening sun just as you want to cool down.
Why is my flat so hot at night?
Brick, stone and concrete soak up heat during the day and release it slowly after dark. A report for the Mayor of London describes how concrete and metal absorb heat by day and give it off at night. In a city the effect adds up: the London Plan says central London can be up to eight degrees warmer than the Green Belt on hot summer nights, so the air outside your window may not cool down much either.
Flats with windows on one side only can't get a breeze through. UKHSA lists these single-aspect homes as more likely to overheat. Many people also keep windows shut at night because of traffic noise or security. Even the building rules for new homes accept that windows are likely to stay closed at night where noise is high.
Why is my house so hot upstairs?
Warm air rises, so heat from the whole house collects on the top floor, and that floor is also the one under the hot roof. UKHSA's keep cool at home checklist names top-floor flats as more likely to overheat because heat rises. A 2024 report for the Mayor of London scored upper-floor flats as the most at-risk homes for the same reasons: more sun and rising heat.
Loft conversions get it worst. Insulation slows heat getting in, but it also slows heat getting out, so a loft that warms up during the day often stays warm well into the night.
New homes and flats that overheat
In 2019 the Climate Change Committee found that around 20% of homes in England, about 4.5 million, overheat even in cool summers. It named mid and top-floor purpose-built flats from the 1960s and 1970s and from after 1990, lacking ventilation and sun protection, among the types most prone to it.
Well-insulated, airtight homes keep heat in during winter, and in summer UKHSA notes that some of them can trap heat inside. Since 15 June 2022, new homes in England have had to meet Approved Document O of the Building Regulations. It requires new homes to limit unwanted solar gain and to have an adequate way of removing heat, with stricter limits on glazing for parts of London. Homes built before then didn't have to meet these rules.
Why does my house feel humid and sticky?
Relative humidity is how much water vapour the air holds compared with the most it could hold at that temperature. The Met Office explains that warmer air can hold more water vapour. On a warm, muggy day the air coming in through open windows is already carrying a lot of moisture.
Humidity makes heat feel worse. The Met Office says high humidity in warm weather is uncomfortable because sweat doesn't evaporate easily, which gets in the way of the body cooling itself. That's the sticky feeling.
Your home adds its own moisture. Government guidance on damp names cooking, bathing, washing and drying laundry as everyday sources of indoor moisture. Drying washing on an airer in a small, closed flat in summer is one of the quickest ways to make it feel close and humid.
How to measure humidity at home
A cheap digital hygrometer, often combined with a thermometer, shows the temperature and relative humidity in a room.
- Put it at about head height in a living room or bedroom, away from direct sun
- Keep it away from the kitchen and bathroom, where steam gives misleading readings
- Check it at the same times each day, for example morning and evening
- Compare rooms: a room that reads much higher than the rest points to a local source, like drying washing
- Watch the trend over days rather than a single reading; cheap meters are a guide, not a lab instrument
- Look for the other signs too: condensation on windows, musty smells or mould spots
Which problem, which fix
| What you notice | Likely cause | First thing to try |
|---|---|---|
| Hot upstairs, cooler downstairs | Heat rising and a hot roof | Shade roof windows by day; open top-floor windows when the air outside is cooler |
| Flat still hot at midnight | Walls and floors giving back stored heat; no through breeze | Keep sun out by day so less heat is stored; ventilate at night if safe |
| One room hot in the afternoon | West or south-facing glass in full sun | Close blinds or curtains before the sun reaches the window |
| Sticky but not that hot | High humidity from outside air or indoor moisture | Extractor fans on, washing dried outside, a dehumidifier or dry mode |
| Condensation or a musty smell | Moisture building up with too little ventilation | Ventilate after cooking and showers; deal with damp early |
Fixes, in the order to try them
Shade the windows
UKHSA advises closing curtains and blinds on windows in direct sun during the day. It also suggests fitting blinds or curtains, or external shutters, roller blinds or awnings, before the hot weather arrives.
Ventilate at the right times
Open windows when the air outside feels cooler than inside, usually late evening, at night and early morning, if it's safe to. Try to get air flowing through the home. Keep them shut on hot afternoons.
Get rid of steam at the source
Use the kitchen and bathroom extractor fans, cover pans and keep doors shut while cooking and showering. Dry washing outside when you can. Turn off lights and appliances you're not using, and cook at cooler times of day.
Use a dehumidifier for damp air
If the air is sticky rather than hot, a portable dehumidifier draws moisture out of one room. Keep doors and windows in that room closed while it runs, and empty the tank regularly.
Use fans sensibly
UKHSA says electric fans help when the air temperature is below 35°C. Don't aim a fan straight at your body, as that can lead to dehydration. A fan moves air but doesn't take heat out of the room.
Then consider air conditioning
If a room still overheats after all that, a split air conditioning unit takes heat out of the room and moisture out of the air. One unit in the bedroom or the hottest room is often enough.
How air conditioning dry mode works
In cooling, warm room air passes over the cold indoor coil. Moisture condenses on the coil and drains away outside, so the air comes back cooler and drier. Dry mode puts the focus on removing moisture rather than lowering the temperature.
All four brands we fit have it. Daikin's guide for its Perfera units says drying mode decreases the humidity in a room, and you can't set the temperature or fan speed in it. Mitsubishi Electric's MSZ-LN manual says DRY mode dehumidifies the room and the room may be cooled slightly. Panasonic's Etherea manual says DRY runs at low fan speed for gentle cooling. LG's DUALCOOL Deluxe manual says you can't adjust the fan speed in dehumidification mode, and the compressor and fan may pause to prevent overcooling.
Dry mode suits muggy days that aren't especially hot. On hot days, normal cooling also takes out moisture as it brings the temperature down.
What it costs to cool a room
A single wall unit in the hottest room is usually the place to start. Every quote is fixed after a free survey.
- from£1,850inc. VAT
A bedroom or study
2.5 kW wall unit, 5 m pipe, fitted in a day
- from£2,050inc. VAT
A living room
3.5 kW wall unit
- from£2,450inc. VAT
Open plan or a loft
5 kW wall unit
Hot and humid home questions
Why is my house so humid in the summer?
Warm summer air holds more water vapour, so air coming in through open windows is often already humid. Cooking, showers and drying washing indoors add more. Use extractor fans, dry washing outside, and if it stays sticky, try a dehumidifier or the dry mode on an air conditioner.
Why does my house feel humid and sticky even when it isn't that hot?
Humidity makes warm air feel worse, because sweat doesn't evaporate easily. A room can feel close at a moderate temperature if moisture builds up and there's little air movement. A hygrometer will show you whether humidity is the problem.
Why is my flat so hot at night?
Walls and floors store heat from the day and release it after dark, and in a city the outside air cools down less at night. Flats with windows on one side can't get a breeze through, and many people keep windows shut for noise or security. Keeping the sun out by day helps.
Why is my house so hot upstairs?
Heat rises, so warm air collects on the top floor, which is also the floor under the hot roof. Lofts and top-floor flats are the hottest rooms in most homes. Shade roof windows by day and ventilate when the air outside is cooler.
Why are new-build flats so hot?
Many are highly insulated and airtight, which keeps heat in, and some have large windows and only one outside wall. The Climate Change Committee found newer and 1960s to 70s purpose-built flats among the homes most prone to overheating. New homes have had to meet overheating rules since June 2022.
Does air conditioning reduce humidity?
Yes. In cooling, moisture condenses on the cold indoor coil and drains outside. Most split units also have a dry or dehumidification mode, which removes moisture with less cooling. Daikin, Mitsubishi Electric, Panasonic and LG all include one.
Is a dehumidifier or air conditioning better for a humid house?
If the problem is damp air in cooler weather, a dehumidifier is the simpler choice. If rooms are both hot and humid in summer, air conditioning deals with both at once and can heat the room in winter too.