The short answer
Humidity changes how warm a room feels at the same temperature. Most people are comfortable with relative humidity of roughly 40–60%; above that, warm rooms feel close and sticky, and air conditioning helps by taking moisture out as it cools.
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The same temperature can feel pleasant one day and oppressive the next. On a warm, still evening the air often feels heavy even when the thermometer reading is modest, and the difference is the moisture in it. Knowing how humidity works helps you keep your home comfortable and makes sense of what air conditioning does to the air.
What relative humidity means
Air always carries some water vapour. Relative humidity (RH) is how much it carries compared with the most it could hold at that temperature, shown as a percentage.
Warm air can hold more water vapour than cold air, which explains two things you notice at home:
- Warm the air without adding moisture and its relative humidity falls. That's why heated rooms in cold weather can feel dry.
- Cool the air without taking moisture out and its relative humidity rises. Cool it far enough, to its dew point, and the water condenses, which is why condensation forms on cold windows and on the outside of a glass of iced water.
Why humid heat feels worse
Your body loses heat partly as moisture evaporates from your skin. In dry air that happens easily. In humid air it happens more slowly, so you stay warmer and feel clammy, and a warm room at high humidity can feel noticeably less comfortable than the same room with drier air.
The effect is strongest in warm weather. When a room is cool, humidity makes much less difference to how warm you feel, and what you notice instead is condensation and washing that takes longer to dry.
Comfortable levels at home
There's no single right figure, and people differ. As a general guide:
| Relative humidity | How it tends to feel | What you might notice |
|---|---|---|
| Below about 30% | Dry | Static shocks; gaps opening in wooden floors |
| About 30–40% | Slightly dry | Usually little |
| About 40–60% | Comfortable for most people | Very little |
| Above about 60% | Close and sticky when warm | Condensation on cold surfaces; washing slow to dry |
A small digital hygrometer, often built into a room thermometer, shows where your rooms sit. Readings rise and fall through the day with cooking, showers and drying washing, so look at the pattern rather than a single number.
Where the moisture comes from
Moisture in the air at home comes from:
- cooking, especially boiling pans without lids
- showers and baths
- drying washing indoors
- breathing, most noticeably in a closed bedroom overnight
- outdoor air, whenever you open windows
Whether ventilation raises or lowers indoor humidity depends on the weather. In cold weather, outdoor air holds little moisture, so once it warms up indoors it lowers the humidity. On a warm, muggy day, open windows can bring moisture in rather than let it out.
Extractor fans in kitchens and bathrooms remove moisture where it's made, before it spreads through the house. Persistent condensation and damp patches on walls are building problems with causes of their own, such as poor ventilation, cold spots or leaks, and need those causes fixing. Air conditioning isn't a fix for damp.
How air conditioning changes humidity
In cooling mode, room air passes over the cold coil inside the indoor unit. Water condenses on the coil and drains away through a pipe, so the air comes back into the room both cooler and drier. That's part of why a cooled room feels more comfortable than its temperature alone would suggest. How air conditioning removes humidity explains the process in more detail.
Dry mode puts moisture first. The system runs gently, typically with a low fan speed, so it takes water out while cooling the room less than full cooling would. It suits muggy days when the temperature is bearable but the air feels heavy. Exactly how it behaves varies by model, and the modes explained covers the differences.
Heating mode takes no moisture out indoors. Warming the air lowers its relative humidity, as any heating does, but the amount of water in the room stays the same. Most home systems can remove moisture but can't add it. For more on this, see whether air conditioning dries the air.
A few habits help in warm, humid weather:
- Choose a moderate setting rather than a very low one. Because drier air feels cooler, a moderate temperature is often comfortable enough.
- Keep windows and doors closed while it runs, or humid outdoor air keeps replacing the air it has dried.
- Use extractor fans while cooking and showering, so moisture leaves at the source.
Related guides
- How air conditioning removes humidityAir conditioning removes humidity as it cools. Room air passes over a coil colder than its dew point, so water vapour condenses on the fins, drips into a tray and drains away through a hose.
- Does air conditioning dry the air?Yes, in cooling and dry modes it takes moisture out, which is part of why a cooled room feels comfortable, but it rarely leaves a UK home uncomfortably dry. Heating mode removes no moisture, although warming air lowers its relative humidity.
- The modes explained: cool, heat, dry, fan and autoCool lowers the temperature and takes out some moisture, heat warms the room, dry mainly removes moisture with gentle cooling, fan only moves air, and auto chooses between cooling and heating for you.
- Sleeping in hot weatherBedrooms stay warm at night because the building gives back the heat it soaked up during the day. Keeping sun out by day, ventilating once it's cooler outside, and quiet cooling or a fan all make a warm night more comfortable.