What should the humidity in your house be?
Damp windows, musty corners, static shocks, a cough that will not shift — most of it traces back to one number on a cheap hygrometer. Here is the number you want, and what to do when yours is wrong.
The short answer: keep indoor relative humidity between 30% and 50%, and aim for 40–50% if you can. Below 30% the air dries out your skin, airways and timber. Above 60%, condensation forms on cold surfaces and mould and dust mites move in.
The catch: that single number behaves very differently at 18 °C than at 24 °C. Use the checker below to see what your reading actually means in your room.
Check your humidity reading
Enter what your hygrometer and thermometer say. You will get the dew point (the temperature at which your air starts condensing), the absolute moisture load, and a mould-risk estimate for the cold surfaces in the same room.
Ideal indoor humidity — 45% RH
You are in the sweet spot.
- Dew point
- 8 °C
- Absolute humidity
- 8.2 g/m³
- Mould risk on cold walls
- Very low
The mould estimate assumes cold surfaces run about 3 °C below room air, which is typical of an uninsulated exterior wall or a window reveal. A poorly insulated corner or a single-glazed window can be colder still, so treat “moderate” as a prompt to check those spots with an infrared thermometer.
The ideal humidity range, room by room
Health authorities and building scientists converge on the same window: 30–50%. It is not arbitrary — it is the band where dust mites, mould, bacteria, viruses and chemical off-gassing are all near their minimum at once. Go lower and your airways and your furniture suffer; go higher and biology takes over.
| Room | Target humidity | Why it differs |
|---|---|---|
| Living room | 40–50% | Comfort baseline — warm air here can carry more moisture without condensing. |
| Bedroom | 40–50% | A sleeping adult adds up to a litre of water a night; ventilate in the morning. |
| Bathroom | Under 60% after 20 min | Spikes to 90%+ during a shower. What matters is how fast it drops back. |
| Kitchen | Under 60% | Cooking and boiling are the second-largest moisture source in most homes. |
| Basement / cellar | 50% or below | Coldest surfaces in the house, so mould appears here at lower room readings. |
| Nursery | 40–50% | Dry air irritates small airways; damp air feeds mite allergens. |
| Wine, books, instruments | 45–55% | Wood, paper and cork move with humidity — stability matters more than the exact value. |
Winter exception: when it is freezing outside, holding 50% indoors can push condensation onto your windows. If water beads on the glass, drop toward 30–40% until the cold snap passes.
Above 60% Your house is too humid
High humidity rarely announces itself with a number — it announces itself with a smell, a stain, or a window you have to wipe every morning.
Signs
- Condensation on windows, especially in the morning
- A musty, earthy smell in one room
- Black speckling in corners, behind wardrobes, on window reveals
- Peeling wallpaper, blistering paint, springy floorboards
- Allergy symptoms that get better when you leave the house
Usual causes
- Drying laundry indoors — up to 2 litres of water per load
- Unvented tumble dryers and long, hot showers
- Cooking without an extractor or with the hood recirculating
- Cold, uninsulated walls that pull moisture out of the air
- Leaks and rising damp — a tide mark low on a wall is the tell
What actually works
- Cross-ventilate 5–10 minutes, twice a day, windows wide rather than tilted
- Run the extractor fan during and 15 minutes after showering
- Dry laundry outside, or in one room with the door shut and a window open
- Pull furniture 5 cm off exterior walls so air can move behind it
- Use a dehumidifier sized to the room — compressor above 15 °C, desiccant in a cold basement or garage
- Heat the cold rooms a little rather than shutting them off entirely
Below 30% Your house is too dry
Dry air is the winter problem. Heating does not remove water from your home — it raises the temperature, and warm air can hold far more moisture, so the same water becomes a much smaller percentage.
Signs
- Static shocks off door handles and clothing
- Dry, tight skin; chapped lips; nosebleeds
- A dry cough or sore throat that is worst on waking
- Gaps opening in wooden floors and furniture joints
- Houseplants crisping at the leaf edges
Usual causes
- Cold weather plus central heating — the classic combination
- Over-ventilating in freezing conditions
- Wood stoves and open fires, which draw air through the house
- Air conditioning in hot, dry climates
- Very high thermostat settings — every degree lowers relative humidity
What actually works
- Run a humidifier and set it to hold 40–45%, not maximum output
- Clean the humidifier weekly — a dirty tank aerosolises bacteria
- Turn the thermostat down 1–2 °C; relative humidity rises on its own
- Leave the bathroom door open after a shower and let the moisture spread
- Dry laundry indoors — in winter this is a feature, not a fault
- Group houseplants; the transpiration adds a modest but real amount
How to measure it properly
Get one honest reading
A digital hygrometer costs very little and is the only way to know. Place it at chest height, away from radiators, windows, exterior walls and doorways, and give it two hours to settle before you trust it.
Cheap units drift. Check yours with the salt test: seal a damp salt slurry and the hygrometer in a sandwich bag for eight hours — it should read 75%. Note the offset and mentally subtract it.
Measure the room that is failing
Humidity is not one number for a whole house. The coldest, least-used, least-heated room is where condensation and mould appear first — usually a north-facing bedroom, a spare room with the door shut, or the basement.
If you are hunting a damp problem, put a hygrometer in that room and check it in the early morning, when surfaces are at their coldest and the reading is at its worst.
Frequently asked questions
What is the ideal humidity level in a house?
Between 30% and 50% relative humidity, with 40–50% the most comfortable band for most homes. Below 30% the air is drying; above 60% you risk condensation, dust mites and mould.
Is 70% humidity too high in a house?
Yes. Sustained readings above 60% let mould colonise cold surfaces, and at 70% dust mites thrive and moisture begins to damage plaster, paint and timber. Ventilate, find the moisture source, and run a dehumidifier.
Why is the humidity in my house so low in winter?
Cold outdoor air holds very little moisture. When it comes inside and is heated, its relative humidity collapses — air at 0 °C and 80% outdoors becomes roughly 20% once warmed to 21 °C. Your heating is not removing water; it is raising how much the air could hold.
How do I measure the humidity in my house?
Use a digital hygrometer at roughly chest height, away from radiators, windows, exterior walls and doorways, and let it settle for a couple of hours. Measure the coldest room as well — that is where condensation and mould show up first.
Does opening windows reduce humidity?
In cool weather, yes: cold outdoor air is dry in absolute terms, so five to ten minutes of cross-ventilation twice a day swaps damp indoor air for drier air. On a warm, muggy day the outdoor air holds more moisture than your indoor air and opening windows makes things worse — dehumidify instead.
What humidity level causes mould?
Mould needs around 80% relative humidity at the surface it grows on. Cold walls, window reveals and corners sit several degrees below room temperature, so a room reading of 60–65% can already mean 80% against the coldest wall. That is why mould appears in corners while the room itself feels only slightly damp.
Dehumidifier or better ventilation — which should I fix first?
Ventilation, every time. A dehumidifier treats the symptom and costs electricity for as long as you run it; ventilation and removing the moisture source (indoor laundry, an unvented dryer, a leak) treat the cause. Use a dehumidifier when ventilation is not enough on its own — a cold basement, a humid climate, or a room you cannot air properly.