Between 40% and 60% relative humidity is the band a British home should hold.
45 to 55% is the comfortable middle, and it is where most homes are quietly aiming without anybody measuring.
In a UK winter the problem is almost never the bottom of that range.
Mould does not need a wet wall, only a cold one
Relative humidity is relative to temperature, which is the source of nearly every misunderstanding about it.
Warm air holds more water. Cool that same air and the percentage climbs without a single drop being added.
The temperature at which it reaches 100% is the dew point, and that is where condensation appears.
Air at 20ยฐC and 60% humidity has a dew point of roughly 12ยฐC.
Any surface in that room colder than 12ยฐC will get wet. A single-glazed window in January sits nearer 6ยฐC.
That is why condensation collects on glass, in the cold corner behind a wardrobe, and on an uninsulated external wall.
Mould germinates where a surface stays above roughly 80% surface humidity for long enough โ days rather than hours.
The room air never has to approach 80% for a cold corner to get there on its own.
Relative humidity on a 0โ100% scale. The green band is the target; the marker is a typical British bedroom on a January morning.
A British winter is dry-feeling and damp-surfaced at the same time
Heating a house raises the temperature and drops the relative humidity, which is why noses and skin feel dry in January.
Then daily life puts the water straight back.
A household of four generates something in the order of 10 to 15 litres of water vapour a day.
Breathing accounts for a surprising share of it. Showers, cooking and drying laundry indoors account for most of the rest.
One load of washing dried on an airer releases around 2 litres into whichever room it is standing in.
So the same house can read 38% in a heated sitting room at eight in the evening and stream with condensation on the bedroom window at seven the next morning.
The bedroom is not damper than the sitting room. It is colder, and that is enough.
Buy a hygrometer before you buy anything else
A digital hygrometer costs about ยฃ10 and settles the argument within a day.
Put it in the room that shows condensation, not in the room that feels dry.
Read it in the early morning, when the house is at its coldest and the number is at its highest.
If it reads above 60%, the answer is ventilation and heat, not a machine that adds more water.
Trickle vents, an extractor fan left running for the full length of a shower, and fifteen minutes of open window after cooking do most of the work between them.
A dehumidifier earns its place where drying laundry indoors is genuinely unavoidable.
Bottom line: running a humidifier in a house that already shows condensation makes the mould worse rather than the cough better.
The genuinely dry home is real, and it is a specific kind of home
Below 35%, static shocks, cracked lips and a dry throat overnight are the usual complaints.
That reading turns up in well-insulated flats with a warm, dry heat source and few occupants.
It also turns up in older homes heated hard through a cold snap, which is a temporary condition rather than a standing one.
If the hygrometer confirms it, adding moisture to reach 45% is sensible, and the target is the middle of the band rather than the top of it.
Size the unit to the room and read the output figure rather than the tank size, which we take apart in our Levoit LV600S review.
Run it against the hygrometer rather than against how the room feels. A built-in humidistat that switches off at a set percentage is worth the extra money.
Keep it away from external walls and windows, which are the surfaces already closest to their dew point.
And clean it. A humidifier is a warm reservoir of standing water, and a neglected one becomes the air quality problem it was bought to solve.
None of this is medical advice. Persistent damp-related symptoms are worth raising with a GP.
