Humidity · Diagnosis
Managing indoor humidity
Humidity is the quieter half of indoor climate. Temperature is easy to feel; moisture announces itself only once it has already settled on a window or a wall corner. A little measurement turns it from a surprise into something you can manage.
Relative humidity describes how much water vapour the air holds compared with the maximum it could hold at that temperature. Warm air can carry more moisture than cold air, which is why the same amount of vapour feels different across the seasons and why it condenses where warm room air touches a cold surface.
What a hygrometer tells you
An inexpensive thermo-hygrometer reports temperature and relative humidity together — and they only make sense together. A reading of "55%" means little without knowing the room is at, say, twenty degrees. For living spaces, a relative humidity roughly in the 40–60% band is a commonly cited comfort orientation; below it the air can feel dry, and well above it the condensation risk on cold surfaces climbs.
A simple way to read condensation
Where moisture appears tells you something about the cause. Treat the stages below as a quick diagnostic walk-through rather than a verdict.
- Observe
- Locate
- Measure
- Adjust
- Review
- Observe when and where droplets form — mornings on bedroom glass are common and often harmless if cleared.
- Locate the coldest surfaces: single-glazed panes, window reveals, and corners on exterior walls condense first.
- Measure with the hygrometer over a few days to see whether the room sits high consistently or only after specific activities.
- Adjust the routine — more airing after moisture-heavy moments, lids while cooking, drying laundry elsewhere.
- Review whether the readings improve; if dampness persists despite consistent airing, it points toward a structural or ventilation question.
Furniture against cold walls
A wardrobe pushed flat against an exterior wall can trap a pocket of still, cool air behind it, where moisture lingers. Leaving a small gap for air to circulate is a low-effort habit that helps in older buildings.
Everyday moisture sources
No single activity is the culprit; the day's small contributions add up. The usual ones in a household are cooking, showering and bathing, drying laundry indoors, indoor plants in number, and ordinary breathing overnight. None of these are problems on their own — they simply mean a home needs a reliable way to move that moisture back out, whether by hand or mechanically.
The honest limit of measurement
A hygrometer shows what is happening, not why. Persistent damp despite good airing habits can have causes a meter cannot see — thermal bridges, rising damp, or a leak. At that point the sensible step is a qualified building inspection rather than more guesswork.
Where ventilation comes in
If manual airing cannot keep humidity in a comfortable range — for example in a tightly sealed new build, or a flat that is empty all day — a mechanical system can carry the load continuously. That is covered in the companion note on ventilation with heat recovery.
References
- Humidity — Wikipedia, including relative humidity.
- Hygrometer — Wikipedia.
- Mould growth, assessment and remediation — Wikipedia, on the link between moisture and mould.