Humidity control is the only mould prevention that genuinely works. And it is cheaper than any treatment.
In living spaces, roughly 40–60% relative humidity. Below 40% the air becomes too dry; above 60% the risk of condensation and mould rises considerably.
The 60% threshold is not a matter of opinion. Mould does not need a puddle — it is enough that the surface of a material stays damp for a long time. Against a cold external wall, the air right at the wall surface is always damper than in the middle of the room, so when a hygrometer in the middle of the room reads 65%, conditions in the external-wall corner may already be those in which fungus grows.
At the other end there is the opposite problem. During the heating season, with sub-zero temperatures outside, humidity in a flat with no additional sources drops to 25–30%. That is not dangerous, but it is unpleasant: dry mucous membranes, static electricity, timber furniture and parquet shrinking. In that case the answer is the reverse of everything else on this page — the air needs humidifying rather than drying.
A hygrometer costs a few euros and is the only way to know your humidity rather than guess it. If you plan to tackle damp, start there — without a reading you are working blind. A simple digital model showing temperature and humidity is entirely sufficient; a dearer device tells you nothing fundamentally better.
How you use it matters more than the device. A few practical points nobody takes into account the first time:
A typical finding from that kind of monitoring: 50% during the day, but 68% in the bedroom in the morning. That means two people breathing in a closed room overnight are pushing the humidity up, and the window needs opening in the morning rather than only in the evening.
An average household releases a considerable amount of water vapour indoors every day. Very roughly, that is several litres a day for a family — and all of it has to go somewhere.
| Source | Note |
|---|---|
| Breathing and evaporation from skin | Continuous, including at night; two people in a bedroom are a noticeable source |
| Cooking | Especially boiling without a lid; an extract deals with most of it |
| Showers and baths | A large amount in a very short time |
| Drying laundry indoors | The single largest source — several litres from one load |
| House plants | Small but constant |
| Washing up | Small |
| Construction moisture in a new build | Considerable in the first year; not a daily source but a background one |
The orders of magnitude are these. A sleeping person releases about a glassful of moisture over a night; two people in a bedroom with the window shut are a noticeable source. A shower delivers far more in far less time — a few minutes of hot water brings bathroom humidity close to saturation. But the single largest source is almost always drying laundry indoors: one washing-machine load after spinning holds several litres of water, and that water goes literally into the air of whichever room the laundry hangs in.
So one of the most effective steps you can take at no cost is to move laundry drying to a room with an open window or working extract — or at the very least never leave it in a bedroom with the window shut overnight.
Moisture does not appear out of nowhere. It comes from your daily activities and stays wherever it is not carried away.
Warmer air can hold more water vapour than cooler air. When warm room air meets a cold surface — a window pane, an external-wall corner, a cold pipe — it cools at that point, and part of the moisture it was carrying can no longer stay as vapour. It settles on the surface as water. The temperature at which that happens is called the dew point.
The practical conclusions matter more than the term itself:
That is exactly why an unheated room with the door closed is the classic mould location in a flat. Damp air from the other rooms flows in, the surfaces are cold, and the condensation settles right there. The answer is not to close the door even tighter but to heat the room at least minimally and air it along with the rest.
On dehumidifiers separately: one works while it is running, and it is a useful answer in a basement, on a balcony or in a room with persistently high humidity. But it does not fix a leak and it does not fix rising damp from the foundations. If a dehumidifier runs constantly and collects a lot of water every day, that is not a solution — it is a symptom that there is a source somewhere that needs finding.
This is where most people do the opposite of what works.
The reason a window on the latch is worse is not obvious. Air exchange through a narrow gap is slow, so moisture is carried away poorly. Meanwhile the wall, the sill and the reveal around that opening are permanently chilled — and condensation forms exactly on a chilled surface. The result is that mould often appears around the very window that was kept on the latch “for ventilation”. Heat losses in that mode are also greater than with short, wide airing.
Short, intensive airing works because the air in the room changes quickly while the thermal mass of walls, furniture and floor is large — a few minutes is not enough for them to cool. Once the window is closed, the new air warms up again from them, and the room is just as warm as before, only with less moisture in it.
In the Latvian climate the damp problem changes character over the year, and what works in one season can be harmful in the other.
The cold outdoor air holds very little water. Brought inside and warmed, it becomes dry, so brief airing in winter removes moisture very effectively. The winter problem is not the air but the surfaces: external walls, corners and window reveals are cold, and that is where condensation settles. So in winter the key is to air and maintain even heating at the same time, rather than economising on temperature.
It is the reverse. On a warm, humid July day the outdoor air holds a lot of water. Let that air into a cool space — a basement, a ground-floor flat in a pre-war building, a north-facing room — and it cools there and the moisture settles on the walls. The result is that airing makes a basement damper rather than drier. So in summer, basements and cool rooms are better aired at night or early in the morning, when the outdoor air is cooler and drier.
The transitional periods — spring and autumn — are when problems appear most often. In autumn the heating is not on yet but the outdoor temperature has already dropped: the walls are cold, and the moisture that open windows carried away in summer suddenly stays in the room. October and November are when we get the most calls, and the typical patch is in the external-wall corner of a bedroom behind a wardrobe.
If the humidity is fine but the problem persists, the next place to look is air exchange: ventilation and mould prevention.
It keeps humidity under control while it runs, and it is a useful answer. But it does not fix rising damp or a leak if there is one.
It is a signal that humidity in the room is too high or air exchange is insufficient. Ignore it and mould is a matter of time.
Short, intensive airing loses little heat, because the walls and furniture do not have time to cool. Dry air is also cheaper to warm than damp air.
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