Ventilation is the least visible thing a room does

This is less a set of instructions about ventilation than an argument, and it is worth saying so at the start.

Why newer homes have more of this problem

Draughty older buildings ventilated themselves continuously through gaps, which wasted heat and quietly removed moisture and stale air. Insulation and draught-proofing removed those gaps, which is good for energy use and removes the accidental ventilation at the same time. Unless deliberate ventilation replaces it, moisture and indoor pollutants accumulate in a way they previously did not.

This is why condensation and mould complaints are common in homes that have recently been improved. The correction is not to undo the insulation but to add ventilation as a deliberate act rather than an accident of construction.

Where the moisture comes from

Cooking, showering, drying clothes indoors and simply breathing all release water vapour into the air of a home in significant quantities. Drying laundry indoors is a particularly large source, and in winter it frequently exceeds what a closed room can absorb.

For most people, warm air holds more moisture than cold air, so vapour generated in a warm room condenses when it reaches a cold surface. That is why condensation appears on windows and in unheated corners rather than uniformly across a room. Persistent condensation is a signal about the balance between moisture production and ventilation, not a defect of the glass.

Damp, mould and when it matters

Mould grows where surfaces stay damp, and it is a consequence of the moisture balance rather than a separate problem to be scrubbed away. Cleaning it without changing the conditions produces a temporary result, since the same surface will stay damp and it will return. Damp problems have several possible causes — condensation, leaks, rising damp — and they require different remedies, which is why identifying the source matters.

The useful part is this: extensive or persistent mould in a home is a health matter as well as a maintenance one, and anyone with respiratory symptoms should speak to a doctor. In rented housing, damp and mould are often the landlord's responsibility, and tenants in many countries have specific rights worth checking locally.

Ventilating without losing all the heat

Opening windows wide for a short period exchanges the air quickly while the walls and furniture retain most of their heat. Leaving a window slightly ajar for hours cools the structure itself and loses considerably more heat for less air exchange.

Opening windows on opposite sides creates a through-draught that clears a room far faster than one window alone. The morning after a night with the bedroom door closed is when a room is most in need of it, which is why the habit is easiest to attach there.

Extractor fans in kitchens and bathrooms are the targeted version, and they only work if they run long enough after the activity finishes.

Stuffiness and how it feels

A closed room with people in it accumulates carbon dioxide, and rising levels are associated with reports of stuffiness, headaches and reduced concentration. The effect on cognitive performance has been studied in offices and classrooms with mixed and sometimes contested results, so strong claims should be treated carefully.

What is uncontroversial is that people notice and dislike it, and that opening a window resolves it within minutes. Meeting rooms, bedrooms and small home offices are the usual offenders because they are occupied for long periods with the door shut. Making it a habit at the start of a working session is easier than remembering once the room already feels wrong.

Practical habits that cover most of it

A few minutes of wide ventilation in the morning, and again in any room used for a long stretch, handles a great deal of the problem. Running the extractor during and after showering, and keeping the bathroom door closed while it works, prevents moisture moving through the home. Drying laundry near an open window or with the extractor running removes the single largest avoidable moisture source in winter.

Where it helps most, leaving a small gap behind furniture on external walls allows air to move and reduces the cold spots where condensation forms. Where these are followed and problems persist, that indicates a building issue and needs someone qualified to look at it rather than more habits.

The takeaway

Open two windows wide for five minutes. It is cheaper than heating a draught all day.

Pick the one that costs you least, and let the rest wait.