Your fogged window is telling you something: six checks for the start of heating season

On the first cool mornings, many homes follow the same pattern: the windows stay shut, the heating goes up, and by evening small droplets appear along the bottom of the glass. Moisture is not the diagnosis by itself. It is a signal that moisture production, air exchange and surface temperatures are no longer in balance.

The answer is neither leaving a window open all day nor simply making the room hotter. Six checks can reveal where action will have the greatest effect.

1. Measure humidity instead of guessing

A basic digital thermometer-hygrometer is more useful than describing the air as “heavy”. The US Environmental Protection Agency recommends indoor relative humidity between 30 and 50 per cent and, in any case, below 60 per cent. This is not a rigid universal target: outdoor conditions, surface temperatures and the building fabric also matter.

Measure at least in the bedroom and in the room where most cooking or showering occurs. Do not place the meter directly on a radiator, windowsill or in the path of bathroom steam. Watch the daily pattern from the occupied part of the room: when does humidity rise, and how long does it take to fall again?

2. Ventilate at the source

Cooking, showering, mopping and drying clothes indoors can add a great deal of moisture in a short time. Brief, intensive airing immediately afterwards is usually more effective than leaving a window tilted for hours. Where the plan allows, a few minutes of cross-ventilation exchanges the air quickly without cooling walls and furniture as much.

Kitchen and bathroom extractors work best when they discharge outdoors, their filters are clean and replacement air can enter. A recirculating cooker hood may capture grease and some odours, but it does not remove water vapour from the home.

3. Read the location of the water

If condensation appears mainly along the lower edge of bedroom glazing in the morning, begin by examining overnight moisture and air exchange. Dampness in an external corner, behind furniture or around a lintel may instead indicate a cold bridge, poor air circulation or a building defect.

Avoid pressing a large wardrobe tightly against a cold outside wall, and do not block a radiator with heavy curtains. Warm air needs to reach vulnerable surfaces. If dampness worsens after rain, leaves a defined stain, blisters paint or returns persistently in one place, do not assume that ventilation is the only cause: rule out water ingress and leaking pipes.

4. Do not let an unused room become completely cold

Saving energy does not mean keeping one room icy while warm, moisture-laden air enters from the next. On a cold wall the air reaches dew point more easily, so condensation forms. Moderate, steadier heating with the door closed is often preferable to a large temperature difference between rooms.

Read humidity together with temperature. The same amount of moisture produces a higher relative humidity in colder air. A reading of 65 per cent on a day without cooking may not indicate a new moisture source; the room may simply have cooled.

5. Room airing and combustion air are not the same thing

This is the most important safety distinction of the heating season. Opening a window from time to time does not replace the permanent air supply required by an open-flued boiler, water heater, stove or gas hob. In its 23 September 2026 advice, Hungary’s disaster-management authority urged regular inspection of both heating appliances and flues, and stressed the importance of adequate ventilation and a reliable carbon-monoxide alarm wherever an open flame is used.

Do not seal an air inlet because it feels draughty, and do not treat a powerful cooker hood as a purely aesthetic kitchen choice. A fan changes air pressure and, in an inadequately supplied or faulty system, can contribute to dangerous back-draught. Appliance maintenance, alarm replacement and placement should follow professional advice and the manufacturer’s instructions.

6. Do not simply paint over visible mould

The WHO’s guidance identifies prevention or minimisation of persistent dampness and microbial growth as the central protective measure. That establishes a practical order: find the cause first, then clean and repair the surface.

A small, one-off patch of condensation may reflect how a room was used. A recurring musty smell, an expanding mark, soft plaster or a damp skirting area is no longer a decorating problem. Bring in a building-diagnostics or services professional. The EPA also advises drying water-damaged areas and items within 24–48 hours; delay increases the chance of mould growth.

A ten-minute autumn checklist

• Check bedroom humidity in the morning and again after cooking or showering.
• Dry the lower edge of the window and note where water returns the next day.
• Clean extractor filters and verify where the extracted air actually goes.
• Clear the space in front of radiators and improve air movement near cold outside walls.
• Test carbon-monoxide and smoke alarms; check their expiry dates and placement instructions.
• If you have an open-combustion appliance, do not alter air inlets or extract systems without expert advice.
• Book maintenance if the heater or flue has not been inspected for some time.

A home ready for heating season is not necessarily warmer. It is more legible, more even and safer: you know where moisture comes from, where air leaves, and which warning signs require a professional rather than another household trick.