Condensation on windows in winter: what to do?

Condensation on windows in winter is often visible when waking up: the bottom of the glazing is damp, sometimes with small droplets. Wiping it down is helpful, but the fog will return if moisture production, ventilation, or surface temperatures remain unchanged.

Here are five actions to understand what is happening: locate the water, identify sources of vapor, check the air, manage cold surfaces, and then monitor the results. A dehumidifier can supplement these actions; it does not repair glazing or fix an infiltration.

1. Identify which side the water is on

Get close to the window and determine if the drops are accessible from inside the room. Interior fog can be caused by humid air coming into contact with cold glass. Exterior condensation is not necessarily addressed in the same way.

If the fog seems trapped between the panes of a double-glazed window and does not disappear when wiping the accessible surfaces, request an inspection of the window unit. An appliance placed in the room will not fix the seal of faulty insulating glass.

Also look for signs that suggest infiltration: marks linked to rain, water running from a specific corner, or localized wall degradation. Note the time it appears and take photographs to facilitate a diagnosis.

Wipe the drops and the sill with a suitable cloth to reduce standing water. Do not scrape the seals or pour harsh products into the orifices. This initial observation avoids buying a solution for the wrong cause.

2. Identify activities that contribute to moisture

Showering, cooking, and drying laundry contribute to indoor humidity. Occupants also produce it. If fog appears mainly in the morning in a closed bedroom, observe the air renewal during the night rather than immediately concluding there is a leak.

Keep a simple log for a few days: room, time, approximate temperature, activities, and duration of fog presence. You will be able to spot a correlation with laundry drying or a shower without resorting to complicated measurements.

A hygrometer is useful for comparing situations. Place it in a representative area and keep it in the same spot. A measurement taken right against wet glass or above a radiator does not correctly describe the state of the entire room.

Reduce avoidable contributions: use the extraction systems provided during humid activities and avoid piling up multiple loads of laundry in a small volume. Do not try to make the air excessively dry; the goal is a balance that is compatible with the home and its occupants.

3. Verify that air can enter, circulate, and exit

Check the air inlets on the windows and the extraction vents. They must not be covered with tape, paper, curtains, or accumulated dust. Do not block a vent because it lets in cool air: it may be essential for the home's proper functioning.

Verify that air circulation between rooms remains possible as originally designed. A modified door or a blocked passage can disrupt the entire system. Do not dismantle the system or change its settings at random.

ADEME recommends airing out the rooms, especially after activities that produce significant moisture. Adapt this ventilation to exterior conditions and the way your home functions. A window left slightly open constantly is not automatically the best strategy.

If an extraction vent seems inactive, have its maintenance and operation checked. A dehumidifier removes water but does not renew air loaded with other pollutants. It should therefore not be used as an argument to stop existing ventilation or block air inlets.

4. Examine cold areas without masking the problem

Condensation forms more easily on cold surfaces. Observe the bottom of the glazing, the corners, and areas behind curtains. A large difference between two windows can help pinpoint the situation, but it is not enough on its own to identify an insulation defect.

Avoid immediately applying a waterproof coating to a damp wall or hiding it behind a piece of furniture. Maintain the possibility of inspection. A decorative treatment does not solve the root cause of the water and can make its progression less visible.

Maintain consistent use of heating, in accordance with the installation, without trying to dry the window with improvised equipment. Do not direct a fan heater at the glazing or a hairdryer at a wet area.

When traces are recurring despite functional ventilation and controlled moisture sources, have the window unit and the building envelope examined. Extensive mold, a persistent smell, or wall degradation justify an appropriate intervention rather than simple daily wiping.

5. Use supplemental equipment only if necessary

A dehumidification appliance can be useful when air humidity remains high and its use complies with the manual. The Humilabs OL20-BD068CF is a mains-powered model; it must remain in a dry, stable, and unobstructed location, without contact with water from the window.

Its advertised capacity of 20 L/day does not correspond to a guaranteed amount in your room. Temperature and humidity influence the result. Do not place it in a small alcove behind a curtain to get it closer to the drops: this would hinder air circulation.

Monitor progress at comparable times. Note the duration of the fog, the visible amount of water, and available measurements. The desired goal is a more stable situation, not a reservoir that is constantly full.

If nothing changes or if the water returns quickly, restart the diagnosis. The detailed selection of an appliance is a separate matter; here, the essential decision is not to confuse supplements with repairs. An infiltration, a glazing defect, or a failing ventilation system must receive its own specific solution.

Key takeaway: To limit condensation on windows in winter, first identify the source of the water, then take action on vapor and ventilation. Supplemental equipment can help, but a persistent anomaly deserves a diagnosis rather than a succession of products.

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Additional resources: ADEME: breathing healthy air at home. The product manual and manufacturer's instructions remain a priority.