Condensation and mould around a window: what you can fix yourself

Why condensation shows up on the window first

Warm air holds more moisture than cold air, and when it touches a cold surface it cools down and releases that moisture as liquid water. Glass and the metal or plastic spacer bar inside a window unit are usually the coldest surfaces in a room, colder than the wall, so water appears there first.

This gets worse in Estonian winters because the temperature difference between inside and outside is large for months at a time. A newer double or triple-glazed window resists this better than an old one, but no window is warmer than the wall around it, so some condensation on the glass on a very cold morning is normal and not a fault.

The frame itself can also be a cold bridge, particularly at the corners and where the frame meets the wall. If the black spots appear specifically along the frame or in the corners rather than spread across the glass, that is usually the cold bridge rather than general room humidity.

Is the fault in the room's ventilation or inside the window

Before treating this as a window problem, rule out the simpler cause: too much moisture in the air with nowhere to go. Cooking, showering, and breathing all add moisture to a flat, and if it cannot escape it will find the coldest surface, which is the window.

A quick way to tell the difference: open the trickle vent or crack the window for ten minutes twice a day for a week and see if the condensation reduces. If it clears up, the room's ventilation was the issue. If water still collects on the glass or sill at the same rate, or if it appears between the panes rather than on the room-facing surface, the window itself is more likely involved.

Trickle vents built into the top of the frame are often painted shut, taped over for winter, or simply closed and forgotten. Check whether yours opens at all; a stuck or blocked trickle vent removes the one deliberate air path the window was designed to have.

Drying laundry and other moisture sources indoors

A load of wet washing drying indoors releases a large amount of water vapour into the room over several hours, and in a small flat with the door shut that moisture has to go somewhere. The window is usually where it ends up.

Other common sources are a kettle or pan left uncovered, an extractor fan that is not switched on or not vented outside, and drying shoes or outerwear on a radiator near the window. None of these are faults in the building; they are just moisture with no exit route.

If you can, dry laundry in a room with an extractor fan or open window, or use a dehumidifier for a few hours afterwards. This alone resolves a large share of the condensation cases we see in flats, without touching the window at all.

Steps to try before ordering anything

Work through these in order over one to two weeks before assuming the window needs work. Most condensation problems are solved at this stage.

Wipe the sill and glass dry each morning rather than letting water sit and soak into the frame or paint. Standing water is what eventually causes the black spots, not the condensation itself.

• Open trickle vents fully, or free them if painted or taped shut
• Air the room for ten minutes, twice a day, even in cold weather
• Move laundry drying away from the window and into a ventilated room
• Wipe condensation off the glass and sill each morning
• Keep furniture and curtains a few centimetres clear of the glass so air can circulate
• Run an extractor fan or open a window while cooking or showering

Black spots inside the frame and how old the seal is

If wiping the sill dry every day still leaves black or grey spots that will not wipe off, that is mould growing in the paint, wood, or sealant, not just water sitting there. It tends to start in the corners of the frame, where air moves least and moisture lingers longest.

Check the rubber or silicone seal around the glass and where the sash meets the frame. Seals harden and shrink with age, typically after ten to fifteen years, and once they lose their flexibility they let cold air in and let condensation form inside the frame rather than only on the visible glass.

A perished seal is a job worth doing rather than living with, because mould in the frame timber will keep coming back regardless of how well you ventilate the room.

When it is the window's problem, not the room's

Some signs point clearly to the window rather than the room. Condensation forming between the panes of a double-glazed unit, rather than on the inner surface, means the seal on the glass unit itself has failed and moist air has got inside where you cannot wipe it away.

A window that feels cold to the touch even when closed properly, or one with a visible gap you can feel a draught through, is losing heat fast enough that condensation on the frame becomes almost unavoidable no matter how well you ventilate.

If mould keeps returning inside the same corner of the frame within days of cleaning it, despite following the ventilation steps above, the frame or seal is the root cause rather than the room's air.

If you decide to repair or replace the window

A worn seal, a warped sash that no longer closes tightly, or hardware that has come loose are usually things a joiner can service rather than replace outright, and servicing is the cheaper route where the window body is still sound. We handle window fitting and servicing jobs from 90 € depending on the work needed.

Where mould has already got into the frame timber or the surrounding plaster, that needs cleaning and repainting once the moisture source is dealt with, which is odd-jobs and painting work rather than window work; odd jobs start from 39 € an hour and painting and decorating from 12 € per job.

If you are not sure whether you are looking at a ventilation habit or a failing window, send us a couple of photos of the affected corner and we can generally tell you which it is before any work is booked.