If the mould appeared after new windows were fitted, that is not a coincidence. It is a direct consequence.
Old timber windows were leaky. A small, unnoticeable air exchange took place through them constantly — and moisture left the flat along with the air. It was a heat loss, but it also kept humidity under control.
A new PVC window stops that exchange almost entirely. The heat stays in the room — and so does the moisture. Ventilation in apartment blocks was designed on the assumption that air comes in through gaps around the windows. When those gaps disappear, the extract no longer works, because there is nowhere for the air to come from.
The result: humidity in the room rises, the coolest surfaces — window frames, sill edges, external wall corners — start to mist, and mould appears on them.
The window has not failed. What has failed is the balance between how airtight the building is and how much air comes in.
The area around a window is the coldest spot in a room even when the window is new and good. The glazing unit, the frame profile and above all the junction between the frame and the wall will always be cooler than the middle of the wall. So condensation appears there first — it is the spot that gives a room its earliest warning.
A misted window is not itself damage. It is a measuring instrument: it shows that there is more moisture in the indoor air than the coldest surface can take. If condensation forms along the bottom edge of the glass on rare, very cold mornings, that is borderline. If it happens every morning all winter and water runs down onto the sill, it has already become the cause of mould rather than just a signal.
Most modern windows have a trickle or winter ventilation setting. It is engaged by turning the handle to a particular position — often upward with a little pressure, or to an intermediate position. The window stays closed but leaves a small, controlled gap.
Many owners simply do not know this function exists. Switching it on often solves the problem at no cost at all.
If the window has no such function, the answer is a trickle vent in the wall or in the window.
| Situation | Solution |
|---|---|
| The window has a trickle ventilation setting | Switch it on — that is often enough |
| The window has no such function | A trickle vent in the window or the wall |
| The windows mist up every morning | Humidity in the room is too high — the airing routine has to change |
| Condensation in one room only | Usually the bedroom — moisture from breathing overnight |
| Radiator blocked by the sill | Warm air does not reach the window, so it cools and mists |
There is one more thing worth checking yourself: the winter and summer setting on the window hardware. On many windows the cams can be turned so that the sash closes more or less tightly. If the window is set to summer mode, cold air comes in past the seal in winter and the frame cools more than it should. The same happens if the seal has hardened and cracked.
Condensation on a window is not a switch that is either on or off. It has degrees, and they mean different things.
There is one more picture that looks more alarming than it is: condensation or frost on the outside of the glass. That is characteristic of good, well-insulating glazing units and means the window is keeping the heat inside — the outer pane stays cool and moisture from the outdoor air settles on it. It is not a problem and nothing needs doing about it.
We measure air humidity and surface temperatures, and check the window settings and the ventilation.
If the fungus has got into the finish at the frame or sill, we remove it.
We treat the frame area, the sill and the wall around the window.
We say exactly what to do — trickle ventilation, an airing routine or a trickle vent.
Around a window the work is usually small in area but detailed: what has to be treated is the bottom edge of the frame, the ends of the sill, the reveal and a band of wall around the reveal. Fungus along the frame edge has often got into the silicone or foam of the installation joint — then that has to be cut out and renewed, because treating over the top will not help there.
Mould around a window is usually a small area, and the work falls within the minimum order of €80. If the walls around the window are affected too, we price by area — from €1.50/m².
No, the windows work as intended — they are airtight. The problem is that the ventilation was not adapted after they were fitted. The answer is to provide an air supply, not to change the windows again.
No. It cools the wall around the window, which can actually make the condensation worse, and it wastes a lot of heat. Short and intensive airing several times a day is better.
Roughly 40–60% in living rooms. If a hygrometer consistently shows more, mould appearing is only a matter of time.
Describe the situation — we will tell you whether treatment is needed at all, and which one.
Fill in the form — we will call you on the number you provide, ask a few clarifying questions and propose a solution.
Thank you! We will contact you on the phone number you provided shortly.
