Condensation on windows: why it appears and how to prevent it

Noticias
Condensation on windows: why it appears and how to prevent it

Equipo de Ventanas Alicante

You get up, raise the blind and there it is: the glass streaming with water, a little puddle in the track and a black shadow creeping along the seal. Before you write your windows off, take a breath. Condensation has a very simple physical explanation and can often be fixed without any building work. Other times, it really is the window asking to be replaced. Let’s see how to tell the difference.

Why windows “sweat”: the dew point

The air inside your home always carries water vapour. When that warm air touches a cold surface, it cools down and can no longer hold all its moisture: the vapour turns into droplets. The temperature at which this happens is called the dew point.

An example: with the living room at 20 °C and 60 % relative humidity, the dew point is around 12 °C. If the inside face of the glass or frame drops below that temperature, water appears. That is why it mostly happens at dawn, after a cold night and hours of breathing with everything shut.

On the Alicante coast there is an extra factor: humidity is high almost all year round. Winters are mild, but the air is so laden with moisture that you don’t need freezing temperatures to see misted-up windows.

Where the water appears and what it is telling you

Pay attention to where it forms, because it is the best diagnosis you can make without any tools.

On the inside of the glass: the most common case. It points to excess humidity, poor ventilation or glass that insulates poorly.
Between the two panes of the double glazing: a bad sign. The seal of the unit has failed and ventilating won’t fix it; the glass needs replacing.
On the outside: strange as it may seem, this is usually good news. The glass insulates so well that its outer face stays cold and collects the morning dew. It disappears once the sun comes out.
On the frame rather than the glass: the profile is to blame, typical of old aluminium frames without a thermal break.
In a strip along the edge of the glass: usually caused by the metal spacer between the panes, which acts as a thermal bridge.

The steam factories in your home

A family of four produces several litres of water vapour a day without realising it. The usual suspects: cooking without a lid or extractor fan, long showers with the door open, drying clothes indoors, sleeping with the bedroom door closed and butane heaters, which release vapour as they burn.

How to prevent condensation without building work

Try this for a couple of weeks. If the problem goes away, it wasn’t the window.

Ventilate briefly and intensely: five to ten minutes with windows open on opposite sides, ideally first thing in the morning.
Measure the humidity: a hygrometer costs very little. The ideal range is between 40 % and 60 %. If you regularly live above 65 %, the problem is the humidity, not the window.
Use the extractor fans in the kitchen and bathroom while producing steam and for a few minutes afterwards.
Let the glass breathe: thick curtains pressed against it trap cold, damp air right on the glass.
Keep a stable temperature: switching the heating off at night and turning it up high in the morning makes condensation soar.
Use a dehumidifier in rooms that are hard to ventilate.
Wipe away the water every morning with a cloth so it doesn’t sit in the seals.

From wet glass to mould is just one step

Occasional condensation is not serious. The problem comes when it happens every day: water soaks the seals and tracks, the silicone turns black and the mould spreads to the wall. If you already have stains, clean them with diluted bleach or an anti-mould product, wearing gloves and with the room ventilated. And tackle the cause, or they will come back.

When it really is time to replace the window

If you have controlled humidity and ventilation and the water is still there, the window is no longer up to the job. The signs:

Single glazing or old double glazing without low-emissivity glass.
Aluminium frames without a thermal break that drip on the inside.
Condensation between the panes.
Dried-out seals, draughts or fittings that no longer close properly.

A multi-chamber PVC profile or an aluminium profile with a thermal break, combined with low-emissivity glass and a warm-edge spacer, keeps the inside face close to room temperature. If that surface doesn’t drop below the dew point, the condensation disappears.

Note: a new, airtight window means your home stops “breathing” through the gaps. After the replacement, keep up your ventilation routine or consider micro-ventilation built into the hardware.

In short

Sweating windows don’t always mean they need replacing. Check where the water appears, measure the humidity and adjust your ventilation. If that isn’t enough, the problem lies in the glass or the profile, and that’s when it’s worth having a technician take a look. At Replus we’ll tell you honestly whether your window can be fixed or needs replacing.