Condensation on windows: What causes it and what actually fixes it
If you have noticed water on your windows, the first question is not simply, “How do I stop condensation?”
Condensation on the room side of the glass, on the outside of the glass, and between the panes are three different things. They have different causes, and they do not all need the same solution.
PVCu sash window
That distinction matters. Opening a window may help with condensation inside your home, for example, but it will not repair a failed sealed unit with misting between the panes.
Three kinds of condensation, and why the difference matters
On the inside of the glass
This is the type most people mean when they search for condensation on windows.
It appears on the room-facing surface of the glass when warm, moisture-laden air inside the house comes into contact with a colder surface. As the air cools, it can no longer hold as much water vapour, so moisture forms as droplets.
Common signs include:
Water on the inside of the glass in the morning.
Droplets around the edges of the window.
Moisture collecting on frames or window boards.
Condensation becoming worse after cooking, showering or drying clothes indoors.
This is usually an indication of a combination of moisture in the air, low surface temperatures and insufficient ventilation.
On the outside of the glass
Condensation on the outside surface can be surprising, especially when you have recently installed efficient double glazing. Unlike internal condensation, it is not usually caused by excessive moisture inside the home.
The outer pane of a highly insulating window can become cold because less heat is escaping from the house to warm it. If the outside air contains enough moisture, condensation can form on that cold outer surface. This is often most noticeable early in the morning and usually clears as the glass warms.
In many cases, occasional condensation on the outside of the window can simply be a sign that the glazing is doing a good job of reducing heat transfer.
In between the panes
This is different. If the condensation or misting is between the two panes of a double-glazed unit, the sealed unit has usually failed. A double-glazed unit is designed to be sealed around its perimeter. If that seal breaks down, moisture can enter the space between the panes.
You may notice:
A permanently misted appearance.
Moisture that cannot be wiped away from either side.
A cloudy patch inside the glazing.
Marks or streaks between the panes.
Depending on the window and its condition, it may be possible to replace the sealed glass unit rather than replacing the complete window. A survey is the best way to establish the cause and the appropriate repair.
Why modern homes often get more condensation, not less
Surely replacing draughty old windows should reduce moisture problems? Better windows can improve thermal comfort, but modern homes are also often more airtight than older properties. Reducing uncontrolled draughts can mean that moisture generated inside the home has fewer routes to escape.
Everyday activities produce a surprising amount of water vapour, including:
Cooking.
Showering and bathing.
Drying washing indoors.
Boiling kettles and pans.
Breathing.
Keeping houseplants.
The solution is not to make a home permanently cold or draughty. It is to provide controlled ventilation while maintaining a comfortable temperature. That is why replacement windows and ventilation need to be considered together.
Current Approved Document F guidance for England explains that when replacing windows, the ventilation of the dwelling should either meet the relevant standard or not be made less satisfactory than it was before the work.
What actually reduces it
There is no single trick that works in every house. Reducing internal condensation usually means managing three things: moisture, ventilation and temperature.
Ventilation and trickle vents
Ventilation allows moist air to leave and drier outside air to replace it.
This does not necessarily mean leaving your windows wide open all day. Depending on the property, ventilation may come from a combination of:
Opening windows.
Extractor fans in kitchens and bathrooms.
Background ventilators (often called trickle vents).
Existing wall ventilators.
Other ventilation measures appropriate to the home.
Trickle vents are small controllable openings designed to provide background ventilation without requiring the window to be left open. They can be useful because condensation often develops over several hours, particularly overnight, when windows are closed.
It is important to remember that a trickle vent is not a substitute for using extractor fans or opening windows when rapid ventilation is needed. If you are showering or cooking, moisture levels can rise quickly, so extraction at the source is important.
Current government guidance also makes clear that background ventilation does not have to be provided by a window trickle vent specifically. Other appropriate methods may be used where they meet the relevant requirements.
Not sure which sort you have?
A misted window can be frustrating, particularly when it is difficult to tell whether the problem is caused by condensation in the room or by a failed double-glazed unit.
Start by checking where the moisture is:
Room side of the glass: Usually linked to indoor humidity, surface temperature, and ventilation.
Outside of the glass: Can occur on efficient glazing and often clears as conditions change.
Between the panes: Usually indicates a failed sealed unit and may require repair or replacement.
If you are planning new windows, it is also worth discussing ventilation at the same time. Better insulation and airtightness should not mean overlooking the need for controlled airflow.
Bedfordshire Windows can survey existing windows and discuss replacement options, glazing and ventilation considerations for your home.
If you are unsure whether you have ordinary condensation on windows, a failed sealed unit or a wider ventilation problem contact our team to arrange a survey or discuss your options.