You wake up on a cold morning and notice water droplets covering the inside of your windows. Sometimes there’s only a light mist; other times, enough water collects to run down onto the windowsill.
This common household problem is condensation.
But where does that water actually come from? Is it caused by the windows, the weather outside, or moisture inside your home?
The answer comes down to a simple combination of warm air, moisture and cold surfaces.
What Is Condensation?
Condensation happens when water vapour in the air changes into liquid water.
Air around us contains water vapour even though we normally can’t see it.
When warm, moisture-containing air comes into contact with a sufficiently cold surface, the air next to that surface cools. If it cools enough, some of its water vapour turns into tiny droplets of liquid water.
Why Does Condensation Form on Windows?
That’s the moisture you see on a window.
It’s similar to what happens when you take a cold drink out of the refrigerator on a warm day. Water droplets may quickly appear on the outside of the glass or bottle.
The water isn’t leaking through the container. It comes from moisture already present in the surrounding air.
Why Does Condensation Appear on Windows?
Windows are often among the coldest surfaces inside a home.
During cold weather, the outside temperature cools the glass. Meanwhile, heating keeps the air inside the house relatively warm.
When that warmer indoor air reaches the colder window, it cools.
If the glass is cold enough, the air immediately beside it can reach its dew point.
At that point, some of the invisible water vapour becomes liquid water.
The result is the familiar foggy window covered in droplets.
What Is the Dew Point?
The dew point is the temperature at which air becomes sufficiently saturated with water vapour for condensation to begin forming.
The exact dew point depends partly on how much moisture is already in the air.
If indoor air contains a lot of moisture, condensation can occur at a relatively higher surface temperature.
That’s one reason homes with high indoor humidity may experience significantly more condensation.
Where Does Indoor Moisture Come From?
A surprising amount of water vapour can be produced through normal everyday activities.
Common sources include:
- Showering and bathing
- Cooking and boiling water
- Drying clothes indoors
- Breathing
- Using certain heating appliances
- Washing dishes
- Houseplants and other sources of moisture
One activity may not seem particularly significant, but moisture can accumulate when a home isn’t adequately ventilated.
Why Is Window Condensation Worse in Winter?
Condensation tends to become more noticeable during colder months because the temperature difference between indoors and outdoors increases.
People also tend to keep windows and doors closed more often during winter.
That creates two conditions that encourage condensation:
colder interior window surfaces and less ventilation to remove moisture.
Heating the house can make the indoor air comfortable, but it doesn’t automatically remove the water vapour being produced inside.
Eventually that warm, moist air reaches cold surfaces such as windows.
Does Double Glazing Reduce Condensation?
Double glazing can help reduce condensation on the room-facing surface of a window because the inner pane generally remains warmer than the inside surface of older single glazing.
A warmer internal surface is less likely to fall below the dew point.
However, double glazing doesn’t guarantee that you’ll never see condensation.
If indoor humidity is high enough, condensation can still form.
Interestingly, condensation can sometimes appear on the outside of efficient windows. This can happen because good insulation prevents as much indoor heat from reaching and warming the outer pane.
External condensation is generally different from the persistent indoor moisture problem homeowners are usually concerned about.
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What If Condensation Is Between the Glass Panes?
This is different.
If you have double- or triple-glazed windows and moisture appears between the panes, rather than on a surface you can wipe, it can indicate that the sealed glazing unit is no longer functioning as intended.
Moisture has entered the space that is supposed to remain sealed.
Simply improving ventilation inside the room will not normally fix condensation trapped between sealed panes.
The glazing unit may need professional assessment and potentially replacement.
Is Window Condensation Harmful?
Occasional light condensation isn’t necessarily a major problem.
Persistent heavy condensation, however, deserves attention.
Repeated moisture can create damp conditions around:
- Window frames
- Seals
- Paint
- Plaster
- Curtains
- Nearby walls
If surfaces remain damp for extended periods, they can provide conditions in which mould can grow.
Therefore, regularly finding large amounts of water on your windows shouldn’t simply be ignored.
How Can You Reduce Condensation on Windows?
The basic goal is to reduce excess indoor moisture while improving ventilation and keeping surfaces from becoming unnecessarily cold.
Improve Ventilation
Ventilation allows humid indoor air to leave and be replaced with other air.
Use extractor fans when cooking and showering where available, and make sure vents aren’t unnecessarily blocked.
Opening windows for appropriate periods can also help exchange humid indoor air, depending on weather and circumstances.
Use Extractor Fans
Bathrooms and kitchens can generate substantial amounts of moisture.
Using extractor fans during and after showers or cooking can remove some of that moisture before it spreads through the rest of the home.
Keep Bathroom Doors Closed
Closing the bathroom door while showering can help prevent large amounts of steam from spreading into cooler rooms.
Use the bathroom’s ventilation system to remove the moisture where it is being generated.
Use Pan Lids When Cooking
Covering boiling pans where appropriate can reduce the amount of water vapour released into the kitchen.
It’s a small change, but it can help.
Be Careful When Drying Clothes Indoors
Wet laundry releases moisture as it dries.
If clothes need to be dried indoors, good ventilation is particularly important.
A suitable dehumidifier can also remove moisture from indoor air.
Maintain Reasonably Consistent Heating
Very cold rooms create colder surfaces, which can make condensation more likely.
Maintaining suitable heating alongside ventilation can help keep internal surfaces warmer.
Heating alone, however, isn’t a complete solution if large amounts of moisture remain trapped indoors.
Can a Dehumidifier Help?
Yes.
A dehumidifier removes water from indoor air, lowering humidity.
This can be particularly useful in homes where ventilation alone isn’t adequately controlling moisture or where drying laundry indoors is unavoidable.
Many dehumidifiers allow you to select a target relative humidity and automatically operate when moisture levels rise.
However, a dehumidifier shouldn’t automatically be treated as a substitute for investigating persistent dampness, leaks or inadequate ventilation.
What Is Relative Humidity?
Relative humidity describes how much water vapour is in the air compared with the maximum amount the air could hold at that temperature, expressed as a percentage.
For example, 60% relative humidity doesn’t mean that 60% of the air is water.
It means the air contains about 60% of the water vapour it could hold at its current temperature before reaching saturation.
Because temperature affects this relationship, relative humidity can change even when the actual quantity of moisture in the air hasn’t changed.
Can You Measure Humidity at Home?
Yes.
A small device called a hygrometer measures relative humidity.
Many inexpensive digital thermometers and home weather stations include a humidity sensor.
Monitoring humidity can help you identify patterns.
For example, you might notice humidity rising sharply after showers, cooking or drying clothes.
That information can help you work out which activities are contributing most to the moisture in your home.
Condensation Isn’t Always the Same as Damp
This distinction is important.
Water appearing around a window may be caused by condensation, but moisture problems in buildings can have other causes.
Examples can include:
Leaking pipes
Rainwater penetration
Roof or gutter problems
Plumbing faults
Building defects
If damp patches persist regardless of ventilation or appear in unusual locations, don’t automatically assume condensation is responsible.
Finding the actual source of the moisture is more important than simply wiping it away.
Why Does Mould Sometimes Appear Around Windows?
Mould needs suitable conditions to grow, and persistent moisture can provide them.
Window edges, seals and surrounding walls may remain cooler than other parts of a room.
If condensation repeatedly wets these areas and they don’t dry adequately, mould growth becomes more likely.
Reducing excess humidity and dealing with persistent moisture can therefore help reduce the conditions that encourage mould.
Existing significant mould problems may require appropriate cleaning or professional assessment depending on their extent and cause.
Why Do Bedroom Windows Get Condensation Overnight?
Bedrooms provide a good example of how condensation develops.
People release moisture into the air simply by breathing.
During the night, bedroom doors and windows may remain closed for several hours while outdoor temperatures fall.
Moisture accumulates in the room while the window glass becomes colder.
By morning, conditions can be ideal for condensation to form on the glass.
This doesn’t necessarily mean anything is wrong with the window itself.
Final Thoughts
Condensation on windows is fundamentally a matter of temperature and moisture.
Warm indoor air contains water vapour. When that air encounters a cold window and cools sufficiently, some of the vapour turns into liquid water.
A small amount from time to time is common, particularly during cold weather.
Persistent heavy condensation, however, can be a sign that too much moisture is accumulating indoors or that ventilation needs improvement.
Understanding where household moisture comes from—and giving it somewhere to escape—is one of the most effective ways to keep condensation under control.
Frequently Asked Questions
Why do my windows have condensation every morning?
Overnight, moisture from breathing and other sources can accumulate while windows become cold. When moist indoor air reaches the cold glass, condensation may form.
Should I wipe condensation off my windows?
Removing accumulated water can help prevent it from sitting on frames, seals and windowsills. However, recurring condensation also means it’s worth addressing the underlying humidity and ventilation.
Does opening windows reduce condensation?
Ventilation can help remove humid indoor air, although its effectiveness depends on indoor and outdoor conditions.
Will turning up the heating stop condensation?
Heating can warm surfaces and affect relative humidity, but heating alone doesn’t remove excess moisture. Ventilation and moisture control are also important.
Why is there condensation between my double-glazed windows?
Moisture trapped between panes can indicate failure of the sealed glazing unit and may require professional assessment.
Can a dehumidifier stop window condensation?
A dehumidifier can reduce indoor humidity and therefore reduce the likelihood of condensation, but any underlying leaks or building problems should still be investigated.
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