Why Do Windows Fog Up? Condensation Explained

You wake up on a cold morning and notice that your bedroom window is covered in tiny droplets of water.

Perhaps the bathroom mirror becomes completely cloudy after a hot shower. Or your kitchen windows mist up while you’re cooking.

Where did that water come from?

It didn’t leak through the glass.

In most cases, the answer is condensation.

Condensation happens when water vapour in the air changes into liquid water. It’s an ordinary physical process, but inside a home, persistent condensation can sometimes signal excess moisture, insufficient ventilation or surfaces that are becoming particularly cold.

Understanding why it happens can help you decide when a foggy window is harmless — and when you should do something about it.

What Is Condensation?

Water can exist in several physical states.

We’re most familiar with:

solid — ice

liquid — water

gas — water vapour

The air around us contains water vapour even when we can’t see it.

When conditions are right, some of that invisible water vapour can change into liquid droplets.

That change from gas to liquid is called condensation.

The droplets on a cold window are therefore not water somehow appearing from nowhere.

The water was already present in the surrounding air as water vapour.

Why Does a Window Fog Up?

Imagine the air inside your home is warm and contains moisture.

Now imagine that same air touching a cold window.

The layer of air immediately next to the glass cools.

Cooler air cannot maintain the same amount of water vapour under the same conditions as warmer air.

If the air next to the glass cools sufficiently, it reaches a point where water begins condensing onto the surface.

Tiny droplets form.

Those droplets scatter light, making the window look:

misty,
cloudy,
wet,
or “fogged up.”

What Is the Dew Point?

The dew point is extremely useful for understanding condensation.

It is the temperature to which air must be cooled, under particular conditions, for it to become saturated with water vapour.

If a surface falls below the dew point of the surrounding air, condensation can form on that surface.

So instead of thinking:

“Cold creates water,”

think:

“A sufficiently cold surface causes water vapour already in the air to condense.”

That’s much closer to what’s actually happening.

Where Does Indoor Moisture Come From?

A surprising amount of water vapour can be produced by ordinary household activities.

Sources include:

showering,
bathing,
cooking,
boiling water,
drying clothes indoors,
breathing,
cleaning,
and certain appliances.

People themselves continuously release moisture through breathing and perspiration.

So even a bedroom with no obvious source of water can become relatively humid overnight.

Why Are Bedroom Windows Wet in the Morning?

This is particularly common during cold weather.

Several things happen overnight.

People spend hours breathing and releasing moisture into the room.

The bedroom door and windows may remain closed.

Ventilation may be limited.

At the same time, the outside temperature falls, making the inside surface of the window colder.

Eventually:

warm moist indoor air + cold glass = condensation.

That’s why a window can be dry when you go to sleep but wet when you wake up.

What Is Relative Humidity?

Humidity describes moisture in the air, but relative humidity gives us more useful information about how close the air is to saturation at a particular temperature.

It’s expressed as a percentage.

For example:

50% relative humidity

doesn’t simply mean “half the air is water.”

It describes the amount of water vapour present relative to the maximum amount the air could contain at that temperature before reaching saturation.

Temperature matters enormously.

Why Does Relative Humidity Rise When Air Cools?

Suppose a quantity of air contains a certain amount of water vapour.

If that air cools without losing moisture, its relative humidity rises.

Continue cooling it and eventually it can reach:

100% relative humidity.

Cool it further under suitable conditions and some water vapour condenses into liquid.

This is why cold surfaces are so important in household condensation.

Why Does a Bathroom Mirror Fog Up?

A hot shower introduces a large amount of water vapour into the bathroom.

The room’s humidity rises rapidly.

The mirror may be significantly cooler than the warm, moist air around it.

When moist air contacts the mirror and cools below its dew point, tiny water droplets form.

Those microscopic droplets scatter light.

Instead of a clear reflection, you see a cloudy surface.

It’s the same fundamental process as a fogged window.

Why Does the Mirror Clear Again?

After the shower ends, conditions gradually change.

Ventilation may remove humid air.

The mirror may warm.

The room’s humidity may fall.

The droplets can then evaporate back into the air.

Once the water layer disappears, the mirror becomes clear again.

So fogging and clearing involve the same water moving between vapour and liquid states.

Why Do Kitchen Windows Fog Up?

Cooking can release substantial moisture.

Common sources include:

boiling pasta,
simmering soups,
steaming vegetables,
kettles,
dishwashers,
and other cooking processes.

When that moisture isn’t removed effectively, indoor humidity rises.

If windows are cold enough, condensation forms.

This is why extractor fans and ventilation can make such a noticeable difference while cooking.

Why Does Drying Clothes Indoors Matter?

Wet clothing contains water.

When the clothes dry, that water doesn’t disappear.

Much of it evaporates into the surrounding air.

If you’re drying a large load of washing indoors, you’re transferring a significant quantity of moisture from the fabric into your home.

Without sufficient ventilation or dehumidification, indoor humidity can rise.

That can increase condensation on:

windows,
walls,
ceilings,
and other cold surfaces.

Why Is Condensation Worse in Winter?

Winter creates an ideal temperature difference.

Inside:

warm air

Outside:

cold air

The window sits between them.

Its interior surface can become much colder than the room air.

The colder that surface becomes, the easier it is for its temperature to fall below the indoor air’s dew point.

That’s why condensation can suddenly become noticeable in autumn and winter even though you use the same house throughout the year.

Why Do Some Windows Condense More Than Others?

Not every window has the same thermal performance.

Factors include:

glazing type,
frame material,
insulation,
seals,
indoor humidity,
outdoor temperature,
curtains and blinds,
and air circulation around the glass.

An older single-glazed window can have a much colder interior surface than a well-performing modern insulated window.

That makes condensation more likely.

Single Glazing vs Double Glazing

A single pane of glass provides relatively little thermal separation between indoors and outdoors.

During cold weather, its indoor surface can become very cold.

Double glazing uses two panes separated by a sealed space.

This reduces heat transfer and helps keep the interior pane warmer.

A warmer interior surface is less likely to fall below the room’s dew point.

So double glazing can reduce condensation on the room-facing surface.

But it doesn’t guarantee that condensation will never occur.

If indoor humidity becomes high enough, even a good window can develop moisture.

What About Triple Glazing?

Triple glazing adds another pane and insulating space.

Depending on its design and installation, it can further improve thermal performance.

Again, the principle is straightforward:

warmer interior glass → lower chance of surface condensation.

But window performance is only one part of the equation.

Indoor moisture levels still matter.

Why Is Condensation Sometimes on the Outside?

This surprises homeowners with efficient windows.

You may install good double or triple glazing and then discover moisture on the outside of the window in the morning.

That can actually indicate that the glazing is insulating effectively.

Because less indoor heat reaches the exterior pane, the outside glass can remain cold.

If outdoor conditions cause that surface to fall below the outdoor air’s dew point, condensation can form there.

External condensation usually isn’t the same problem as persistent moisture on indoor surfaces.

Why Is There Condensation Between the Glass Panes?

This is different.

Modern insulated glazing units are designed with a sealed space between panes.

If moisture repeatedly appears inside that sealed cavity, the unit’s seal may have failed.

Moisture has entered an area that should normally remain sealed.

Cleaning the inside or outside surface won’t remove condensation trapped between the panes.

A failed glazing unit may need professional assessment or replacement.

Is Window Condensation Dangerous?

A little temporary condensation isn’t automatically a serious problem.

For example, a bathroom mirror fogging during a shower is completely expected.

The concern is persistent moisture.

If windows, frames or surrounding walls remain wet regularly, that moisture can contribute to conditions favourable to:

mould,
damaged paint,
deteriorating timber,
staining,
peeling wallpaper,
and other moisture-related problems.

The objective isn’t necessarily to eliminate every droplet from your home.

It’s to prevent excessive moisture from remaining on vulnerable surfaces for prolonged periods.

Condensation vs a Leak

These problems can sometimes look similar.

Condensation usually appears where warm humid air contacts cold surfaces.

A leak, however, involves water entering from another source.

Possible signs of a leak can include:

moisture appearing after rain,
isolated damp patches,
damaged roofing or plumbing,
water staining,
or moisture in locations not easily explained by surface temperature.

Don’t automatically assume every damp patch is condensation.

Likewise, don’t assume every wet window means the window itself is leaking.

Finding the source matters.

What About Rising Damp?

Rising damp is another different moisture mechanism.

It involves moisture moving upward through porous building materials from the ground.

Condensation originates from water vapour in the air turning into liquid on sufficiently cold surfaces.

These issues require different solutions.

Misidentifying the cause of dampness can lead to spending money on work that doesn’t address the actual problem.

Can Condensation Cause Mould?

Persistent condensation can contribute to conditions in which mould grows.

Mould needs suitable:

moisture,
temperature,
nutrients,
and time.

Many building materials and household surfaces provide enough organic material or dust for mould to use.

Moisture is often the factor homeowners can most readily control.

This is why recurring condensation shouldn’t simply be wiped away indefinitely without considering why it’s happening.

Why Does Mould Often Appear in Corners?

External corners, areas behind furniture and poorly insulated surfaces can be colder than the surrounding room.

Air circulation may also be limited.

That creates a combination of:

cold surface + trapped humid air + limited drying.

Those areas can therefore be particularly vulnerable to condensation and mould.

Furniture Can Affect Condensation

A large wardrobe pushed tightly against a cold external wall can reduce air movement behind it.

The wall may remain colder, while moisture becomes trapped in the small space.

Moving furniture slightly away from cold external walls can sometimes improve air circulation.

But if there’s a serious underlying damp or insulation problem, rearranging furniture alone won’t solve it.

How to Reduce Condensation on Windows

The solution is based on controlling three things:

moisture

ventilation

and

surface temperature.

Let’s look at each.

1. Use Ventilation

Ventilation helps replace humid indoor air with air that can carry moisture away.

Depending on your home, this can involve:

opening windows appropriately,
using trickle vents,
running bathroom extractor fans,
using kitchen extraction,
or relying on a mechanical ventilation system.

Don’t permanently block ventilation openings simply because they let some cool air in.

They may be serving an important moisture-control function.

If your windows have trickle vents, avoid blocking them without understanding their purpose. These small openings provide background ventilation, helping moisture escape even when windows are closed. Blocking them can make condensation worse, particularly in rooms where humidity is already high.

2. Use Bathroom Extraction

During and after a shower or bath, use the bathroom extractor fan if you have one.

Keep the bathroom door appropriately closed while generating large amounts of steam so moisture doesn’t immediately spread throughout the rest of the home.

Allow extraction to continue long enough to remove excess humidity.

3. Use Kitchen Extraction

When cooking, use an extractor that actually removes moist air outdoors where available.

Some cooker hoods merely recirculate air through filters.

Recirculation can help with grease and odours depending on the system, but it doesn’t necessarily remove water vapour from the building.

Know what type of extractor you have.

4. Cover Pans

Putting lids on pans can reduce the amount of water vapour released into the kitchen.

It can also reduce the energy needed to maintain cooking temperatures.

It’s a small habit, but repeated every day it can make a difference.

5. Be Careful When Drying Clothes Indoors

Where practical, avoid spreading moisture from wet laundry throughout the entire home.

If clothes must be dried indoors, consider using a well-ventilated space or an appropriate dehumidifier.

Don’t simply close a large amount of wet laundry into an unventilated room and expect the moisture to disappear.

6. Keep the Home Reasonably Heated

Very cold interior surfaces increase condensation risk.

Maintaining reasonable indoor temperatures can help keep surfaces warmer.

However, simply turning up the heating while continuously adding large amounts of moisture isn’t an efficient solution.

Heating and moisture control need to work together.

7. Improve Air Circulation

Avoid completely trapping cold surfaces behind heavy furniture where possible.

Curtains and blinds can also affect airflow around windows.

If condensation is severe behind closed blinds every morning, reduced air circulation may be contributing.

8. Wipe Up Persistent Water

If substantial condensation has already formed, remove standing water rather than allowing it to remain on:

wooden frames,
seals,
sills,
walls,
or painted surfaces.

Wiping the water doesn’t solve the underlying humidity problem, but it can reduce how long vulnerable materials remain wet.

Should You Buy a Dehumidifier?

A dehumidifier removes water from indoor air.

It can be useful in homes or rooms where humidity is difficult to control through ventilation alone.

The appliance draws in air, removes moisture and collects water in a tank or sends it to a drain.

But a dehumidifier shouldn’t automatically replace investigating the underlying cause.

If the moisture is coming from:

a plumbing leak,
roof damage,
failed drainage,
or another building defect,

you need to address that problem.

A dehumidifier can’t repair a leaking pipe.

What Is a Hygrometer?

A hygrometer measures humidity.

Small digital temperature-and-humidity meters are inexpensive and can help you understand what’s happening inside different rooms.

Instead of guessing that a bedroom “feels humid,” you can observe how relative humidity changes:

overnight,
after showering,
while cooking,
when drying clothes,
or after opening windows.

That makes moisture management much more objective.

Why Opening a Window Can Help Even When It’s Damp Outside

This seems contradictory.

If outdoor air feels damp, why bring it indoors?

The key is that relative humidity depends on temperature.

Cold outdoor air can contain relatively little total water vapour even when its relative humidity is high.

When that air enters the house and warms, its relative humidity can fall considerably.

It can then absorb additional indoor moisture before being ventilated out again.

This is one reason short periods of effective ventilation can help during cold weather.

Does Turning the Heating Off Prevent Condensation?

Usually not.

Allowing the home and its surfaces to become very cold can actually increase the likelihood that surfaces fall below the dew point.

However, heating alone isn’t sufficient either.

The more useful strategy is to combine:

reasonable heating + controlled moisture production + appropriate ventilation.

Why Does Your Car Windshield Fog Up?

The same physics applies.

People inside the car release moisture through breathing.

Wet clothing, shoes and umbrellas add more.

The glass may be cold because of outdoor conditions.

Warm humid air contacts the cold windshield and water condenses.

The result is a foggy interior surface.

Why Do Cold Drinks “Sweat”?

Take a cold drink from the refrigerator on a warm humid day.

Soon, droplets appear on the outside of the glass or can.

The drink isn’t leaking through the container.

The cold surface cools nearby air below its dew point.

Water vapour from the surrounding air condenses on the outside.

Exactly the same physical principle explains:

wet windows, foggy mirrors and a “sweating” cold drink.

Why Does Your Breath Look Like Mist in Winter?

The air you exhale is warm and moist.

When it mixes with sufficiently cold outdoor air, some of the water vapour condenses into tiny droplets.

Those droplets scatter light and become visible as a small cloud.

Again, you’re seeing a phase change involving water.

Read:Why Does a Mirror Fog Up After a Shower?

When Should You Investigate Further?

Occasional window condensation is common.

But consider investigating if you notice:

heavy condensation every day,
persistent mould,
damp patches that don’t match normal condensation patterns,
peeling paint or wallpaper,
rotting window frames,
musty smells,
water between sealed glazing panes,
or moisture appearing after rainfall.

Those signs may indicate that more than ordinary temporary condensation is happening.

A qualified building professional may be appropriate if the source isn’t clear.

For further practical advice, see the Energy Saving Trust’s guide to ventilation. It explains how ventilation helps reduce condensation and why removing moisture with a dehumidifier is not a substitute for fresh air.

Condensation Is a Symptom, Not the Water Source

This is perhaps the most useful way to think about the issue.

When water appears on your window, the window usually hasn’t created the moisture.

The glass has simply provided a cold surface where moisture already present in the air can become visible.

So asking:

“How do I stop the window getting wet?”

is less useful than asking:

“Where is the moisture coming from, and why is this surface cold enough for it to condense?”

That question gets much closer to the real cause.

The Bottom Line

Windows fog up because water vapour in the air condenses onto a surface that is cold enough.

Warm indoor air can contain substantial moisture from everyday activities such as:

breathing, cooking, showering and drying clothes.

When that humid air reaches cold glass and cools to its dew point, water vapour turns into tiny liquid droplets.

Those droplets create the familiar foggy appearance.

A small amount of temporary condensation is normal.

Persistent heavy condensation, however, can contribute to mould and damage and may indicate that your home has too much moisture, inadequate ventilation, cold surfaces or another dampness problem.

The solution is rarely one magic product.

Instead, think in terms of:

reducing excess moisture + providing appropriate ventilation + keeping vulnerable surfaces reasonably warm.

Once you understand the dew point, foggy windows stop being mysterious.

They’re simply physics happening on your glass.

Frequently Asked Questions

Why do my windows fog up inside?

Indoor condensation forms when humid room air contacts glass cold enough to cool the air below its dew point, causing water vapour to become liquid droplets.

Why are my windows wet every morning?

People release moisture while sleeping, while bedroom ventilation may be limited overnight. During cold weather, the glass can become cold enough for that moisture to condense.

Does condensation mean my windows are faulty?

Not necessarily. Indoor condensation can occur even on functioning windows if humidity is high enough. However, persistent moisture between sealed panes can indicate a failed glazing unit.

Can condensation cause mould?

Persistent moisture can create conditions favourable to mould growth, particularly on cold surfaces with limited air circulation.

Will opening windows reduce condensation?

Appropriate ventilation can help remove humid indoor air. The effectiveness depends on indoor and outdoor conditions and the building’s ventilation system.

Will a dehumidifier stop window condensation?

A dehumidifier can reduce indoor humidity and therefore reduce condensation risk, but it won’t repair leaks or other building defects.

Why does condensation form on the outside of double glazing?

Efficient glazing can keep the exterior pane relatively cold because little indoor heat reaches it. Under suitable outdoor conditions, moisture can therefore condense on the exterior surface.

What’s the difference between condensation and damp?

Condensation is one possible cause of dampness. Moisture can also come from leaks, water penetration and other building problems, so persistent damp should be correctly diagnosed.