Fogged windows on the inside are common in the fall and winter. You see moisture on the glass, drops on the windowsill or even mold on the sealant edges. Sometimes it looks like the window is leaking, but most of the time it is condensation.

The short rule: windows fog up on the inside due to a combination of moist indoor air and cold glass. The solution is better ventilation, producing less moisture and, if necessary, installing better insulating glass. In this article we explain why windows fog up on the inside, when it is normal and what you can do to reduce condensation.

What does condensation on the inside of windows mean?

Condensation on the inside means that moisture from the indoor air settles on the glass. Warm air can contain more moisture than cold air. When warm moist air hits a cold window, the air cools. The moisture then precipitates as water droplets.

You mainly see this with:

  • single glazing
  • old double glazing
  • bedrooms
  • bathrooms
  • kitchens
  • poorly ventilated areas
  • homes with high humidity
  • windows without a radiator underneath
  • cold nights
  • many people in the house

Condensation is therefore not a glass leak. It is moisture from the home that ends up on the cold glass.

Image: close-up of condensation drops on the inside of a window, with a snow-covered garden behind it

Why do windows fog up especially in winter?

In winter the glass is colder. As a result, moist indoor air reaches the dew point more quickly: the point at which air can no longer hold moisture and water droplets are formed.

Several factors play a role in winter:

  • outside air is cold
  • glass surface becomes colder
  • windows remain closed more often
  • ventilation is less
  • heating is running
  • humidity inside can rise
  • laundry is dried indoors more often
  • bathrooms stay moist longer

That's why you often see condensation in the morning, especially on the bedroom window or bathroom window.

Is condensation on the inside normal?

A little condensation on the inside can be normal, especially on cold nights and high humidity. But a lot of condensation, wet windows every day or mold around the frame are signals that something needs to be improved.

Normal Not ideal
Light haze in the morning Wet windows every day
Small amount of moisture on cold night Water drops on the windowsill
Condensation that disappears quickly after ventilation Mold on sealant or frame
Moisture spots on the wall
Musty smell
Condensation that lasts for hours
Wooden frame that gets wet

For the signals on the right, it is wise to address ventilation, insulation and moisture sources.

Condensation on the inside or between double glazing?

It is important to determine where exactly the moisture is.

Where is the moisture? Meaning
On the inside of the window Condensation due to moist indoor air
On the outside of the window Often normal with good insulating glass
Between the glass sheets Leaky double glazing
At the glass edge or sealant Possible condensation, sealant problem or cold bridge
On the frame Cold frame or high humidity

You cannot brush away moisture between the glass sheets. Then the insulating glass is leaking and the window usually needs to be replaced. Read here how to recognize and replace a leak in double glazing.

How do you recognize condensation on the inside?

You can wipe away condensation on the inside with a cloth. It's on the side of the room.

Features:

  • moisture is on the inside of the glass
  • you can wipe it away
  • especially at the bottom of the window
  • often in the morning
  • more often in the bedroom or bathroom
  • can form droplets
  • windowsill can get wet
  • disappears after ventilation or heating

Does the moisture remain visible between the glass layers? Then it is not internal condensation.

Rule of thumb: If you can wipe away the moisture, then it is indoor condensation and it concerns your indoor climate. If the moisture is between the glass sheets and you cannot reach it, then the window is leaking.

Why do bedroom windows fog up?

Bedrooms often have condensation because people exhale moisture while sleeping. If the door is closed and the window remains closed, the humidity rises. The window is often cold, especially with single glazing or old double glazing.

Causes in bedrooms

  • breathing during sleep
  • door closed
  • little ventilation
  • cold windows
  • heating low or off
  • curtains closed against the window
  • laundry drying in the bedroom
  • poor air circulation

Solutions

  • ventilation grille open
  • Air the window briefly in the morning
  • bedroom door ajar
  • heating not extremely low
  • curtains not completely against wet glass
  • consider better insulating glass

Why do bathroom windows fog up?

Bathrooms produce a lot of moisture. Showering, bathing and hot water create high humidity. If moist air is not quickly removed, it will settle on cold glass.

Solutions:

  • fan on during and after showering
  • window open after showering
  • Only open the door when the ventilation is running
  • dry shower walls
  • dry wet surfaces
  • let ventilation run longer
  • remove condensation from the window
  • Check mold-sensitive areas

Bathrooms without good ventilation quickly develop condensation and mold.

Why do kitchen windows fog up?

A lot of moisture is created in the kitchen by cooking. Cooking without an extractor can quickly cause windows to fog up.

Sources of moisture in the kitchen:

  • cooking
  • boil water
  • dishwasher
  • washing dishes
  • wet cloths
  • steam
  • poor extraction
  • insufficient ventilation

Solutions:

  • use extractor hood
  • lid on pans
  • window or grille open
  • use mechanical ventilation
  • let wet cloths dry outside the kitchen
  • cooking with sufficient air supply

Condensation from laundry drying indoors

Drying laundry in the house puts a lot of moisture in the air. That moisture seeks out cold surfaces, such as windows. You notice this:

  • foggy windows
  • moist air
  • musty smell
  • mold on window frames
  • condensation in bedrooms
  • wet walls

Tips:

  • dry laundry outside if possible
  • use a dryer with good drainage or a heat pump dryer
  • ventilate extra
  • dry in a separate, well-ventilated area
  • use a dehumidifier if necessary

Drying laundry indoors is one of the biggest causes of high humidity.

Condensation due to insufficient ventilation

Ventilation removes moist air and brings in drier air. Without ventilation, moisture remains in the house.

Signs of insufficient ventilation:

  • windows often fog up
  • musty smell
  • mold on windows
  • damp bathroom
  • condensation remains for a long time
  • heavy air in the house
  • headache or feeling of shortness of breath
  • clothing or bedding feels clammy

Ventilating does not mean that you have to open windows wide all day. Continuous basic ventilation via grilles or mechanical ventilation is often more important. A ventilation grille in the glass is a neat way to always maintain sufficient air supply; You will find a suitable model at our ventilation grilles.

Ventilate without much heat loss

Many people keep ventilation grilles closed because they are afraid of heat loss. But insufficient ventilation can actually cause moisture problems and an unhealthy indoor climate.

Smart ventilation:

  • leave grilles ajar
  • breathe briefly and powerfully
  • use mechanical ventilation
  • extra ventilation in the bathroom
  • Air the bedroom daily
  • extra extraction after cooking and showering
  • place furniture slightly away from cold outside walls

Dry air also warms up more pleasantly than moist air.

Does heating help against condensation?

Yes, heating helps to some extent. Warm air can retain more moisture and warm surfaces are less likely to condense. But heating without ventilation is not enough. Then the moisture remains in the room.

Good approach:

  • sufficient heating
  • ventilate at the same time
  • limit moisture sources
  • improve cold surfaces
  • Remove condensation daily

Do not set the heating to extremely low levels in damp areas. Cold rooms are more likely to develop condensation and mold. A cold window can also cause cold trap, making a room feel drafty and uncomfortable.

Does HR++ glass help prevent condensation on the inside?

HR++ glass often helps against condensation on the inside, because the inner pane remains warmer than with single glazing or old double glazing. This means that moisture precipitates less quickly.

Advantages:

  • warmer glass surface
  • less condensation
  • less cold
  • more comfort
  • lower heat demand

Please note: HR++ glass does not completely solve moisture problems. If the humidity is very high, HR++ glass can also fog up. Ventilation remains necessary. Read more about what exactly HR++ glass is, or customize your HR++ window immediately.

Does triple glazing help even better?

Triple glass insulates better than HR++ glass. The inside remains even warmer, which reduces the chance of condensation on the inside.

Triple glass is especially interesting for:

  • new construction
  • large glass areas
  • very well insulated homes
  • homes with underfloor heating
  • high desire for comfort

Triple glazing does not always fit in existing window frames due to its thickness and weight. View when triple glazing is and is not useful or put together your custom triple glazing.

Does vacuum glass help against condensation?

Vacuum glass can help against condensation on the inside, especially in existing or older window frames where thick glass does not fit. Because vacuum glass insulates well, the inside of the glass remains warmer.

Interesting at:

  • old wooden frames
  • monumental homes
  • narrow rebates
  • renovation
  • replace single glass
  • comfort improvement without a bulky package

Here too, ventilation remains important. Are you unsure between the options? Compare vacuum glass and HR++ glass or view the full overview to compare insulating glass.

Condensation on new HR++ glass on the outside

Sometimes people see condensation on the outside of new HR++ glass and think something is wrong. But condensation on the outside is often a sign that the glass insulates well.

With well-insulating glass, the outside window remains colder, because heat from the home leaks out less. As a result, outside moisture can condense on the outside window. That is different from condensation on the inside. You can find more information in our article about condensation on windows inside, outside and between glass.

Condensation between double glazing

Condensation between the glass sheets usually means that the insulating glass is leaking. The seal of the window is then no longer good and moisture has entered the cavity.

Recognition:

  • moisture is between the glass layers
  • you can't wipe it away
  • blurry spots in the cavity
  • condensation remains visible
  • sometimes white deposits
  • insulation value decreases

The solution is to replace the window. You cannot clean or dry the cavity properly. See how to have double glazing replaced and immediately put together your new custom insulation window.

What can you do immediately against foggy windows?

Quick Actions:

  1. Remove condensation with a cloth.
  2. Open the ventilation grille.
  3. Ventilate the room briefly.
  4. Increase the mechanical ventilation.
  5. Heat the room slightly.
  6. Dry the wet windowsill.
  7. Dry the shower.
  8. Use the extractor hood when cooking.
  9. Do not dry laundry in a poorly ventilated area.

Important: do not leave water on wooden frames or windowsills every day. That can cause damage.

Measure humidity

A hygrometer measures the relative humidity in your home. This helps to see if your home is too humid. If the humidity is often high, you will get condensation and mold more quickly. High humidity for long periods of time is particularly unfavorable, especially in winter.

Place a hygrometer in the:

  • bedroom
  • living room
  • bathroom
  • kitchen
  • space where laundry dries

This way you can see better when ventilation is necessary.

Use dehumidifier

A dehumidifier can help in rooms where there is a lot of moisture and ventilation is difficult.

Useful for:

  • laundry drying inside
  • cellar
  • poorly ventilated area
  • temporary moisture problems
  • condensation in bedroom
  • bathroom without window

A dehumidifier is not a substitute for good ventilation, but can help temporarily.

Mold due to condensation

If windows are often wet, the sealant, frame and wall also become damp. That can cause mold.

NB:

  • black dots on the kit
  • mold in corners
  • damp windowsill
  • peeling paint
  • musty smell
  • dark spots on the wall
  • soft sealant
  • wood rot in window frames

Mold means that the moisture problem is structural. Cleaning alone is not enough. Read how to remove mold in the sealant edge.

Image: black mold spots on a sealant edge along a window frame

Wooden frames and condensation

Wooden frames cannot withstand long-term moisture. If condensation runs on the wood every day, it can damage paint and cause wood rot.

Prevent damage by:

  • Remove condensation daily
  • good ventilation
  • maintain the paintwork
  • check sealant seams
  • do not leave water on the windowsill
  • check glazing beads and sills

When moisture returns, better glass or better ventilation is often necessary.

Common mistakes with foggy windows

  1. Leave ventilation grilles closed — this keeps moisture inside.
  2. Only heating without ventilation — warm, moist air will still remain in the house.
  3. Leave condensation behind — it can damage sealant, paint and wood.
  4. Thinking that every foggy window is leaking — condensation on the inside is usually internal moisture, not a glass leak.
  5. Dry laundry indoors without ventilation — this brings a lot of moisture into the house.
  6. Leave curtains tightly against wet glass — mold can grow behind them.
  7. Don't look at glass quality — single glazing and old double glazing give condensation more quickly.

Selection aid: what do you do if there is condensation on the inside?

Situation Best solution
Light condensation in the morning Ventilate and dry
Wet windows every day Improve ventilation
Condensation in bedroom Open grille, air, heat
Condensation in bathroom Mechanical ventilation and drying
Single glazing HR++ glass, vacuum glass or monument glass
Old double glazing HR++ consider glass
Condensation between glass sheets Replace window
Mold in sealant Remove mold or replace sealant
Wooden frame gets wet Remove condensation immediately and address the cause
Laundry dries inside Additional ventilation or dehumidification

Checklist for foggy windows

Check these points:

  • Is the moisture on the inside?
  • Can you wipe away the moisture?
  • Is there moisture between the glass?
  • Does it mainly cover bedrooms or the bathroom?
  • Is there sufficient ventilation?
  • Are the ventilation grilles open?
  • Is laundry dried indoors?
  • Do you cook a lot without extraction?
  • Is the glass single, old double or HR++?
  • Is there mold in the sealant or frame?
  • Does condensation remain for a long time?
  • Is the room sufficiently heated?
  • Is a hygrometer useful?
  • Is better insulating glass possible?

Conclusion

Windows fog up on the inside because moist indoor air hits cold glass. This is especially noticeable in winter, with single glazing, old double glazing and rooms with a lot of moisture, such as bedrooms, bathrooms and kitchens.

The solution consists of three steps: ventilate, reduce moisture and improve cold glass surfaces. Open grilles, use extraction, dry wet areas and heat rooms sufficiently. With single glazing or old double glazing, HR++ glass, triple glass or vacuum glass can make a big difference.

You can wipe away condensation on the inside, but not moisture between double glazing. If there is moisture between the glass sheets, the window is leaking and usually needs to be replaced. By taking condensation seriously you prevent mold, wood rot and permanent damage to window frames.

Do you want to switch from single or old double glazing to better insulating glass? Calculate your custom glass immediately, put together your custom insulating glass or contact with us for personal advice.