What Causes Condensation — And What You Can Do About It

Waking up to misty bedroom windows is incredibly common in UK homes, but it’s more than just “cold glass.” If you’re noticing droplets on the sill, fog on your double glazing or streaks of water running down the panes, the real issue is usually a mix of high indoor humidity and poor ventilation.

The good news: once you understand what causes condensation, you can fix it.

What is condensation?

Condensation forms when warm, moisture-laden air hits a cold surface and cools down. As the air cools, it can no longer hold as much water, so it releases it as droplets.

Many people assume double glazing eliminates condensation altogether, but that’s a misconception. Even modern windows still have cold surfaces — and if the indoor air is humid enough, moisture will form on them.

Why bedrooms are condensation hotspots?

Bedrooms combine several everyday factors that make condensation almost inevitable:

1. You breathe out moisture all night

Every adult releases water vapour while sleeping. In a closed room, humidity builds up surprisingly quickly.

2. Warm air meets cold glass

In winter, heating raises the temperature — and the amount of moisture the air can hold. When that warm air reaches the cold window surface, it cools rapidly and deposits moisture.

3. Poor ventilation

Modern homes are well insulated and tightly sealed. Great for efficiency, not so great for airflow. Moisture from cooking, showers, drying laundry and breathing often gets “trapped,” eventually settling on the coldest surface: the windows.

Because bedrooms stay closed for long periods, they’re prime locations for this type of moisture build-up.

What the location of your condensation tells you

Where the moisture appears can reveal the root cause.

Condensation on the outside of the window

This is actually a good sign. It shows your double glazing is performing well and retaining heat. No action needed — it will clear as the outside temperature rises.

Condensation on the inside of the window

This is the classic sign of excess indoor humidity and not enough ventilation. If left untreated, it can lead to black mould on frames and nearby walls.

Condensation between the panes

This means the sealed unit has failed. Moisture has entered the air gap and the insulating gas has usually escaped. The unit will need repairing or replacing.

How to reduce bedroom condensation

You’re aiming for two things:

1. Lower the humidity, and
2. Improve the airflow.

Here’s what works.

Short-term fixes

• Wipe condensation each morning
Use a microfibre cloth or window vac. This prevents mould from taking hold.

• Use a dehumidifier
Helpful for damp rooms, though they can consume a noticeable amount of electricity if run constantly.

Long-term fixes

1. Increase ventilation

• Open windows for 10–15 minutes each morning to flush out humid air.
• Keep trickle vents open if your windows have them.
• For recurring problems, consider Positive Input Ventilation (PIV) to provide steady, filtered airflow throughout the home.

2. Control humidity at the source

• Avoid drying laundry in bedrooms.
• Use extractor fans during and 20 minutes after cooking or showering.
• Keep bathroom and kitchen doors closed when generating steam.

3. Optimise heating and airflow

Leave small gaps behind furniture to encourage air movement and prevent cold spots.

Maintain a consistent indoor temperature to prevent cold surfaces.

Ensure curtains don’t trap cold air against the glass.

Why double glazing alone won’t solve condensation

Double glazing reduces heat loss — but it doesn’t stop condensation if your indoor air is too humid. In well-insulated, draught-proof homes, moisture can linger indoors unless there’s proper ventilation.

Think of it this way:
Good windows keep the heat in. Good ventilation keeps the moisture out.
You need both for a healthy, comfortable home.

Condensation on bedroom windows isn’t just a nuisance — it’s a sign that humidity and airflow need attention.

When you see condensation:

• Check where it’s forming (outside, inside, or between panes).
• Improve ventilation through window opening, trickle vents or PIV.
• Reduce moisture sources like drying clothes indoors or unvented showers.
• Use good habits like steady heating and keeping air moving behind furniture.

Double glazing helps, but it’s not a magic bullet. Managing humidity and ensuring steady airflow is the real key to clear windows… and a healthier home.