Condensation on Windows and What It Means

Water running down the inside of a window on a cold morning is one of the most common household complaints, and it is a symptom rather than a fault. What it indicates determines what to do about it.

Why It Forms

Air holds water vapour, and how much it can hold depends on its temperature. Warm air holds more; cold air holds less.

When warm humid room air meets a cold glass surface, it cools, its capacity drops, and the excess water condenses out onto the glass. The window is not leaking; it is simply the coldest surface in the room.

Where Blinds Fit In

This is worth being honest about. An insulating shade keeps the room warmer and makes the glass colder, because it reduces the heat reaching the pane.

That can increase condensation on the glass behind a closed shade rather than reduce it. It is a real effect and it surprises people.

Manufacturers who publish the limitations of their products alongside the benefits, as at lazblinds.net, are more useful than those who imply a shade solves every window problem, because condensation is a ventilation issue rather than a covering one.

Where the Moisture Comes From

• Cooking, particularly boiling and simmering without an extractor
• Showers and baths, which release a substantial volume
• Drying laundry indoors, which is a major and underestimated source
• Breathing, which in a closed bedroom overnight is significant
• Unvented tumble dryers and portable gas heaters

The Three Remedies

Reduce moisture at source: extractor fans during and after cooking and showering, lids on pans, laundry dried in one ventilated room rather than throughout the house.

Increase ventilation: trickle vents open, windows opened briefly even in cold weather, which is more effective than it feels because cold air is dry.

Raise surface temperature: better glazing, or keeping the room from getting very cold overnight.

Managing It With Shades

Raise blinds during the day so air can circulate at the glass and moisture can evaporate.

Leave a small gap at the bottom overnight where condensation is severe. Wipe the glass and sill in the morning rather than leaving water to sit, since standing water on a sill is what causes rot and mould.

When It Is a Bigger Problem

Condensation between panes of a sealed unit means the seal has failed, and no amount of ventilation addresses it. That is a glazing repair.

Damp on walls rather than glass, particularly at floor level or in a pattern, may be rising or penetrating damp rather than condensation. Those are building problems requiring professional assessment.

Mould

Persistent condensation produces mould on frames, sealant and adjacent walls. Extensive damp and mould in a home is a recognised health concern, particularly for children, older people and anyone with asthma.

Where mould is widespread or keeps returning after cleaning, that warrants proper assessment. In rented properties it is generally the landlord’s responsibility and should be reported in writing.

The Cheapest Fix

Fifteen minutes of ventilation in the morning, particularly in bedrooms, removes most of the overnight moisture before it can cause a problem.