Condensation on windows in winter UK

Why winter makes window condensation worse

In winter, the glass of a window is the coldest surface in a heated room because it is in direct contact with cold outside air. Warm indoor air holds moisture in suspension. When that warm, moist air reaches the cold window glass, it cools down. Cooler air cannot hold as much moisture, so the excess water vapour condenses into liquid water on the glass.

This is physics, not a sign of a dirty house or poor habits. The question is whether the amount of condensation is manageable or excessive.

Single-glazed windows are the worst affected the inner pane stays close to outside temperature. North-facing windows are worse because they receive no direct sunlight to warm them during the day. Poorly maintained double-glazed windows with failed seals can also have cold inner panes, making them nearly as bad as single glazing.

Normal vs excessive window condensation

Some morning window condensation in winter is normal. It becomes a problem when:

• Condensation does not clear within an hour or two of waking, even with ventilation
• Water pools on window sills and soaks into the surrounding plaster or wooden frame
• Black mould grows on the rubber window seal, the UPVC frame, or the wall below the window
• Condensation runs down the glass and causes damp patches on the wall or floor below
• You need to squeegee the windows every morning just to see through them
• More than two panes of glass in the same room are fogged up every morning

If window condensation clears quickly with a bit of ventilation and does not leave damp marks, it is within normal range. If it causes damage or takes hours to clear, it is worth addressing more seriously.

How to reduce window condensation in winter

These steps make a measurable difference without requiring major home changes:

• Open a window slightly while heating is on: Even a 5cm gap on one window in the room allows moisture to escape. It sounds counterintuitive in winter but the small heat loss is far less damaging than the damp caused by persistent condensation.
• Use extractor fans ? in kitchen and bathroom every time you cook or shower, and leave them running for 15 minutes after. These are the two biggest sources of moisture in most UK homes.
• Wipe condensation from glass each morning: Use a squeegee, a dry cloth or kitchen roll. Do not let it evaporate back into the room air wiping removes the moisture rather than redistributing it.
• Keep furniture away from windows: Sofas, curtains and bookcases against external walls block airflow and create cold spots where condensation is worse and harder to dry out.
• Run a dehumidifier ? in rooms with persistent window condensation. Set the hygrostat to 50-55% and run it overnight in bedrooms where condensation is worst.
• Open windows during the day: Even on cold days, opening all windows for 10-15 minutes in the morning clears stale, moisture-laden air and replaces it with drier outside air. Do this before heating comes on if possible.

Does double glazing stop window condensation?

Double glazing reduces window condensation significantly because the inner pane stays warmer than a single-glazed pane, making it less likely to reach the dew point. However, double glazing does not eliminate condensation entirely.

If the room humidity is high enough from cooking, showering or indoor drying condensation can still form on the inner pane, particularly on north-facing windows or windows with failed seals. The gas between the panes in a double-glazed unit is supposed to provide insulation; if the seal has failed, the unit loses this and the inner pane gets almost as cold as the outer one.

The most effective single step is reducing room humidity, regardless of window type. Even single glazing will stay mostly clear of condensation if room humidity stays below 55%.

Window condensation and mould

The rubber seals around windows and the UPVC frames themselves are the places where window condensation causes the most damage. When condensation runs down the glass and sits in the seal or frame, it creates the damp conditions where black mould grows. This is particularly common on north-facing windows and in bedrooms where ventilation is lowest.

To prevent mould on window frames: wipe the rubber seals dry each morning, keep the frame wiped down, and ensure the seal is not holding pooled water. A small dehumidifier in the room reduces the amount of condensation forming in the first place. If mould does grow on seals, treat it with a mould remover spray but stop it returning by fixing the humidity problem, not just the mould.