How to Reduce Condensation on Sash Windows

Condensation on sash and case windows is usually a symptom of warm, moisture-laden indoor air meeting a cold glass surface, rather than a fault with the window itself. The right fix depends on where the condensation is forming — inside the glass, between the panes, or on the outside.

Condensation on the inside of the glass (most common)

This is caused by excess humidity inside the home. Try these measures first:

• Ventilate daily. Open windows for 10–15 minutes each morning to exchange moist indoor air for drier outdoor air.
• Use extractor fans. Run kitchen and bathroom extractors during cooking and showering, and leave them on for 15–20 minutes afterwards.
• Maintain even heating. A steady indoor temperature keeps the glass warmer, reducing condensation.
• Use a dehumidifier. Aim to keep indoor relative humidity between 40% and 60%.
• Avoid drying clothes indoors — or use a vented tumble dryer or a dehumidifier if you must.

Condensation between the panes

If you have double-glazed sash windows, moisture between the panes usually means the sealed unit has failed. The insulated glass unit normally needs replacing — but the timber frame is often still perfectly serviceable, so this is a glass replacement rather than a new window.

Condensation on the outside

This is actually a sign that modern double glazing is insulating well. The outer pane stays cool because less heat is escaping from inside the home. It normally disappears as the day warms up and needs no action.

Improvements for traditional sash windows

If you have original timber sash and case windows, the following upgrades reduce condensation while preserving the character of a period property:

• Professional draught-proofing
• Secondary glazing
• Slimline heritage double glazing (where suitable)
• Repairing damaged putty or seals
• Ensuring the sash opens and closes correctly

Everyday habits that help

• Keep trickle vents open if your windows have them.
• Leave a small gap between curtains and the glass so warm air can circulate.
• Don't block radiators beneath windows.
• Wipe away condensation promptly to reduce the risk of mould and timber damage.

When to seek professional advice

Have the windows inspected if:

• Condensation persists despite good ventilation and humidity control.
• Black mould is appearing around the window.
• The timber shows signs of rot.
• Cold draughts are coming through the window.
• Double glazing has misted internally.

In many cases — especially with traditional sash and case windows — draught-proofing and secondary glazing deliver a significant reduction in condensation without needing full window replacement.

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