Condensation inside windows forms when moisture-laden indoor air meets a glass surface that is cold enough for water vapour to condense into droplets. It is especially common on single-glazed sash windows in colder weather, but the window is rarely the only cause: indoor humidity and ventilation matter too.
If the problem keeps returning despite sensible moisture control, a free sash window survey can help establish whether maintenance, draught-proofing or a glazing upgrade is appropriate. If condensation keeps returning and you are unsure whether the cause is ventilation, the condition of the windows or the glazing itself, give our team a call on 02080 163298 to talk through the options.
Why Does Condensation Form on the Inside of Sash Windows?
Warm air can hold more moisture than cold air. When humid indoor air reaches colder window glass and cools to its dew point, some of that moisture becomes liquid water.
Condensation often appears overnight or first thing in the morning. Cooking, showering and drying washing indoors all add moisture to the air. If it cannot escape, it is more likely to collect on cold surfaces.
In many period London homes, single-glazed sashes can make the effect more noticeable because the room-facing glass is colder than the inner surface of insulated glazing.
How Can You Reduce Condensation on Sash Windows?
Start with moisture and ventilation. Use extractor fans while cooking or showering and keep kitchen or bathroom doors closed so humid air does not spread through the house. Keep trickle vents open where fitted and make sure extractor grilles are unobstructed.
Where possible, dry laundry outside or use a suitable tumble dryer. If clothes must dry indoors, choose a ventilated room rather than hanging them over radiators throughout the home.
A dehumidifier can help in persistently humid rooms, but it should support rather than replace effective ventilation.
Wipe heavy condensation from timber frames and sills instead of allowing water to sit there. This will not cure the cause, but it helps protect finishes and timber while you address it.
If condensation keeps returning, treat it as a balance problem: moisture levels, ventilation and surface temperature all need to be considered before blaming the window alone.
Can Draught-Proofing Help with Internal Condensation?
Professional sash window draught-proofing can improve comfort by sealing unnecessary gaps around moving sashes and reducing uncontrolled cold air leakage. It is useful where windows rattle or obvious draughts are present.
However, draught-proofing does not warm single glazing in the same way as insulated glazing. The glass can still become cold enough for condensation to form. Draught-proofing reduces uncontrolled air leakage, so adequate background or mechanical ventilation becomes particularly important for removing moisture.
When Is a Glazing Upgrade Worth Considering?
If condensation remains frequent after you have addressed ventilation and obvious moisture sources, cold glass may be contributing significantly.
Double-glazed sash windows have an insulating space between the panes, helping the room-facing glass stay warmer than single glazing. This can reduce the conditions that cause internal condensation. Depending on the existing window and the proposed design, an upgrade may involve new double-glazed sashes while retaining the existing box frame.
Where retaining the original window is particularly important, Selectaglaze secondary glazing may be another option. An additional internal glazed layer can improve thermal performance while leaving the external sash in place. Secondary glazing also needs to be designed with condensation control in mind; simply sealing the original sash as tightly as possible is not always appropriate, so the ventilation and sealing arrangement should be assessed as part of the installation. The right solution depends on the window condition, property requirements and any heritage constraints.
Even upgraded glazing can develop room-side condensation if indoor humidity is high, so ventilation remains part of the solution.
How Do You Know Whether the Window Itself Needs Attention?
Look beyond the droplets. Difficulty opening or closing the sash, damaged timber, failed putty, persistent rattling or obvious gaps can point to repair or maintenance needs. Condensation alone does not prove the window needs replacing.
If you already have double glazing, check where the moisture appears. Condensation on the accessible room-side surface usually relates to indoor humidity and glass temperature. Moisture trapped between the panes is different and can indicate a failed sealed glazing unit.
A professional assessment is useful when the cause is unclear or you are deciding between repair, draught-proofing, secondary glazing and double glazing.
Reduce Condensation Without Losing the Character of Your Windows
Condensation on sash windows occurs when indoor moisture meets a surface that is cold enough for condensation to form, with high humidity and inadequate ventilation increasing the risk. Reducing humidity and improving airflow should come first; draught-proofing or better glazing may then improve comfort where the existing windows contribute to cold surfaces.
If condensation remains a regular problem, contact The London Sash Window Company to arrange a free survey and discuss whether repair, draught-proofing, secondary glazing or double-glazed sashes are the most suitable next step for your windows.
Frequently Asked Questions
Is condensation on the inside of sash windows normal?
A small amount can occur during cold weather, especially on single glazing. Frequent or heavy condensation deserves attention because repeated wetting can affect timber and finishes. Check ventilation and moisture-producing activities before assuming the window needs replacement.
How do I stop condensation on single-glazed sash windows?
Reduce indoor moisture, use kitchen and bathroom extraction, keep ventilation routes open and remove standing water. If the glass remains very cold, draught-proofing may improve comfort, while secondary or double glazing can create a warmer internal surface and reduce condensation risk.
Why is there condensation on the inside of my double glazing?
Condensation on the room-facing surface usually means humid indoor air is meeting a cold pane. Improve ventilation and reduce humidity first. If moisture is trapped between the panes, the sealed glazing unit may have failed.
Can condensation damage timber sash windows?
Persistent water sitting on timber can damage protective finishes and may contribute to deterioration over time. Wipe heavy condensation away, but also address why it is forming. Repairing finishes without dealing with the moisture problem may only provide a temporary fix.