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Condensation on a sash window is not always a problem — and it is not always caused by the window. Where it forms tells you everything about the cause and the fix. This guide explains each type clearly.
Condensation forms when moist air meets a surface that is cold enough to bring it below its dew point. On a sash window, this can happen in three distinct locations — inside the room on the glass face, between the panes of a double glazed unit, or on the external face of the glass. Each is a different phenomenon with a different cause and a different solution.
This is the most common type and the one most people mean when they say their windows are “running with condensation.” It forms on the room-side surface of the glass — typically on cold mornings — and drips down onto the sill. On single-glazed sash windows this is extremely common in winter because the glass is cold enough to condense moisture from the room air almost continuously overnight.
High indoor humidity combined with a cold glass surface. Cooking, bathing, breathing, and drying laundry all add moisture to the air. If that moisture cannot escape — and the glass is cold enough — it condenses.
Improved ventilation (trickle vents, extractor fans, opening windows), reduced indoor humidity, and warmer glass. Double glazing significantly reduces this by keeping the inner pane warmer.
Important: This is not a fault with the window. It is a ventilation and humidity problem. Replacing the window with a double-glazed unit will reduce it significantly but will not eliminate it if the underlying ventilation issue is not addressed. Draught proofing can reduce condensation by eliminating uncontrolled cold air ingress that creates temperature differentials at the glass.
A hazy or misty appearance between the two panes of a double glazed unit is a completely different problem. The cavity between the panes is sealed and filled with argon or dry air at manufacture. When the hermetic seal fails — typically after 10–20 years — moist external air enters the cavity. As the temperature changes, moisture condenses between the panes where it cannot be wiped off.
Failed hermetic seal on the double glazed unit. The sealant around the spacer bar has deteriorated, broken down due to UV exposure, or the desiccant material inside the spacer bar is saturated.
The failed unit must be replaced. The seal cannot be repaired and the cavity cannot be dried out. The frame itself does not need to be replaced — just the glass unit within it. See our glass replacement page.
This is the only type of condensation that requires professional window work. A misted double glazed unit is beyond repair and must be replaced. Do not be persuaded by any product claiming to drill and dry a failed unit — the desiccant is exhausted, and any improvement is temporary at best.
External condensation appears on the outside surface of the glass and is most common on clear, still mornings in spring and autumn. Many homeowners worry when they notice this — but it is actually a sign that the window is performing well.
A well-insulated double glazed unit keeps the outer pane cold (because less heat is escaping through it). On a clear night, the outer pane radiates heat to the sky and drops below the dew point of the outside air, causing external condensation at dawn.
Nothing — this is not a fault. External condensation clears naturally within an hour or two of sunrise. If it bothers you, external condensation-resistant glass coatings are available as an option on new units.
Good news: If you have recently had new double glazing fitted and are now seeing external condensation for the first time, this is confirmation that your new units are performing as they should. Single glazing almost never shows external condensation because it is so poorly insulated that the outer pane stays warm.
Persistent internal condensation on a single-glazed sash window is a risk factor for the timber frame. Water that drips repeatedly onto the sill, collects in the corner joints of the lower sash, or soaks into cracked putty creates the damp conditions that wet rot requires. Over time, what starts as a condensation problem becomes a decay problem.
Improving ventilation addresses the condensation. Inspecting and maintaining the paintwork and putty prevents moisture ingress at the points where decay starts. If there is existing rot in the lower sash or sill, addressing that at the same time prevents the cycle from continuing. See our sill replacement and sash window repair pages.
Use extractor fans in kitchens and bathrooms. Open a window briefly when cooking or bathing. Trickle vents in the sash frame allow low-level ventilation without draughts and can be retrofitted.
Draught proofing reduces uncontrolled cold air infiltration that chills the glass and frame, which reduces condensation as well as heat loss. Brush pile seals fitted to the meeting rails and jambs are the most effective method.
Slimline double glazed units keep the inner glass pane significantly warmer, reducing the condensation surface dramatically. Combined with draught proofing, this is the most effective solution for a sash window with persistent condensation.
On single-glazed windows during winter, wiping the glass and sill dry each morning prevents water from pooling in joints and soaking into the timber. A squeegee or microfibre cloth takes 30 seconds and significantly reduces the risk of rot in the long term.
Intact paintwork is what protects the timber. Cracked or flaking paint allows condensation water to soak directly into the wood. Inspect and repaint every 5–7 years, paying particular attention to the bottom rail and putty lines.
Avoid drying laundry indoors where possible. Keep lids on pots when cooking. A dehumidifier in rooms with persistent condensation will noticeably reduce the problem within days.
Internal condensation on single-glazed sash windows in winter is very common in London, but it is not something that should simply be accepted. It indicates that humidity levels or ventilation need attention, and over time it risks damaging the timber frame. External condensation on double-glazed units is completely normal and not a concern.
This is a common concern and it is partially valid. Sealing a window reduces uncontrolled air infiltration, which means slightly less ventilation through the window itself. However, draught proofing also keeps the room warmer and reduces the cold spots at the glass that cause condensation to form. The net effect is usually a reduction in condensation, provided other ventilation (extractor fans, trickle vents) is maintained.
No — only the glass unit itself needs replacing, not the frame. We remove the failed unit from the existing sash and fit a new one of the correct specification. This is significantly cheaper than a full window replacement and is usually straightforward on a timber sash window.
Yes, over time. Persistent water dripping onto the sill and collecting in the corner joints of the lower sash creates the sustained damp conditions that wet rot requires. The bottom rail of the lower sash is the most vulnerable point. Regular wiping down, maintaining the paintwork, and addressing ventilation all reduce this risk significantly.
It will dramatically reduce internal condensation by keeping the inner glass surface significantly warmer. However, if humidity levels in the room are very high, condensation can still form on a double-glazed surface in extreme cold. Double glazing removes the window as the primary cause; ventilation addresses the underlying moisture source.
We replace failed double glazed units and carry out draught proofing and double glazing upgrades across London. Free site survey, no obligation.
Slimline double glazed units retrofitted into original timber sash frames across London.
Brush pile seals fitted to eliminate draughts, reduce heat loss, and lower condensation risk.
Failed double glazed unit replacement — new unit fitted into the existing sash frame.
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