Noticed more condensation on the outside of your sash windows since upgrading them? In many cases, that is entirely normal. Efficient glazing reduces heat transfer from inside the home, allowing the external pane to remain colder and making condensation more likely on cool, humid mornings.
If you have recently installed double-glazed sash windows, external condensation can therefore be consistent with improved thermal performance rather than a problem. If you are unsure what you are seeing, give our team a call on 02080 162058 and we can help you understand it.
Why Can New Sash Windows Have More Condensation Outside?
It can seem backwards. You replace or upgrade an older window to make your home warmer, only to find beads of water covering the outside of the glass the following morning.
The explanation comes down to surface temperature. With older single glazing, more heat passes from the room through the glass. That heat warms the external surface to some extent. Older, poorly fitting sash windows may also lose additional heat through gaps around the sashes and frame.
Energy-efficient glazing is designed to reduce that heat transfer. Less heat reaches the external pane from indoors, allowing its surface temperature to fall further overnight.
If its temperature drops below the dew point, moisture naturally present in the outdoor air condenses on the glass. This is most noticeable during calm, humid conditions when the glass has cooled overnight.
That does not mean every efficient window will be covered in condensation every morning. Temperature, humidity, wind, orientation, shelter and how much of the window faces the open night sky can all affect whether dew forms.
Does External Condensation Mean Your New Windows Are Working?
External condensation is generally compatible with good insulating performance because efficient glazing allows the outside pane to remain colder. It is a recognised characteristic of well-insulating low-emissivity glazing.
However, it should not be treated as a performance test on its own. A window without visible dew is not necessarily inefficient, and condensation on another morning does not provide a measurement of how well a window is performing.
External condensation can occur with conventional double glazing and high-performance options such as FINEO vacuum glazing, which provides high thermal performance in a slim unit suitable for traditional timber windows.
Professional sash window draught-proofing can also reduce uncontrolled heat loss through gaps around moving sashes. However, external condensation is primarily related to the temperature of the glazing surface, so it should not be used as proof that draught-proofing itself is working.
How Can You Tell External, Internal and Between-Pane Condensation Apart?
Where the moisture appears matters much more than the fact that condensation exists.
External condensation forms on the outside surface of the external pane. You cannot wipe it away from inside the room. It often develops overnight and disappears as sunlight, rising temperatures or air movement warm and dry the glass.
Internal condensation appears on the room-facing surface when humid indoor air meets glass that is cold enough for moisture to condense. Cooking, showering, drying laundry indoors and inadequate ventilation can all contribute.
Condensation between the panes of a conventional sealed double-glazed unit is different again. If mist or droplets are trapped between the panes and cannot be wiped away from either surface, the sealed unit may have suffered seal failure. That should be assessed rather than dismissed as normal external dew.
Being clear about the location prevents homeowners from worrying unnecessarily about harmless external condensation while overlooking moisture that may need attention.
Why Might Some Windows Condense While Others Stay Clear?
It is common for one window to be misted while another on the same property remains completely clear.
A window facing an open section of night sky can lose heat differently from one protected by an overhang, tree or neighbouring building. Air movement also matters: a light breeze may prevent condensation forming or help existing moisture evaporate more quickly.
Orientation and local temperature differences can even mean that adjacent panes behave differently.
For that reason, patchy external condensation does not necessarily suggest inconsistent installation or glazing performance. Before assuming something is wrong, check whether the moisture is definitely on the external surface and whether it clears as conditions change during the morning.
Do You Need to Do Anything About External Condensation?
Usually, no. External dew is generally temporary and should disappear naturally once the glass warms or the surrounding air becomes less humid.
There is normally little benefit in trying to prevent it by increasing your heating. The purpose of thermally efficient glazing is to keep that heat indoors rather than send more of it through the window simply to warm the external pane.
What matters is recognising when the situation is different. Moisture trapped between panes, persistent condensation on the room side, water affecting timber or an unexplained change in window performance deserves further investigation.
If you have recently upgraded your timber sash windows and the moisture is clearly on the exterior, occasional morning condensation is generally something to understand rather than something to fix.
A Colder Outer Pane Can Be Part of a Warmer Home
External condensation after a sash-window upgrade can be surprising, but it can be a normal result of the outer pane remaining colder because less heat is transferred through the glazing from indoors.
The key is knowing where the moisture is forming. External dew is usually temporary, while room-side or between-pane condensation needs a different assessment. If you are unsure which you have, contact The London Sash Window Company to discuss your sash windows.to discuss your sash windows.
Frequently Asked Questions
Why do my new windows have condensation outside but my old ones did not?
Older glazing typically allows more indoor heat to reach the external surface. Efficient glazing reduces that heat transfer, allowing the outer pane to become colder overnight. If it falls below the dew point, moisture from the outdoor air can condense on it.
Will external condensation damage my sash windows?
Occasional external condensation is normally short-lived and evaporates as conditions change. Well-maintained exterior timber should be protected with an appropriate coating system, but if water persists, paintwork is failing or timber shows signs of deterioration, the window should be inspected.
Why does external condensation usually appear in the morning?
The outer glass can cool considerably overnight, particularly during clear, still conditions. If humid outdoor air then meets glass below its dew point, condensation forms. Morning sunlight, warmer temperatures and wind usually help the moisture disappear.
Should I worry about condensation between double-glazed panes?
Yes, this is different from harmless external condensation. Moisture that is visibly trapped between the panes of a conventional sealed double-glazed unit can indicate that the seal has failed. If it persists, arrange for the glazing to be assessed.