Flagship guide · 7 min read
U-Values and Thermal Performance Explained
The single figure most quoted in glazing brochures, and the one most often misunderstood. Here is what a U-value actually measures, and what to do with it.
Condensation on Windows Explained
Condensation on a window is not always a fault. What matters is where it forms and what that tells you about ventilation, insulation or seal failure.
3 min read
Condensation is one of the most common concerns homeowners raise about their windows, and one of the most misunderstood. It occurs when warm, moist air meets a surface cold enough to bring it below its dew point, and the exact location of the condensation tells you a great deal about the underlying cause.
This guide separates the three distinct types of window condensation, explains what each one means, and sets out when it is a genuine fault worth addressing versus a normal seasonal effect.
The three types of window condensation
| Type | Where it forms | What it usually means |
|---|---|---|
| Internal condensation | Inside face of the inner pane | High indoor humidity relative to ventilation, or a cold internal surface |
| External condensation | Outside face of the outer pane | High-performance glass working well — a good sign, not a fault |
| Between-pane condensation | Inside the sealed unit cavity | Failed seal — the unit has lost its gas fill and needs replacing |
What causes internal condensation
Internal condensation is driven by the relationship between indoor humidity, ventilation and surface temperature. Modern airtight homes with insufficient background ventilation, bathrooms and kitchens without adequate extraction, and cold surfaces such as an uninsulated reveal or a thermally bridged cill are the most common contributing factors.
- Insufficient background ventilation (trickle vents blocked or absent)
- Extractor fans not used, undersized, or vented into a loft rather than outside
- Drying laundry indoors without ventilation
- Cold surfaces from thermal bridging at reveals, cills or lintels
- A general increase in air-tightness elsewhere in the home without a corresponding increase in ventilation
Between-pane condensation and seal failure
If condensation, or a persistent hazy film, appears inside the sealed unit itself, this indicates the edge seal has failed and the inert gas fill and desiccant have been compromised, allowing moist air into the cavity. This is a manufacturing or age-related seal failure rather than a ventilation issue, and the affected sealed unit needs replacing — the whole frame is not usually affected, so in most cases only the glass unit needs changing.
- 01Confirm the locationCheck whether the moisture is genuinely between the panes rather than on an internal surface that is hard to see clearly.
- 02Check the unit's ageSealed units typically carry a 10 to 20 year guarantee; failure outside this period is more common with age and UV exposure.
- 03Arrange a like-for-like replacementThe frame and hardware are usually unaffected; only the glazed unit itself needs replacing in most cases.
Reducing internal condensation
Since internal condensation is primarily a ventilation and humidity issue rather than a window fault, addressing it usually means improving background ventilation and moisture control rather than changing the glazing specification, though warm-edge spacer bars and well-insulated frames do help by keeping the coldest surfaces warmer.
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