Condensation on windows is one of the most common concerns homeowners raise, and one of the most misunderstood. It can appear on the inside of the glass, on the outside, or between the panes. Each type has a different cause, a different implication, and a different solution. Treating them all the same leads to frustration, unnecessary expense, and in some cases, ignoring the one type that actually does indicate a problem.
Internal condensation
Condensation on the inside surface of the glass is the most common type. It forms when warm, moist indoor air comes into contact with a glass surface that is cooler than the dew point of the air. The moisture in the air changes from a gas to a liquid, and water droplets appear on the glass. It is the same process that causes a cold glass to “sweat” on a warm day.
This is not a fault with the window. It is a sign that the humidity level inside the home is high relative to the surface temperature of the glass. It is most common in kitchens and bathrooms where cooking, showering, and boiling water produce large amounts of moisture, and in bedrooms overnight where breathing and reduced ventilation allow humidity to build up.
The solution is a combination of ventilation and glazing performance. Opening windows, using extractor fans, and keeping trickle vents open all help remove moisture-laden air before it has a chance to condense. On the glazing side, windows with lower U-values keep the internal glass surface warmer, which reduces the likelihood of condensation forming. Triple glazing outperforms double glazing in this regard because the inner pane stays closer to room temperature. Warm-edge spacer bars also play an important role by reducing heat loss at the glass perimeter, which is where condensation typically appears first.
External condensation
Condensation on the outside of the glass surprises many homeowners, particularly those who have recently upgraded to high-performance windows. It appears as a fine mist or water film on the exterior surface, usually in the early morning, and it clears as the temperature rises.
This is actually a sign that your windows are performing well. It occurs because the outer pane of a highly insulated glazing unit stays cool, since very little heat is escaping through the glass from inside. When the outside air temperature drops overnight and humidity is high, moisture condenses on that cool outer surface in the same way that dew forms on a car windscreen.
External condensation requires no action. It is a natural consequence of good thermal performance and will clear on its own as conditions change. It is worth understanding this before contacting your supplier, as it is often mistakenly reported as a defect.
Condensation between the panes
This is the one type of condensation that does indicate a genuine problem. If moisture appears inside the sealed unit, between the two or three panes of glass, the seal around the edge of the unit has failed. The inert gas fill has escaped and moist air has entered the cavity. Once this happens, the unit has lost a significant portion of its insulating performance, and the condensation will not clear.
A failed sealed unit cannot be repaired. The glazing unit needs to be replaced. On modern windows, this is usually a straightforward process where the sealed unit is swapped within the existing frame, without removing or replacing the window itself. However, it should be addressed promptly. A failed unit contributes to increased heat loss, persistent moisture on the glass, and in some cases mould growth around the window reveal.
Common myths
Several misconceptions about condensation persist and are worth correcting.
The first is that new windows cause condensation. In reality, new windows are more airtight than the old ones they replace. Older windows with gaps and worn seals provided uncontrolled ventilation that carried moisture out of the home, even though they also let heat escape. When those gaps are eliminated by well-sealed modern windows, the moisture that was previously escaping now stays inside the room. The solution is not to make the windows less airtight. It is to ventilate properly using trickle vents, extractor fans, and good ventilation habits.
The second is that condensation means the windows are faulty. As outlined above, internal and external condensation are not defects. Only condensation between the panes indicates a failed unit.
The third is that opening a window in winter wastes energy. Brief, focused ventilation, such as opening a window fully for five to ten minutes, is far more effective at removing moisture than leaving a window slightly open for hours. It exchanges the moist air quickly without allowing the walls, floors, and furniture to cool down significantly.
Reducing condensation in your home
Managing condensation effectively comes down to two things: controlling indoor humidity and ensuring the glass surface temperature stays as high as possible. Ventilate high-moisture rooms during and after cooking, showering, and drying clothes. Keep trickle vents open. Use extractor fans where fitted. And if your glazing is old or underperforming, upgrading to modern units with low U-values and warm-edge spacer technology addresses the root cause by keeping the glass warmer.
If you are experiencing persistent condensation issues and are unsure whether the cause is ventilation, glazing performance, or a failed sealed unit, book a free consultation with the Fairco team. We can assess the situation and recommend the right solution for your home.
