Why is there condensation inside my double glazing?

A double glazed unit is two panes with a sealed gap between them, usually filled with dry air or gas. Once the seal fails, moist air gets in and condenses on the cold inner surface, where you cannot reach it.

What you'll need

• Tape measure (optional)
• Photographs of the frame (optional)

Step by step

1. Confirm where the condensation is.Between the panes means a failed seal. On the room side means a ventilation and humidity problem. On the outside face means a well-insulated window working correctly.
2. Understand that it cannot be cleaned.The surface is inside a sealed unit. Any product claiming to clean it either does not work or involves drilling the unit.
3. Know what has actually failed.The perimeter seal and the desiccant that keeps the gap dry. Once it is saturated, moisture condenses whenever the temperature drops.
4. Replace the sealed unit, not the whole window.The glass unit can be replaced within the existing frame in most cases. It is far cheaper than a new window and takes under an hour.
5. Measure the unit or have it measured.Width, height and thickness, plus whether it is toughened, low-emissivity or patterned. Glaziers usually measure it themselves.
6. Check the warranty first.Sealed units often carry a warranty of five to ten years. If the window is newer than that, the failure may be covered.
7. Treat repair services with caution.Drilling and drying a unit removes the visible misting but the seal remains failed and the insulating gas is gone. It usually returns.
8. Consider whether other units are close to failing.If several are misting, they were probably installed at the same time. Replacing them together is often cheaper per unit.

Tips

• Condensation on the outside of double glazing in the morning is a sign the window is working well — the outer pane is cold because heat is not escaping.
• Condensation on the room side is a humidity and ventilation problem, not a window fault, and is dealt with quite differently.
• Photograph the frame and any markings on the spacer bar. Glaziers can identify the unit specification from them.

Common mistakes

• Assuming the whole window needs replacing — In most cases the sealed unit can be swapped within the existing frame at a fraction of the cost.
• Paying for a drill-and-dry repair — It removes the visible misting but the seal is still failed and the insulating gas is gone. The problem generally returns.
• Confusing internal condensation with a seal failure — Condensation on the room-facing surface is a humidity problem. Only misting between the panes indicates a failed unit.

Questions people ask