Condensation between the panes cannot be wiped away and does not go away on its own. We explain why double glazing mists up, how to tell it apart from surface condensation and whether replacing just the glass is enough.
If you notice mist on your windows on winter mornings or after rainy days, the first thing to do is work out where the mist is. Condensation on the room-facing surface is usually not a fault but a matter of indoor humidity. If, however, the mist is between the two panes, where you cannot reach it by wiping, the insulated glass unit has lost its seal. In this article we explain the types of condensation, why double glazing fails and how to decide on replacement.
Where is the mist: inside, outside or in between?
Start with a simple test: wipe the misted area first on the inner surface, then on the outer surface, with a dry cloth.
- Mist on the inner surface: If it wipes away, the problem is humidity in the room. It is common in bathrooms, kitchens and crowded bedrooms, especially on cold mornings. Changing ventilation habits is usually enough.
- Mist on the outer surface: Especially in humid areas such as around Lake Köyceğiz and the Dalyan channel, dew can form on the outer pane on cool mornings. This is actually a sign of good insulation; because indoor heat does not escape, the outer pane stays cold.
- Mist between the panes: If it does not wipe away, and there are white marks, droplets or dirty streaks between the panes, the seal of the insulated glass unit has failed.
Why does double glazing mist up?
An insulated glass unit is made by placing an aluminium or warm-edge spacer between two or three panes and sealing the edges with a dual sealing system (primary butyl, secondary polysulphide or silicone). Desiccant granules (molecular sieve) inside the spacer absorb the moisture remaining in the air between the panes. Over time, the seal can fail for the following reasons:
- Ageing of the edge seal or a manufacturing defect,
- Glass fitted without packers or with the wrong packers, putting the edge under stress,
- Water collecting in the sash standing at the glass edge for long periods because of blocked drainage,
- High temperature differences on panes exposed to the sun,
- Harsh chemicals or incompatible silicone applied to the glass edge.
Once the desiccant is saturated, moisture entering the unit can no longer be absorbed and condensation begins on the glass surface. From this point the process is irreversible.
What do you lose with misted double glazing?
Misting is not just a cosmetic problem. When the dry air or argon gas between the panes is replaced by humid air, the thermal insulation value drops. In Low-E coated glass, moisture can oxidise the coating over time and cause permanent stains. Even if the mist disappears on some days and returns on others, the seal does not recover; the marks usually become a little more visible every season.
| Situation | Cause | What to do |
|---|---|---|
| Mist on the inner surface that wipes away | High indoor humidity | Ventilation, humidity control |
| Morning dew on the outer surface | Cool, humid outdoor air | No action needed |
| Condensation at the glass edge on the inner surface | Cold edge, aluminium spacer | Consider a warm-edge spacer unit |
| Mist or marks between the panes | Unit seal has failed | Replace the glass unit |
| Cracked glass | Impact, thermal stress, packing error | Replace the unit, check packing |
Replace just the glass or the whole window?
If the profile, hardware and gaskets are sound, replacing only the insulated glass unit is enough. The glazing beads are removed, the old unit is taken out, the new unit is placed on the correct packers and the beads are refitted. This is usually done quickly without removing the sash. After measuring, you may need to wait a few days for the unit to be manufactured.
Replacing the glass is also an opportunity to upgrade the insulation. Instead of old plain double glazing, you can choose a Low-E coated, argon-filled unit with a warm-edge spacer. On facades with heavy sun exposure, solar control coated glass markedly reduces indoor heat gain during the Muğla summer. For glass near floor level with a risk of impact, toughened or laminated glass should be chosen. Remember that the new unit’s thickness must suit the glazing rebate of the existing sash, and that a much thicker unit requires different glazing beads.
However, if the profiles are yellowed or cracked, the hardware is worn and the overall insulation of the window is poor, replacing only the glass may not be a lasting solution. In that case renewing the entire window makes more sense.
Tips to prevent misting
- Clean the drainage channels twice a year so water does not stand at the glass edge.
- Do not apply solvent-based cleaners to the glass edge and gaskets.
- Ventilate briefly but frequently in winter to keep indoor humidity low.
- In holiday homes closed out of season, plan for a dehumidifier or regular airing.
- Make sure correct packing is used when replacing glass and during installation.
Conclusion
Mist on the inner surface of the glass is mostly a humidity problem solved by ventilation. Mist between the panes, however, shows that the insulated glass unit has reached the end of its life, and the only lasting solution is replacing the unit. Choosing better-insulating glass during replacement also brings energy savings. Our team inspects on site to identify the cause of the condensation and, with a free site survey and measurement, presents suitable glass options.
