Double glazing mists up between the panes when the seal around the sealed unit has failed, letting moist air into the gap between the two sheets of glass. Once that happens it cannot be wiped away, but in most cases you only need to replace the sealed unit, not the whole window.
Here is how to tell what kind of misting you have, why it happens and what the repair involves.
Where is the condensation?
The first thing to check is which surface the moisture is on. That tells you whether anything is actually wrong.
- On the inside face, in the room: normal condensation from cooking, showers, drying washing and breathing. Better ventilation helps. The unit is fine.
- On the outside face: common on cool mornings, especially on newer, better insulated glass. It clears as the day warms up. Nothing is wrong.
- Between the two panes: the seal has failed. This is the one that needs fixing.
If you are not sure which side the moisture is on, wipe the glass on both sides. If it is still there, it is between the panes.
Why the seal fails
A sealed unit is two (or three) panes of glass held apart by a spacer bar and sealed around the edge. Inside the spacer is a drying material called desiccant, which soaks up any moisture in the gap.
Over time the edge seal can break down. Age, sun, frames that have moved, water sitting in the bottom of the frame and poor original fitting all play a part. Once the seal goes, damp air gets in, the desiccant fills up and the glass starts to mist.
Misting often starts small, as a faint haze on cool mornings, and gets worse over the following months as the desiccant fills up.
Signs a unit has blown
Misting is the obvious sign, but there are others:
- Mist or droplets between the panes that come and go with the weather
- White or cloudy staining inside the unit that will not clean off
- A unit that looks permanently hazy even on a dry day
- Water marks or tide lines on the inside surfaces of the glass
Can it be repaired, or do I need a new window?
In most cases you do not need a new window. If the frame is sound and the hinges and handles work, the failed sealed unit can be measured and a new one made to fit the existing frame.
That avoids the cost and mess of taking out the whole frame, and there is no making good to the walls afterwards. If the frame itself is damaged, warped or rotten, it may make more sense to replace the window.
A new unit can usually match the glass you had before. Where glass needs to be safety glass, such as in many doors and low-level panes, the new unit should be toughened or laminated to suit. Our guide on where you need safety glass explains which panes this applies to.
Some companies offer to drill and dry out a misted unit. That does not restore the original edge seal, so replacing the unit is generally the more reliable fix.
Does it matter if I leave it?
A misted unit is not dangerous, and it will not damage the frame on its own. But it does not get better. The view gets worse, the staining can become permanent, and a unit with a failed seal will not insulate as well as it did.
If the glass is cracked rather than misted, get it looked at sooner, as a crack can spread and may need making safe.
Do I need Building Regulations approval?
In England, replacing only the glass in an existing frame does not normally need Building Regulations approval. Replacing the whole window, frame and all, does, which is why window installers use a competent person scheme or the local council building control.
If you are not sure, ask before the work is booked.
Need it looked at?
SWM Locks, Glass & Double Glazing Repairs measures and replaces blown, misted and cracked sealed units in existing frames across Leominster, Hereford, Ludlow and the villages in between. Steve is a qualified glazier as well as a locksmith, and every job is priced before the work starts.
Read more about misted & broken double glazing and window repairs & replacement, or see the work Steve does in Leominster and Hereford.