Why does double glazing mist up between the panes?
A double glazed unit is two sheets of glass held apart by a spacer bar round the edge, sealed all the way round and usually filled with a gas that keeps heat in better than air. The spacer bar holds a drying material that soaks up the tiny amount of moisture sealed in when the unit was made.
Every sunny day the unit warms up and every cold night it cools down, and over the years that movement works at the seal. Once the seal lets go, damp air gets in, the drying material fills up, and the moisture shows as mist on the inside faces of the glass. A frame whose drainage channels have blocked makes it happen sooner, because the edge of the unit ends up sitting in water.
Two things follow. The mist comes and goes with the weather and gets steadily worse, and the insulating gas has gone, so that window is no longer doing much to keep the heat in.
Misted double glazing: new glass or a new window?
If the frame is sound, the glass is all that needs changing. The old unit comes out of the frame, a new one made to the exact size goes in, and the frame, the handles and the paintwork around it are left alone. One misted pane in a house full of good windows is not a reason to replace the lot.
A new window is the better answer when the frame itself has had it: rotten or split timber, a uPVC frame that has twisted so the sash will not shut, or hinges and locks that have worn out along with the glass. If several things are going at once, the windows section covers full replacements, and Luke and Dale will say which way your window falls when they see it.
There is one more reason glass only is the easy route. Replacing just the sealed unit in an existing frame is a repair, so it is not notifiable building work and it does not need a FENSA or building control certificate. A whole new window does.
Can you upgrade the glass while it is being replaced?
Yes, and it is the cheapest time to do it, because the unit is being made anyway.
- Warmer glass. An old unit from the nineties or earlier is likely to be plain glass. Its replacement can have a heat reflecting coating and a warm edge spacer, which makes a real difference on the cold side of the house.
- Obscure glass for a bathroom, a landing or a front door, in a range of patterns from nearly clear to fully private.
- Safety glass where it belongs. Glass in doors, in panels within 300mm of a door, and in any pane that comes lower than 800mm from the floor should be toughened or laminated, so that it breaks safely. Older windows often have ordinary glass in those places, and a replacement unit is the moment to put that right.
One honest caution: a patterned or leaded design that is decades old may not be made any more, so a single replacement can look slightly different from its neighbours. It is worth seeing a sample before deciding whether to do one pane or the matching set.
A single misted unit is a small job, and Luke and Dale are happy to come out for one. Ring 07487 719666, or send your photos through the contact page. If the misted panes are in a conservatory, the conservatory repairs section covers that too.
