What you have actually lost
The gap between the two panes is not empty air. It is filled with argon, a heavier gas that conducts heat far less readily, and it is held there by the same edge seal that keeps moisture out. When that seal fails the argon leaves and ordinary damp air takes its place, so the unit stops insulating anything like as well as it did. The fogging you can see is the symptom; the heat going out through it is the part nobody sees. It is why a room with a blown unit in it often feels colder near the window than the rest of the house, and why the draught people go looking for frequently is not a draught at all.
It gets worse rather than levelling off
A failed seal does not settle into a steady state. Moisture keeps cycling in and out with the temperature, and over a few years it etches the inside faces of the glass, leaving a permanent tidemark that stays even on a dry day. At that point the unit is not going to look better whatever the weather does, and the insulation has long gone.
The replacement is a better unit than came out
New units are made to a standard the originals were not, with a warm-edge spacer rather than bare metal and a better gas fill. So changing a blown unit is not just getting back to where you started — the glass in the old frame performs better than the window did when it was new.
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Common questions
Does a misted unit actually cost me anything, or does it just look bad?
Both. The gap was filled with argon to insulate, and when the seal fails the argon goes and damp air replaces it. The unit stops insulating properly, which is why the room often feels colder by the window.
Will it get worse if I leave it?
Yes. Moisture cycles in and out with the temperature and over a few years etches the glass, leaving a permanent tidemark. It will not settle down on its own.
Which parts of Smethwick do you cover?
Smethwick, Bearwood, Cape Hill, Warley, Langley, Rood End, Soho, Uplands and across to Oldbury.
Everything else we do in Smethwick
Repairs
Something's broken or stuck
Emergencies
Broken glass, made safe
Glass cut to size
Supplied, and fitted
Windows & doors
When repair isn't the answer