You wipe the window, and the mist is still there. If that sounds familiar, the moisture is not on the glass you can reach. It is inside the sealed unit, and no amount of cleaning will shift it.

The good news is that this does not usually mean you need a new window.

What is happening inside the unit

A double glazed window is a sealed unit made up of two panes of glass, a spacer bar around the edge and a cavity in between. An edge seal keeps damp air out of that cavity, and the spacer bar contains a drying material that deals with the small amount of moisture present when the unit is made.

Over the years, that seal can fail. Damp air then finds its way into the cavity. When the temperature drops, the moisture condenses on the inside faces of the glass, where you cannot touch it.

Why seals fail

Seals are under constant stress. Glass and frames expand and contract as the temperature changes, and windows that catch the full sun or sit in exposed spots tend to work the hardest. Age is the most common factor, although a knock to the frame or poor drainage around the unit can also play a part.

How to tell which kind of condensation you have

Not all condensation means a failed unit. Try this:

  • Wipe the room side. If it clears, the moisture was on the surface, and the cause is most likely humidity in the room. Better ventilation should help.
  • Check the outside. Condensation on the outer pane on a cold morning is usually harmless, and often a sign that your glazing is keeping heat in.
  • Look between the panes. If the mist, droplets or white marks stay put whatever you do, the seal has gone.

What can be done about it

Once moisture is in the cavity, the dependable fix is a new sealed unit. It is made to the size of your window and fitted into the existing frame, so if the frame is still sound there is no need to replace it.

That costs far less than a new window and causes much less disruption. It is also a good moment to think about the glass itself, as a Low E coating or a different specification may improve on what you had.

Mist in one window does not always mean the others are failing, because each unit is sealed separately. Windows of the same age are worth keeping an eye on, though.

Find out how we replace misted glass, or get in touch for a free quote.