Condensation in double glazing worries people because it looks like a fault and often is not one. Water can appear in three places on a sealed unit and they mean completely different things: on the room side of the inner pane, sealed between the two panes, or on the outside face of the outer pane. Only the middle one is a failed window. The first is a fact about the air in your house and the third is usually a sign the window is working well.
On the room side: the air, not the window
Water forming on the inside face of the glass is warm moist room air meeting the coldest surface in the room. The glass is doing nothing wrong; it is simply the place the air reaches its dew point first. That is why it appears in bedrooms in the morning, in kitchens while cooking and in bathrooms after showers, and why it is worse in a house that has recently been draught proofed. The answers are about moisture and ventilation: extract at the source, use trickle vents rather than blocking them, and keep temperatures more even. Replacing the window moves the cold surface but does not remove the moisture.
Between the panes: the unit has failed
Misting, streaking or standing water sealed between the two panes means the edge seal has failed and moist air has been drawn into the cavity. Nothing can be done from outside it, because the space is not accessible: the unit is a manufactured, sealed item. The remedy is a new sealed unit fitted into the same frame, which is a glazier's visit rather than a window replacement provided the frame is still sound. Drilling and venting a failed unit is offered occasionally and it does not restore the cavity or the gas fill.
On the outside face: usually a compliment
Dew on the outer pane, typically on clear still mornings in spring and autumn, happens when the outer glass falls below the dew point of the outside air. It does that because so little heat is escaping through the window to warm it, which is what a good modern unit is supposed to do. It clears as the sun rises and it is not a defect. It is more common on newer, better performing units than on older ones, which is why people who have just had windows fitted are the ones most likely to report it.
Condensation at the edges and on the frame
Water that gathers in a band around the perimeter of the glass, or on the frame itself, points at the cold bridge at the edge of the unit. Traditional aluminium spacer bars conduct heat straight across the junction between inner and outer panes, making the edge the coldest part of the window. Warm edge spacers in less conductive materials reduce this. If a house has persistent edge condensation with no other damp problem, that is worth knowing when the units are eventually replaced.
When condensation is a symptom of something else
Persistent condensation with black mould at the reveals, or damp that does not follow the pattern of cooking and washing, is worth investigating beyond the window. Cold reveals where a cavity closer is missing, a blocked or removed extract fan, and a genuine damp problem in the wall all produce condensation that looks like a glazing fault. Replacing windows in that situation is expensive and does not fix it, which is a good reason to establish the cause before ordering anything.
Questions people ask about condensation in double glazing
Can a misted unit be repaired rather than replaced?
Not meaningfully. The cavity cannot be resealed to its original condition and the gas fill cannot be restored, so the accepted answer is a new sealed unit into the existing frame if the frame is sound.
Is condensation covered by a window guarantee?
Sealed unit failure often is, within a stated period, and it is worth checking what that period is and whether it covers labour as well as the unit. Condensation on the room side is not a fault and is not covered by anything.
Do trickle vents make condensation worse?
They do the opposite. They provide the background ventilation that carries moist air out of the room, which is why replacement windows are generally required to include them. Taping them shut is a common and counterproductive response to a draught.