Misted and blown double glazing explained
If your double glazing has clouded up between the panes and no amount of wiping clears it, the sealed unit has failed. This is what people mean by misted double glazing or a “blown” unit. It is a common fault, it is not dangerous, and in most cases you can replace just the glass rather than the whole window. This page explains what has happened and what your options are.
What is actually happening
A sealed unit relies on a continuous edge seal to keep the cavity dry and, where fitted, to hold in the gas fill. Over years of daily heating and cooling the seal expands and contracts, and eventually it can fail. Once it does, moist air seeps into the cavity, the desiccant in the spacer bar becomes saturated, and water vapour condenses on the inner faces of the glass whenever the temperature drops. Because it is sealed inside the unit, you cannot reach it to clean it — the only fix is a new unit.
Misting versus surface condensation
It is worth separating two different things. Condensation on the room-facing surface of the glass is about humidity and temperature in the room, and it wipes away. Misting between the panes is a failed seal and does not. A warm-edge spacer helps reduce the surface kind by keeping the glass edge warmer, but only a replacement unit resolves the sealed-in kind.
Get a misted unit measured
Compare free, no-obligation quotes from vetted local installers who confirm the fault and the fix on survey.
Can you repair it, or must it be replaced?
You will sometimes see “de-misting” services that drill and flush a failed unit. This may clear the visible fog for a time, but it does not restore the original sealed, gas-filled construction, so the unit's insulating performance stays reduced. Replacing the sealed unit is the durable answer, and because the unit is separate from the frame, it is usually a straightforward job. Our guide to replacing glass units covers how the replacement is measured and quoted.
Common questions
Why has my double glazing misted up on the inside?
Misting between the panes means the sealed unit's edge seal has failed and moisture has entered the cavity. It is not condensation on the room surface; it is trapped inside the sealed unit, which is why you cannot wipe it away.
What does a blown double glazing unit mean?
A blown unit is a sealed unit whose edge seal has failed, letting moist air in and, over time, any gas fill out. The result is misting, sometimes with visible water droplets or a hazy film that comes and goes with temperature.
Can you replace just the glass in a misted unit?
Usually yes. When only the sealed unit has failed, the glass can often be replaced without changing the frame or the whole window, which is a smaller and less costly job. An installer measures the unit and confirms this on survey.
Is a misted unit an emergency?
No. A misted unit is a performance and appearance problem rather than a safety one, so there is no rush, but it will not clear on its own and its insulating performance is reduced, so it is worth planning a replacement.
Fix a blown unit properly
Free, no-obligation quotes from vetted local installers — installers have survey appointments in many areas this month.