The short answer
Mist or droplets between the two panes of a Velux mean the seal around the double-glazed unit has failed. Damp air has got into the gap that should be sealed, and it cannot be wiped away. The fix is usually a new sealed glass unit in the existing frame. If the frame is worn or old, replacing the whole window can be the better buy.
A film inside the glass, where no cloth can reach, is a sealed-unit failure. It is a glazing fault and not a roof leak, though the two need telling apart.
What a failed seal looks like
A double-glazed unit is two panes held apart by a spacer bar and sealed round the edge, with dry air or gas trapped between them. That sealed gap is what gives the window its insulation. Once the edge seal fails, humid air gets in, and with it a fine mist that shifts from day to day.
How can I tell it is between the panes?
Wipe the inside face and the outside face of the glass. If the cloudiness does not move, it is inside the unit. Condensation on the room-side pane wipes away, and so does dew on the outside. A failed unit often looks worst on cold mornings and clears a little in dry, warm weather, but it will not clear for good. You may also see a ghostly stain or a dried tide mark once the moisture has dried out.
It is not a leak from the roof. A leak shows as drips or staining on the lining. If you are unsure, our answer on Velux condensation explains how to tell room damp from a glazing fault, and how each is treated.
Why sealed units fail
Time and movement are the usual causes. A roof window sits in direct sun, takes heavy rain and goes through a wide swing between a freezing night and a hot afternoon. Glass and frame expand and contract, and over many seasons the edge seal can crack or let go. Some units simply reach the end of their working life.
Is a failed unit dangerous?
No. It is a loss of performance, not a safety fault. A unit with a blown seal insulates less well, so the room can feel colder near the window and the inner pane may mist more often. The window still opens, closes and keeps rain out as long as the frame and flashing are sound.
A knock, a poorly fitted blind pressing against the glass or a damaged frame can also hasten a failure. If the glass has cracked, it needs attention sooner. Lives vary widely and no one can name an exact figure, so judge by what you can see: how the unit looks, how old the window is and what the frame is doing. If you know the window's age, tell us, and send a photo.
Replacing the glass or the whole window
The usual repair is to fit a new sealed unit in the existing sash. The old unit is taken out, the frame is checked and cleaned, and a new unit is fitted in its place. It saves the cost of a new frame and avoids disturbing the flashing and the roof covering. Our page on replacing Velux glass goes through it step by step.
When is a whole new window the better choice?
When the frame is rotten or warped, the opening mechanism is worn, the flashing is old or failing, or the model cannot be fitted with a replacement unit. A new window then buys you a fresh frame, a fresh flashing kit and a current product. MyJobQuote puts a mid-range Velux at £1,100, and about £100 less where it is a plain swap (Sep 2026). Take those as typical UK ranges for a whole window. A new unit alone sits well below that, so the gap is the real question.
A replacement element is also covered by Approved Document L, so ask your building control body whether a glass-only repair is treated differently from a new window, and read about replacing a Velux in the same opening before you decide.
Deciding what to do next
Start with the facts you can gather in ten minutes. Check whether the mist is between the panes, note the age of the window if you can find it, and photograph the label on the frame, which carries the product code. Then check the frame and the surrounding lining for stains, soft timber or flaking paint.
Should I wait before replacing it?
A misted unit does not usually need emergency action, but it does not improve. If the window is working well otherwise, a glass-only repair is a measured choice. If you are already planning other roof work, or the window is old, combining the jobs saves a visit and lets us check the flashing at the same time.
We survey from outside as well as inside, send you photos of the frame and flashing, and put one written total on the option that suits. Where a small repair is enough, that is what we propose, and if a whole new window is the sounder buy we explain why. The main roof windows and skylights page shows how a visit runs, and a photo of the window and its label is a good first step.
More on misted roof windows
Can the mist be cleaned out of the glass?
Not reliably. Once the edge seal has gone, the gap cannot be made dry and sealed again from the inside, so the haze returns. The sounder repair is a new sealed unit. Cleaning the outer and inner faces is still worthwhile first, so you know exactly where the haze sits.
Does a misted Velux let rain in?
Not by itself. Mist between the panes is a glazing fault and the unit still keeps rain out. If you see drips, staining or soft lining round the frame, that points to the flashing or the frame, and we would check the outside of the roof around the window.
The full picture: Velux roof window and skylight installation in Taunton · More short answers