The short answer
Not by itself. Mist between the two panes means the sealed glass unit has let moisture into its cavity, which is a fault inside the glass. A misted skylight leaking is a different fault: water entering from outside past the flashing, the kerb or the roof covering. The two can sit in the same window, and either needs attention soon, but a cloudy pane with a dry lining points to a seal failure.
- Free survey, fixed written quote
- 10-year workmanship guarantee
Reading the evidence in the room
Stand under the window with a torch and look at three places: the glass, the lining around it, and the ceiling or wall beside it. Where the water shows tells you where it came from.
A haze or a row of droplets that sits between the sheets of glass, with the lining dry and the paint unmarked, is the seal. Water that appears as a brown ring on the plaster, a bubbled patch on the reveal or a damp line along the bottom of the lining is coming from outside and running down behind the finish.
Look at the sill too. A pool of water on the inside sill after heavy rain could be condensation or an entry point. If the glass is clear and the sill is wet only on stormy days, suspect the flashing. If the sill is wet on cold mornings after a shower is used, suspect ordinary condensation on the room face of the window.
| What you see | Where it usually comes from |
|---|---|
| Cloud or beads between the panes, lining dry | Failed edge seal in the sealed unit |
| Brown ring or bubbled plaster beside the frame | Water getting past the flashing or kerb |
| Wet sill on cold mornings, glass clear | Condensation on the room-side face |
| Drip at the head of the frame during a downpour | Entry point above the window, such as a lifted tile or a lapped joint |
Does it follow the weather?
Timing separates the faults better than any other clue. Keep a note for a fortnight of when the window looks worst. Mist that thickens on cold, clear mornings and thins by afternoon is a sealed unit on its way out. Mist that stays put whatever the day is a sealed unit that has finished failing.
Water from outside follows rain. It shows during or just after a shower, grows in a long wet spell and dries over a sunny week. If the stain on the ceiling is bigger every time the wind drives rain up the slope, the entry point is usually on the side facing the weather.
A flat-roofed kitchen extension with a rooflight over the island shows the same split. A clouded unit is a nuisance at breakfast but dry underneath. A wet ceiling after a downpour means the upstand or the covering dressed up it needs looking at.
Take dated photos each time. They help us at survey, and they stop a patch you painted over from hiding a pattern. The cause of a stain is not always in line with the stain, because water can run along a rafter before it drops.
Why both faults need action soon
A failed seal will not mend itself, and a misted unit will not stay clear again once moisture is inside. We explain the reasons in our answer on clearing misted glass. The view and the light you get from the window drop away over time, and the unit is closer to replacement each season.
Water from outside is the urgent one. It wets the timber round the opening, the insulation and the plasterboard, and it keeps doing so until the entry point is dealt with. A mark you can see on the ceiling can mean a larger damp patch in the roof space above.
When both appear together, the order matters. A new sealed unit in an old frame will not stop an entry point that is somewhere else, so the survey looks at the glass, frame, flashing and kerb as a whole before anything is priced. The guide to a glass swap versus a full replacement shows how that decision is made.
While you wait for a survey
Put a towel on the sill and a bowl on the floor if water is dripping. Move furniture and anything electrical away from the area. Take photographs of the window, the lining and the ceiling, and note the date and the weather.
Do not go onto the roof to look, and do not smear sealant round the frame: it can trap water behind it and hide the real entry point from the survey. If the unit is an opening one, keep it shut in the meantime.
We book a free survey, look at the whole unit and the roof around it, and give a fixed written quote for the work. Other faults that people mistake for a leak are covered in common VELUX window problems, and the full picture sits in our misted rooflight replacement section. When you are ready, send us the photographs.
