A roof window mists up for one of two reasons, and they are completely different. Condensation on the room-side face of the inner pane wipes off with a cloth and is a ventilation problem: warm, moist air meeting cold glass. Misting sealed between the two panes does not wipe off, because it is inside the glazing unit, and that means the unit has blown and needs replacing.
Which one you have decides everything about what to do next, so the first job is to work out which side the moisture is on. It takes ten seconds and a dry cloth.
How to tell which you have
Reach up and wipe the inside of the glass with a dry cloth or your hand.
- If the mist clears, the moisture was on the room-side surface. This is room-side condensation. It is a ventilation and warm-air issue, not a fault in the window.
- If the mist stays, the moisture is inside the sealed unit, between the two panes, where no cloth can reach it. This is a blown unit. Nothing you do from the room will clear it, because the problem is inside the glass.
A second check: room-side condensation appears on cold mornings and clears through the day as the room warms and the glass warms with it. Sealed-unit misting comes and goes with temperature too, but it never fully clears, and over time it leaves permanent milky staining and fine mineral tide-marks between the panes that a cloth passes straight over.
Room-side condensation, and what it means
If the mist wiped off, the glass is fine. What you are seeing is warm indoor air, carrying moisture from showers, cooking, drying washing and simply breathing, meeting the coldest surface in the room. On a roof window that cold surface is the inner pane, and in a loft it is often the coldest thing up there.
The cures are all about air and warmth, not the window. Use the trickle vent or the night-latch position on the sash to let moist air out. Run an extractor in a loft bathroom and let it overrun. Keep some background heat in the room so the glass never falls to the dew point. A window with a vertical reveal at the head and a sloped reveal at the sill also helps, because it lets warm room air wash up the glass rather than sitting still against it.
One nuance worth knowing: a modern, well-sealed replacement window can actually show more room-side condensation than the draughty old one it replaced, because the draughts used to ventilate the room for you. That is not a fault. It is the room telling you it now needs deliberate ventilation.
A blown sealed unit, and why it means replacement
If the mist stayed, the sealed unit has failed. A sealed unit is two panes of glass with a spacer bar between them, holding a desiccant that absorbs residual moisture, and usually an argon fill and a low-emissivity coating. The perimeter seal degrades over roughly fifteen to twenty years. Once it goes, the desiccant saturates, the argon escapes, and moisture cycles in and out between the panes. That is the haze you cannot wipe.
Roof windows blow earlier than vertical windows, and the reason is thermal cycling. A south-facing roof window on a Suffolk slope can swing from below freezing at dawn to well over 50 degrees at the glass surface by midday in June. That daily expansion and contraction works the edge seal far harder than any wall window sees. Loft-conversion windows fitted across Ipswich between 2000 and 2010 are reaching this point together, which is why we see them in batches.
There is no wiping, drilling or resealing that brings a blown unit back. We do not offer glass swaps or resealing on roof windows, and on a unit old enough to have blown the sealed unit is usually no longer available anyway, because the glazing bead and glass thickness changed with the frame generation. By then the frame timber and flashing are the same age as the glass. The honest answer is a new window: whole unit, new collars, flashing matched to your covering, with a 10-year workmanship guarantee. The mechanics of why the seal fails are set out on our knowledge page, misted VELUX glazing.
A blown pane is also a performance loss, not just a cosmetic one. Once the argon has gone and the low-E coating is degrading, a unit that started around 2.8 to 3.2 W/m2K is worse than that now, against roughly 1.1 to 1.3 for a modern replacement. You are heating the loft through the glass. That is part of the wider case for replacing rather than nursing an old window, on our replace not repair page.
What it means locally, and when to act
If the mist wipes off, you have a ventilation job and there is no rush, though a persistently damp loft is worth sorting before mould follows. If it does not wipe off, the glass has failed and the only question is timing. A blown unit is not an emergency the way a leak is, so there is no need to rush a decision, but it will not improve and the frame beneath it is ageing at the same rate.
The judgement of whether a single blown window is worth replacing on its own, or whether it is a signal that the whole set is due, is covered on when to replace a roof window. And if you are weighing the glass against the whole window on cost, our glass versus whole-window cost page sets out why we fit the complete unit rather than the pane alone. Roof window installation and replacement is the whole of what we do.
Request a fixed quote. Send a photo of the misting and we can usually tell you from the image whether it is condensation or a blown unit before anyone comes out.
Thinking about this job? We survey across Ipswich, Suffolk and north Essex and give you a fixed price before any work starts, with a 10-year workmanship guarantee.