Why is my roof window misted up inside the glass?
If the haze is genuinely between the two panes, where a cloth cannot reach it, the sealed unit has lost its edge seal and its desiccant is saturated. That is the end of the unit’s working life. It is not a fault in the installation, it is not caused by anything you did, and it does not come back from it.
The first job, though, is being certain which surface the water is actually on, because two of the three possibilities are ordinary condensation and mean something completely different. It takes ten seconds to check and it changes the answer entirely.
The ten second test that settles it
A double glazed unit has four glass surfaces. Number one faces the sky, number four faces the room, and numbers two and three face each other across the cavity.
Wipe the room side with a dry cloth. If the haze clears, it was surface four and you have condensation in the room, which is a ventilation matter. If nothing changes, look at the outer face on a clear morning: dew on surface one burns off by mid morning and is a sign of a well insulated unit rather than a failing one. If the haze survives both, it is on surface two or three, sealed inside, and out of reach of anything short of a new unit.
What has actually stopped working inside the cavity
The cavity between the panes is dry air or argon, held dry by desiccant packed inside the perforated spacer bar. Around the outside of that spacer sit two seals: a thin butyl primary seal that does the vapour work, and a polysulphide or silicone secondary seal that holds the assembly together mechanically.
Vapour crosses the primary seal slowly for the whole life of the unit, and the desiccant absorbs it. There is a finite amount of desiccant. Once it is loaded, the first cold night puts the inner face of the outer pane below the dew point of the trapped air and water forms where nobody can get to it. The consumable ran out. Nothing snapped.
Why it appears one winter and seems to clear by spring
People often describe the haze as coming and going for two or three years before it settles in permanently, and that pattern is exactly what a saturated cavity does.
Warm daytime air drives the moisture back into vapour and it disperses across the cavity, so the glass looks clear at two in the afternoon in April. Cold, still nights bring it back onto the coldest surface. As the desiccant becomes more fully loaded the clear spells shorten, and eventually there is enough free water that it is visible year round. By that point the unit has usually been finished for a couple of seasons.
Tide marks, and reading how long it has been happening
Once liquid water has been running down the inside of the cavity for a while, it leaves permanent marks and those marks tell a story.
Fine white deposits at the bottom edge are minerals left behind as water evaporated repeatedly from the same place. A distinct horizontal line partway up the pane shows where standing water sat. A cloudy grey bloom across the whole surface is chemical etching, where the low emissivity coating has been attacked by the water and cannot be recovered even in theory. Etching means years, not months.
Why roof windows reach this point sooner than wall windows
Two units of identical build, one in a bedroom wall and one in the slope above it, will not fail at the same time. The one in the roof goes first, usually by several years.
A sloping or horizontal unit takes the full solar load in summer, so the outer pane runs far hotter than air temperature, and on a clear winter night it radiates to the open sky and drops below it. That daily swing expands and contracts the seal edge harder than any vertical plane. On a flat unit there is a second mechanism, because rainwater sits against the edge seal rather than draining off it, and prolonged wetting is what degrades polysulphide.
What a misted unit is costing you while it sits there
The obvious loss is the view, and that is the one people mention. The measurable losses are elsewhere.
A saturated cavity conducts heat far better than a dry one, so the unit’s thermal performance falls away and the inner pane runs colder, which produces more surface condensation in the room below. Water in contact with the low emissivity coating degrades it, so the solar and thermal behaviour drifts. And on a timber sash, persistent moisture at the glazing rebate eventually reaches the timber itself. Whether that adds urgency is covered on whether a misted rooflight is an emergency.
Whether the glass alone can come out
It depends entirely on how the window was built, and roof windows are usually the awkward case.
In most centre pivot and top hung roof windows the pane is bonded into the sash as a factory assembly rather than held by a removable bead, so the sealed unit and the sash are effectively one component. Site glazed frames, common on lanterns and some flat units, are different, and the pane is a separate part with its own gaskets. The glass only question goes through which is which.
What happens from here
For a roof window in a tiled or slated slope, a like for like replacement into the existing timber trimmed opening is an ordinary day’s work. The outer frame sizes of the common ranges have been reasonably stable for decades, so a unit from the nineteen nineties usually has a current equivalent that lands in the same hole.
The flashing kit is always new, because kits are matched to a specific frame profile and covering depth. The tiles or slates around the opening come off in courses, are stacked and go back. The room below is protected first, the old unit is lifted out from above rather than pushed through, and the roof is closed and watertight at the end of the working day. The full sequence sits on the skylight replacement page.
Related questions
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- Do I need building regulations approval for a rooflight?
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- Do rooflights add value to a house?
- Is a rooflight cheaper than a dormer?
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