Why is my roof window misted up inside the glass?

Why it happens

The perimeter seal has a life, typically twenty to twenty five years overhead. Roof glazing fails sooner than vertical glazing for two reasons: more thermal cycling, from cold at dawn to hot by midday, and water sits on it rather than running off, so the seal is wet more of the time.

It cannot be fixed

There are companies who will drill the unit, dry it and fit vents. It clears the visible misting temporarily. It does not restore the seal, the argon has already gone, the insulating performance does not come back, and the unit continues to breathe moist air.

We do not offer it, because on an overhead unit at height the labour to do it is most of the labour to replace it properly.

What replacement involves

Depends on the model. Some roof windows have a replaceable sealed unit, so the glass alone is changed and the frame stays. On others the glass and sash are one bonded component and the sash is replaced.

Where the frame is more than about twenty five years old, replacing the whole window is usually the better value: you get current thermal performance, new seals, new hinges and a warranty on all of it.

It is not urgent

Nothing is getting into the building. The unit has lost most of its insulating value and it looks poor, so it is worth planning rather than rushing.

If several windows went in at the same time they will fail at about the same time, and doing them in one visit shares the access cost. That is the main argument for doing them together.

The perimeter seal has failed

If the moisture is between the two panes and cannot be wiped off either face, the sealed unit has gone. That is not a leak, not condensation in the room, and not something a cleaning or a repair will fix.

What has happened

A sealed unit is two panes with a gas filled cavity, held apart by a spacer bar containing a desiccant that absorbs the small amount of moisture sealed in at manufacture. A perimeter seal keeps the gas in and the weather out.

On a rooflight that seal works harder than on any window in the building. It sits horizontally or near horizontally under standing water rather than shedding it, and it faces the sky, so the cavity gas expands and contracts through the full temperature swing of a clear night and a sunny afternoon, every day, for decades. That daily pumping is what eventually works it loose.

Once air can get in, the desiccant absorbs moisture until it is saturated. That takes a while, which is why misting appears suddenly years after the seal actually went.

What follows

The argon has escaped, so the unit is performing like plain double glazing rather than to its original specification. And once the cavity is wet the low emissivity coating starts to degrade, which produces the permanent bloom that does not clear even on a warm dry day.

Is it urgent

No. It is not a safety issue and nothing is going to fall out of the roof. It is a performance and appearance problem, and it does not improve on its own.

What it does mean is that the room is colder than it should be and the view is spoiled.

How old is it likely to be

Fifteen to twenty five years is a normal life. Earlier than that usually points at something specific: standing water from a kerb with no fall, an installation that clamped the unit rigidly so it could not move thermally, or a manufacturing batch problem.

Coastal and exposed positions tend toward the shorter end of the range.

Glass or whole window

On some models the sealed unit is held by a removable bead and can be changed with the frame in place. Whether it is worth doing is a separate question, because most of the cost of this job is getting to the window rather than the component.

Frame under fifteen years and sound: replace the glass. Over twenty: replace the window. Any timber decay: replace the window, always.

What not to do

Drilling and drying. The visible misting clears, the seal is not restored, the argon is gone, and the unit then deliberately breathes outside air with a spent desiccant. At height, the labour to do that is most of the labour to replace it properly.

The seal has failed

A sealed unit is two panes held apart by a spacer, with a desiccant inside the spacer to absorb any residual moisture, and a perimeter seal keeping the cavity closed.

When that seal lets go, outside air enters and leaves the cavity as the unit warms and cools each day. It brings moisture with it. The desiccant absorbs it until it saturates, and after that the moisture condenses inside the cavity where nothing can reach it.

That is what you are seeing, and it is why the misting comes and goes with the weather.

The causes behind it

Age, mostly. Sealed units have a service life of fifteen to twenty five years, and a rooflight works harder than a vertical window: greater temperature swings, direct sun, and standing water on shallower installations.

Also poor drainage at the sill, which keeps the bottom edge of the unit permanently wet and attacks the seal from outside.

What it is not

Not a leak, not a health risk, and not urgent. The unit still keeps rain out.

What it costs to leave

Thermal performance, because the gas fill has gone.

And light, permanently. As moisture repeatedly condenses and evaporates it leaves mineral deposits that etch the inner faces. That staining does not come off even if the unit is replaced, so the longer it is left the worse the glass looks in the meantime.

The choice

Under about fifteen years with a sound frame, a replacement sealed unit is usually right. Over about twenty, hinges and gaskets are the same age and the whole window is the better buy, since the access cost is identical.

What is not worth doing is drilling and venting the unit, which permanently ends its thermal performance and does not stop the staining.