If the mist is genuinely between the two panes, where you cannot wipe it from either side, the sealed glazing unit has failed. Its perimeter seal has let go, the dry gas and the desiccant inside are finished, and moist air is now moving in and out with every temperature change. It is not something that can be dried out and it is not caused by anything you have done. The glass has to be replaced. Everything else that looks like it is a different problem entirely, and most of the time it is not a problem at all.
Three mists, and telling them apart in ten seconds
Put a finger on the mist from inside the room. If it wipes clear, the moisture is on the room-side surface and this is ordinary condensation. If it does not wipe, rotate the window or reach the outer face and try again from outside. If it wipes from there, the moisture is on the outer surface and there is nothing wrong. If it wipes from neither side, it is between the panes and the unit has failed.
That test takes less time than reading about it, and it saves people a great deal of unnecessary worry, because two of the three answers cost nothing.
What has actually happened inside a failed unit
A sealed glazing unit is two panes held apart by a spacer bar around the edge. The spacer is filled with a desiccant that absorbs the small amount of moisture sealed in at the factory, and the perimeter is closed with a primary butyl seal and a secondary structural seal, usually polysulphide or silicone. The cavity holds dry air or argon.
When that perimeter seal is breached anywhere, the cavity starts breathing. Warm damp air enters as the unit cools and contracts, then cannot leave as fast as it came in. The desiccant absorbs it until it is saturated, which can take months or years, and after that the water has nowhere to go. Every cold night it condenses on the inner face of the outer pane. Every sunny morning it evaporates and the glass looks clear again, which is why people keep thinking it has cured itself.
Why the mist leaves a permanent mark
The cycle does not stay cosmetic. Condensed water sitting on glass leaches sodium out of the surface, and over a few years that etches a fine haze that will not clean off even after new glass is fitted, if you were minded to keep the old pane. On the roof it goes faster than on a vertical window, because a roof window gets far more direct sun and far bigger daily temperature swings.
Once the etching stage is reached, the pane looks permanently cloudy in raking light even on a dry day. That is the point at which people stop calling it condensation and start calling it damage, though it has been the same fault throughout.
Why the drying schemes do not solve it
There are outfits that drill the pane, flush the cavity, apply a chemical dryer and fit a small breather valve. It clears the visible mist for a while. What it does not do is restore the seal, the argon fill or the desiccant, so the unit is now permanently vented to the outside air and its insulating performance is gone. On a roof window, where the whole point of the glass is a low U-value over a large area of exposed roof, giving that up is a poor trade.
It is also worth knowing that it does nothing about the etching, and the valve becomes another opening on a surface that sits in weather. We do not offer it and we would not recommend it.
Why roof windows fail earlier than the ones in your walls
A roof window’s glass leads a harder life than any vertical pane in the house, and four things account for most early failures.
- Ultraviolet exposure. A south-facing pitched pane takes far more direct sun than a wall, and ultraviolet light degrades the secondary seal.
- Thermal cycling. The pane can swing from below zero at dawn to well above forty degrees by early afternoon. The seal is worked hard every single day.
- Standing water at the bottom edge. Blocked frame drainage keeps the lower edge of the unit wet, and constant wetting of the edge seal is its single worst enemy.
- Edge shading and hot spots. An external awning or a badly fitted blackout blind can hold heat against part of the pane and leave the rest cold, which stresses the seal unevenly.
The first two you cannot change. The third you can, with the yearly clear-out described on do roof windows need servicing, and it is the most useful thing an owner does for the glass.
Mist on the room side is the house talking
Condensation on the inner face is a humidity and surface temperature story, not a fault. Roof glass is the coldest surface in most rooms, so it is where the air gives up its moisture first. A misted roof window in a bathroom in January is behaving exactly as physics requires.
What reduces it is background ventilation, using the trickle flap rather than sealing it, running extract fans for their full overrun, keeping the heating on low in unused rooms rather than off, and making sure any blind is not held tight to the glass overnight. A window that has been fitted with too much plasterboard closing in on the reveal will also condense more, because warm air never reaches the pane properly.
Mist on the outside face is good news
Dew on the outer pane on a clear autumn morning means the outer glass has radiated its heat to a cold sky and dropped below the dew point of the air around it, which it can only do if very little warmth is coming through from the room. It is a sign of a well insulated unit. It clears within an hour or two of the sun reaching it and it needs no action at all.
New glass, or a new window
Both are possible and the choice is usually decided by age. On a unit under about fifteen years old, where the frame is sound and the model is still supported, replacement glass is available as a spare part and goes in from inside without disturbing the flashing or the covering. That is the cheaper answer and often the right one.
On an older window, particularly one where the timber has darkened at the lower corners, the sash no longer pulls up square, or the model is out of production, new glass into a tired frame is money on the wrong side of the problem. A full unit change into the same opening gets you current thermal performance, a new gasket line, a new flashing kit and a fresh start on the perimeter detailing.
| Situation | Usual answer |
|---|---|
| Under 15 years, frame sound, model current | Replacement sealed unit |
| Over 20 years, or model discontinued | New window into the same opening |
| Timber soft at a bottom corner | New window |
| Two or more units misted on the same roof | Change them together, one visit, one scaffold |
What we do about it
We come and look. We confirm which of the three mists you have, read the spacer bar for the manufacturing date, identify the model from the data plate on the sash, and tell you whether the glass alone will settle it. If it will, we fit the glass. If it will not, we quote a replacement unit and say why.
We install and replace roof windows and rooflights across Oxford and up to twenty five miles by road. Fifteen years in the trade, VELUX, Fakro, Keylite and Roto units fitted, Building Control notification handled where it applies, the roof never left open overnight, and ten years on the workmanship. You get a range by email and a fixed price after a survey. Call 01865 704245.
Helpful reading on this
Costs, comparisons and the questions we are asked most.
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