The short answer
Usually not. If the mist is between the two panes of a sealed unit, no amount of wiping will touch it, because the moisture is inside a cavity that was closed at the factory. The lasting fix is a new sealed glass unit, or a new rooflight if the frame has had its day. If the mist is on the room side or the outside face, though, that is a different matter, and a cloth can clear it.
First, find out which surface is misted
Run a dry finger across the glass on the room side, then clean the outside if you can reach it. Mist that wipes off is condensation or grime on the surface, and it says something about the room rather than about the glass. Mist that stays put, with a cloudy film, droplets or a pale ring of dried residue that you can see but not touch, is inside the unit.
The two look alike from across the room, so it is worth the thirty seconds. Another clue is timing. Surface condensation comes and goes with cold mornings and steamy rooms. Mist between the panes tends to be there day after day, though it can look worse after a cold night and fade a little in dry afternoon sun.
Why mist between the panes cannot be wiped away
A double or triple glazed unit is two or three sheets of glass held apart by a spacer around the edge, with the gap sealed and filled with dry air or gas. A desiccant in the spacer soaks up the small amount of moisture trapped at manufacture. When the edge seal fails, humid air gets in, the desiccant fills up, and moisture condenses on the cold glass inside the cavity.
At that point there is nothing to reach. The cause is a failed seal, and the seal cannot be reached from either side of the glass. You can read more on the cause in our guide to why rooflights mist between the panes.
What about the products that claim to demist units
You may come across services that drill small holes in the glass, clean the cavity, and fit tiny vents. Some of them can clear the visible mist. We do not recommend relying on this for a rooflight, for three reasons.
- The seal is still gone
- The unit has been opened rather than resealed, so the cavity breathes with the room and the outside air. Moisture will return.
- The performance is lost
- Once the gas fill has gone and the cavity is vented, the unit insulates like a plain pair of panes, and any low-emissivity coating inside the cavity can be marked by the process.
- It is overhead glass
- A drilled hole in a pane above your head is a break in the glass. Toughened glass cannot be drilled at all, because it shatters, and drilling a laminated safety pane overhead defeats the point of it; most rooflight glass is one or the other.
What does clear it
There are two routes, and the right one depends on the state of the frame.
- A new sealed unit in the existing frame. If the frame, seals and flashing are sound, the glass alone can be replaced. This does not usually need a Building Control notification, because it is only the glass and not the whole window.
- A new rooflight or roof window. If the frame is tired, the unit cannot be re-glazed, or the rooflight is old enough that you want better performance, replacing the whole thing is the better route. That is notifiable under the Building Regulations, and we handle the notification.
Our guide on replacing just the glass in a rooflight explains how to tell which route fits yours. Glass-only replacement works when the unit can be made to the same size and the frame will take it. It does not work when the rooflight is a single-piece moulded unit, as many small domes are, or when the frame has warped.
Why replacing the glass is not the end of the job
A glazing swap on a roof window or a lantern is neat work, but the room underneath may have had years of dripping condensation or staining beside the frame. When we replace a unit, we look at the lining and the plaster beside it, and note anything that wants making good, so the visible edge is finished well and not just the glass. If an old stain or a cracked ceiling line bothers you, tell us at the start so it can be written into the finish schedule alongside the quote.
Why it happens to some units sooner than others
Seals age. Sunlight, heat swings and the slow flexing of a frame all work on the sealant at the rim of the unit, and a rooflight gets more of all three than a wall window because it faces the sky. A unit that sits in standing water, where a flat rooflight kerb does not drain well, tends to fail earlier than one that sheds water freely. A poorly made unit may fail early; a well made one can last a long time. That is why two rooflights of the same age on the same house can behave differently, with one clear and one cloudy.
What to do in the meantime
A misted unit is usually a nuisance and a slow loss in performance, and it rarely needs hurried action. While you arrange a replacement, keep the room ventilated and the surface of the inner pane dry, so you can tell what is going on between the panes and what is only steam from a shower. Take a few photos of the glass in daylight, from inside and out, with a coin or a tape in the shot for scale, and note the size of the rooflight if you can read it from a label on the frame.
It also helps to read about whether a failed unit is a safety concern, in our answer on whether a misted rooflight is dangerous, and what it costs you in heat, in does a misted rooflight lose heat.
When you want a new unit priced, send those photos with the quote form. We will survey the opening, tell you whether the glass alone will do, and write it all into the misted and failed rooflight replacement quote.