Can a Misted Rooflight Be Cleared?

The short version

People usually ask this because a new unit sounds like a big job for what looks like a stain. It is a fair thing to ask, and there are services advertised that promise to clear misted double glazing without replacing it. The usual method is to make small holes in the unit and treat the gap between the panes. We do not suggest it for a rooflight, for the reasons below.

Cleaning the glass

Washing and wiping only deal with dirt on the outer faces. The mist is on the two inner faces, in a sealed gap, so a cloth never touches it. If you wipe both sides and the haze is unchanged, it is inside the unit. No cleaning product changes that.

Drilling and defogging

Some firms drill tiny holes in the glass, dry out the gap and fit small vents. The idea is that the space breathes and the moisture escapes. It can make the glass look clearer for a while, but it takes away the thing a sealed unit is for. The gap is no longer sealed, so the insulating value is gone, and damp air can return on any humid day.

On a wall window you might shrug at that. On a rooflight, which faces the sky and swings through large temperature changes, the haze tends to return, and the holes leave marks in the glass. You also end up paying for a job that has to be done again.

Waiting for it to clear by itself

Misting that comes and goes with the weather can give the impression that the glass is recovering. On a warm, dry afternoon the moisture evaporates and the glass looks fine. That is the unit breathing, which is exactly the fault. The moisture comes back with the next cold night, and each cycle leaves a little more mineral staining on the inner glass.

What clear glass actually needs

There are two routes, and they are set out fully in new sealed unit or a whole new rooflight.

• A new sealed unit. The old glass comes out of the existing frame, the rebate is cleaned and a unit made to the measured size is set in. It suits a frame that is square and sound.
• A whole new rooflight. The frame and glass are replaced together on a checked upstand. It suits a frame that has bowed, cracked or corroded, or a rooflight that predates modern insulation standards.

A point about plastic domes

If the misting is in a plastic dome or a twin-wall sheet, the answer is different again. Those are usually yellowed and brittle by the time they mist, and a glass rooflight is the sensible replacement. Our page on replacing a polycarbonate rooflight with glass covers what changes on the roof.

Why a proper replacement tends to be better value

A temporary fix costs less on the day, but it leaves you with a unit that is no longer insulating and a glass that may need replacing anyway. A new sealed unit restores the insulation as well as the view. There is a fuller breakdown of what drives the price in what it costs to sort a misted rooflight.

What happens at the survey

We check the frame, the upstand and the weathering, then tell you which route fits and why. You then get a written quote. Inside the house, floors and furniture are covered while the glass comes out and goes in. Each evening the opening is closed against the weather, and the old glass leaves with us. Photographs of the concealed parts are handed over at the end.

If you are tempted to wait

A misted unit is not urgent, and nobody needs to rush. The glass still keeps the weather out. But the longer it stays misted, the more the inner faces stain, and a unit that could have been replaced on its own may, if the frame is left to deteriorate beside it, turn into a whole new rooflight job.

It is worth having a look at the frame sooner rather than later. A frame in good order means the cheaper route remains open to you. A frame that has begun to rot, corrode or split narrows the choice.

A note on private sales and landlords

If you are selling, a buyer's survey will often note misted glass. If you let the property, tenants notice a cloudy rooflight in the room. In both cases clear, properly made glass settles the point quickly, whereas a drilled unit raises questions about what else has been done to the building.

What you will see when it is done

The difference is easy to spot as soon as the new glass is in. The haze has gone, the room is brighter, and the outside edge of the glass is crisp where the old unit looked soft. Winter mornings are the real test, because a sound unit stays clear when the temperature drops and a failed one does not.

A small detail worth asking about is the label. Many sealed units carry a mark on the spacer recording the maker and the date. Keep a note of it with your house papers, since it identifies the unit if you ever need to ask the maker about it.