First, find where the haze sits
Before deciding anything, work out which surface is cloudy, because a skylight that has gone cloudy on the outside, the inside or between the panes is telling you three different things.
Start from inside the room on a dry, bright day. Wipe a small area of the inner face with a soft damp cloth. If it comes up clear, the haze was condensation, cooking film or dust, and the unit itself may be fine. If it stays cloudy, look at the outer face from a window or from a ladder at the edge of a flat roof, never by standing on the glazing. Dirt, algae and hard-water marks on the outer face build up on shallow flat rooflights, because rain runs off a low pitch slowly and leaves behind whatever it carries.
If both faces are clean and the view is still milky, the haze is inside the material or inside the sealed unit. That is where replacement comes in.
| Where | What it looks like | Usual cause |
|---|---|---|
| Outer face | Grey or green film, streaks | Dirt, algae, lime scale |
| Inner face | Fog that wipes off | Room condensation |
| Between panes | Milky film, white spots | Failed edge seal |
| In the plastic | Yellow tint, fine cracks | UV ageing |
Plastic that the sun has worn out
Polycarbonate sheet and acrylic domes are the usual culprits when a rooflight turns yellow or frosted, and the cause is ultraviolet light slowly breaking down the surface of the plastic.
Good polycarbonate is made with a thin UV-resistant layer on the outer face. That layer protects the sheet beneath it, but it is only a fraction of a millimetre thick, and years of sun, wind-blown grit and hard brushes wear it away. Once it has gone, the plastic underneath starts to yellow and turn brittle. Sheets fitted the wrong way up, with the protected face towards the room, age far faster, and that mistake is easy to make on a DIY job. Multiwall sheet adds another problem: dirt, moss and moisture get into the open flutes at the ends, and the channels go grey or green from the inside, where no cleaning can reach.
Acrylic domes, the bubble rooflights on many flat-roofed extensions and garages from the 1970s and 1980s, age in a similar way. The surface dulls, then fine cracks called crazing spread across it, and the dome looks frosted even when clean. Polishing can lift a light surface bloom, but it cannot remove yellowing that runs through the material or crazing that has gone below the surface. If your skylight is one of these, our guides to replacing polycarbonate rooflights and replacing dome rooflights set out what moving to glass involves.
Coatings that have worn or been scoured away
Modern rooflight glass often carries a coating on its outer face, and when that coating is damaged the glass can look smeared or hazy however often it is cleaned.
Self-cleaning glass has a thin layer, usually based on titanium dioxide, that breaks down organic dirt in daylight and lets rain sheet off rather than bead. It works well on a clean surface, but abrasive pads, scouring cream, silicone sprays and some window products damage or mask it. The result is uneven: parts of the pane clear after rain and parts stay dull, with streaks where the coating has been rubbed. Hard-water marks from a garden hose, left to dry on warm glass, can etch into the surface too.
The low-emissivity and solar control coatings that keep heat in or out sit on a face inside the sealed cavity, so they do not wear from cleaning. If they look discoloured or blotchy, that points back to the seal, because moisture in the cavity attacks them. A scratched or etched outer pane on a sealed unit usually means a new unit rather than a new frame; glass only or full unit replacement explains how that choice is made.
A seal that has let go
When the haze sits between two panes of glass and cannot be touched from either side, the sealed unit has failed, and the cloudiness is what repeated condensation leaves behind.
At first a failed unit mists on cold mornings and clears by the afternoon. Each cycle leaves a trace of minerals on the inner faces of the glass, and over months those traces build into a permanent milky film, sometimes with white spots or tide marks near the lower edge where water collects. At that stage the glass looks cloudy even on a warm, dry day. The mechanism behind it, the edge seal, the spent desiccant and the lost gas, is covered in why rooflights mist between the panes, and whether misted glass can be cleared looks at demisting services and why they do not bring the unit back.
What clear glass will change in the room
A cloudy rooflight acts like a diffuser, so replacing it with clear glass changes the character of the light as well as the amount.
Hazy plastic scatters sunlight into a soft, even glow. It is dull, but it rarely throws a hard sun patch or glare. Clear glass lets the sun straight through, so on a south or west facing roof you will see a bright shape move across the floor and walls through the day, and the room will warm faster in summer. In a Cambridge side-return kitchen, where an old dome has sat over the worktop for decades, the change can be striking. That is why we look at the orientation before choosing the glass: a solar control coating or a light diffusing interlayer can keep the glow you liked while giving back the brightness you lost. On a north facing roof, clear glass simply gives a brighter, steadier light. Will a new rooflight be brighter than the old one and our page on how we plan the light go further into this.
If the old unit is also in the wrong place, replacement is a good moment to move it. Every quote for a misted or failed rooflight replacement comes with a written light plan setting out the position, size, glass and shading. When you are ready, book a light survey or message us on WhatsApp.
Questions people also ask
Can a yellowed polycarbonate rooflight be cleaned back to clear?
No. Surface dirt washes off, but yellowing is a chemical change in the plastic itself, caused by UV. Once the protective outer layer has gone, the colour runs into the sheet and cleaning cannot reach it. Replacing the sheet or the whole rooflight is the lasting answer, and moving to glass stops it happening again.
How quickly does a skylight go cloudy?
It depends on the material and the exposure. Glass does not yellow at all. Plastic glazing on an open, south facing roof ages faster than the same sheet on a shaded north slope. A sealed glass unit usually lasts many years before its seal gives out; how long rooflight glass lasts covers the factors.
Is the cloudiness on the inside of my rooflight just condensation?
If it wipes away, yes. Condensation on the room side forms when warm, moist air meets a cold surface, and it is common over kitchens and bathrooms. If it keeps returning, better ventilation or a warmer, better insulated unit will help. If it does not wipe off, the haze is inside the glass or the plastic.
Does a cloudy rooflight lose more heat?
Cloudy plastic was never a good insulator, and a sealed unit that has gone cloudy between the panes has lost its gas fill and dry cavity. In both cases a modern double or triple glazed unit will keep the room noticeably warmer.
