Why Has My Skylight Gone Cloudy Between the Panes?

Rooflight glass is almost always a sealed unit, two or three panes held apart by a spacer bar and bonded at the edge. The gap is filled with dry air or gas, and a drying agent in the spacer keeps it dry. The cloud you see is moisture condensing inside that gap.

A quick test tells you where the problem lies. Wipe the face you can reach with a dry cloth. If the haze moves, it is ordinary condensation from the room. Look at the outer face from a safe place, or ask someone to check. If it is clean there and the fog stays, the moisture is trapped between the panes.

In a misted rooflight the fog often clears a little on a warm, dry afternoon and returns on a cold night. That pattern is a giveaway. You may also see a fine tide mark at the edge of the unit, or white mineral deposits left behind where the moisture has dried away. Those marks live inside the glass. They never wash off.

A twin-skin plastic dome can fog in a similar way when its outer skin has cracked. That is a different cause, covered in our guide to replacing a cracked or yellowed polycarbonate dome.

The edge seal is the weakest part of a sealed unit, and a rooflight puts it under more strain than a wall window does. It faces the sky. Every day it heats in the sun, cools at night, and sheds rain and frost, so the sealant is stretched and relaxed year after year.

Standing water makes things worse. A flat rooflight where water lies along the lower edge keeps the sealant wet for long stretches, and that wears the bond sooner. Frames that flex in the wind or settle with the house put extra pull on the edge. A unit that was poorly made, or glazed without proper support blocks, may fail early.

Eventually the seal opens by a hair. Air passes in and out as the temperature changes, and each breath leaves a little more moisture behind. The drying agent can only hold so much. Once it is full, the fog begins.

It happens in roof windows, roof lanterns and flat rooflights alike. Age brings it on faster, but a young unit can fail too if the seal was flawed. It is a wear-and-tear fault, and it says nothing about how clean you have kept the glass.

A misted pane is not dangerous, and the roof does not suddenly fail. The glass still keeps the weather out. What you lose is clarity, some daylight at the spot beneath, and a fair share of the unit's insulating value, because the gas fill and the dry gap have gone. Whether any of this is a rush is covered in is a misted rooflight urgent.

The remedy is a new sealed unit. In some roofs that is all that is needed. In others the frame, flashings or opening mechanism have aged alongside the glass, and a whole new rooflight is the sounder choice. How that call is made is explained in our guide to glass against a new unit.

Whatever goes in has to satisfy the Part L heat-loss limit for its type, which Approved Document L sets out for existing homes. We fit to current Building Regulations.

Note which pane is affected and take a clear photo of the whole rooflight from inside. Mark on a rough sketch of the room the place you use beneath it, since that spot decides how much the cloud bothers you. Check the ceiling around the opening for brown marks, which would suggest a leak as well.

Then ask for a survey. Room, spot and roof are all seen, and the unit is measured, before any price is written. A fixed written quote follows, and a 10-year workmanship guarantee in writing backs the job.

You can start with the free survey form or send the photo on WhatsApp on 07880 284133. Our wider page on misted rooflight replacement shows how the whole process runs.