Why Is My Rooflight Misted Between the Panes?

A double glazed rooflight is a sandwich. Two panes of glass sit a short distance apart, kept there by a spacer bar that runs around the edge. The cavity is sealed and filled with dry air or an inert gas such as argon. That still layer of gas is what slows the passage of heat.

The spacer bar is hollow and filled with a drying agent, called desiccant, which absorbs any trace of moisture that was in the gap when it was sealed. The edge of the sandwich carries two seals. An inner seal, usually a butyl strip, stops vapour moving in. An outer sealant bonds the panes to the spacer and holds the unit together.

While both seals hold, the gap stays clear for years. When one fails, the gap starts to breathe.

A rooflight is hard on its seals. Several causes act together, and often more than one is behind a single misted unit.

• Temperature swings. The glass heats in summer sun and cools overnight. The panes expand and contract by small amounts, and the edge seal flexes with them, day after day.
• Standing water. On a flat rooflight, water that sits against the edge of the glass keeps the sealant wet. A frame that drains poorly, or a unit fitted without enough fall, puts the seal under constant strain.
• Ultraviolet light. Over many seasons the outer sealant can harden and crack under sunlight.
• Pressure changes. A change in air pressure or temperature pushes air in and out of the cavity if there is any tiny path.
• A fault in making the unit. A seal that was not fully bonded at the factory can fail early, in which case the maker's guarantee may apply.

Once vapour gets past the seals, the desiccant takes up what it can. When it is full, moisture sits on the inner faces of the glass and shows as mist.

Mist on the room-side face of the glass is ordinary condensation from the air in the room, and we deal with it in our condensation answer. Mist between the panes is a seal failure. A dry cloth settles it.

Wipe the inside face, then the outside. If the cloudiness stays, it is in the gap. Other signs support this reading.

• The mist comes and goes with the weather but the glass is never fully clear.
• Streaks, droplets or a pale film appear along the lower edge, where moisture collects.
• A white mineral bloom forms on the inner faces after the mist has dried, a stain that no cleaning removes.
• A rooflight that was clear last winter is cloudy this one.

Our guide to the signs a rooflight needs replacing covers the other tell-tales, including a cold frame and a worn handle or hinge.

Once air and vapour enter the gap, some of the insulating gas is likely to have escaped too. A misted unit lets heat through faster than it did when new, so the room is colder in winter and the glass is cold to the touch.

The glass stays safe, and the mist is not a risk to people in the room. The loss is in clarity and insulation. Neither gets better with time. The mineral deposit on the inner faces can become permanent, and the view out is clouded.

Some firms sell a drilling treatment, in which small holes are made in the glass to clear the mist and a vent is fitted. It does not restore the gas fill or the seals, so the insulation does not come back.

A few details speed up a quote and make it accurate before we get on the roof.

• The room the rooflight lights, and whether it is a roof window, a flat rooflight or a lantern.
• Roughly how long the glass has been misted, and whether it is one pane or several units.
• Any stamp on the glass edge, or a plate on the frame, which gives the maker and the model.
• Photographs of the glass from inside and of the frame, so we can see its condition.

We still measure on the roof before we price anything, because the size of the replacement unit comes from the opening and not from the photographs.

There are two routes, and the choice depends on the state of the frame.

Our answer on new glass or a whole new rooflight sets out how we judge this at the survey, by measuring the frame, the kerb or rafters, and the glazing itself.

If the frame is sound and the maker still supplies a matching sealed unit, new glass is the smaller job. If the frame is rotten, out of square or from a discontinued range, a whole new rooflight is the route that lasts.