Misted and failed rooflight replacement in Exeter

A sealed glass unit is two or three panes held a few millimetres apart by a spacer bar, with the gap filled with dry air or an inert gas such as argon. The spacer holds a drying agent, a desiccant, that keeps the cavity dry. The whole sandwich is closed by a primary seal against the glass and a secondary seal around the outer edge. While that seal holds, the cavity stays clear for the life of the unit.

When it fails, the first sign is usually a faint haze in one corner or along one edge, often seen on a bright morning when the sun comes through at a low angle. Then a film of tiny droplets appears on the inside faces of the glass, and as the weeks pass the haze spreads. On a warm afternoon the droplets can vanish and the pane looks clear for a few hours. By the next cold night the moisture is back. That coming and going is the mark of a failed seal: the cavity is breathing, taking damp air in as the glass cools and pushing some out as it warms.

In heavier cases, mineral deposits dry out on the inner faces and leave a white or grey bloom, like limescale on a kettle, that stays when the droplets have gone. At that stage the unit is permanently cloudy. The glass is intact and still keeps the rain out. What has gone is its clarity and, more quietly, a good share of its insulation.

A rooflight can fail in other ways too, and it is worth separating them early. A cracked pane, a frame that has warped or corroded, a hinge that no longer holds, an opening mechanism that jams, and a covering of green or black growth over a flat rooflight dome are all signs that a unit has reached the end of its life. We cover those in our guide to the signs a rooflight needs replacing. This page concentrates on the commonest failure and the one people most often ask about: a misted unit.

Before anything is ordered, check that the unit has really failed. Moisture on a rooflight is not always a failed seal, and the position of the moisture tells you which you have.

If the haze wipes away, it was condensation.

On the room side of the glass

Droplets on the inside face of the inner pane are ordinary condensation. They come from humid room air meeting cold glass, and they are most common in bathrooms, kitchens and rooms with drying laundry. The unit is fine. Better ventilation, a trickle vent or an opening rooflight usually helps, and a modern unit with a warm-edge spacer and a coated inner face resists it better.

On the weather side of the outer pane

A film of dew on the outside of the roof glass on a clear, still night is the unit doing its job. The outer pane is so well insulated from the room that it falls colder than the air outside, and dew forms on it. It clears as the morning warms up. This is more likely with a good, energy efficient unit than with a poor one.

Between the panes

Moisture, haze or a bloom you cannot reach with a cloth, on the face of a pane you can see is inside the unit, is a failed seal. You can wipe both outer faces dry and it will still be there.

One quick test is to wipe the inner face of the glass with a dry cloth on a morning when the haze is showing. If the haze wipes away, it was condensation. If it stays exactly where it was, the moisture is inside the cavity. The test is simple, and it saves a good many people from replacing glass that did not need it.

If the answer is still unclear, our answer on whether misted rooflight glass can be cleared explains why the cloudiness between the panes will not shift, and what the alternatives to a new unit really amount to.

Misting looks like a cosmetic problem, but a failed unit is also a thermal one. Once the seal has gone, the cavity that was meant to hold still air or gas now holds damp air that moves. The gas fill, if there was one, has largely gone, and the low emissivity coating on the inner face, which reflects heat back into the room, can degrade in contact with moisture. The result is a unit that loses more heat than it should and feels colder to stand under.

You may notice it as cold air falling from the ceiling on a winter evening, as a rooflight that mists on the room side more readily than it used to, or as a bedroom that is harder to heat in a loft conversion. Our answer on whether a misted rooflight loses heat goes into the loss, and our page on energy efficient rooflights covers choosing a better unit this time.

A misted rooflight is not a danger to the people under it.

A misted rooflight is not a danger to the people under it. The glass is as strong as it was, and the moisture sits in a closed gap. Our answer on whether a misted rooflight is dangerous sets out the one situation where it deserves attention sooner: a cracked pane, which is a different fault from a misted one and is covered under the signs of failure.

There is one more cost, and it is the plainest. A rooflight that is meant to bring light into a room, now a milky panel with a bloom on it, brings in less of it, and what it brings is dull. The room often looks noticeably brighter once the new unit is in, because the dullness had crept up slowly.

A replacement is a chance to improve the unit, not only to copy it. Glass has moved on, and a unit made today will usually outperform one from a few decades ago, even in an old frame. The choices are these.

Double or triple glazing

Most replacement units are double glazed. A triple unit is thicker and heavier, and it only suits a frame with a deep enough rebate. Where a frame cannot take triple glazing, a good double unit with a low emissivity coating and argon is usually the better choice than forcing the fit.

Low emissivity coating

A thin, invisible metal oxide coating on one of the inner faces reflects heat back into the room. It is the single largest improvement over an uncoated old unit.

Warm-edge spacer

The spacer bar at the glass edge is where an old unit loses most heat and where condensation starts. A warm-edge spacer in a composite or plastic material cuts the cold bridge and keeps the edge of the inner pane warmer.

A laminated inner pane

Overhead, a laminated inner pane is good practice and industry guidance, because laminated glass holds together if it breaks. It is not a requirement of Part K, which deals with glazing in doors, beside them and at low level, but it is a sensible choice where people sit or walk beneath the glass.

Solar control and self-cleaning coatings

A south or west facing rooflight over a kitchen can overheat in summer. A solar control coating cuts the heat that comes in, at a small cost to winter warmth and a slight tint. A self-cleaning coating helps rain wash a low pitched pane, though it does not remove the need for an occasional wash.

Acoustic glass

A thicker or laminated outer pane damps rain noise, and it can be chosen as an option where a loft bedroom sits right under the roof.

If your frame is going too, the choices widen to the rooflight itself. Our pages on roof windows, VELUX windows, flat roof rooflights and electric opening rooflights cover what is available. A failed opening mechanism is often a good moment to move to an electric or a better manual one, since the sash is coming out anyway.

A glass-only replacement is usually a short visit. The sash or beads are opened, the failed unit is lifted out, the frame's rebate is cleaned and checked, and the new unit goes in on fresh setting blocks and glazing gaskets. The beads or clips are refitted and the weather seal tested. The old unit is taken away with the rest of the debris.

Replacing the whole rooflight is a bigger job. The covering and flashings are lifted around the frame, the old unit is released and removed, the new one is fitted on its kerb or upstand, and the flashings or flat roof covering are dressed back to it. The lining is checked, adjusted where the new frame needs it, and finished. Because the roof is opened, the work is planned around the weather, which is why many people ask us whether replacement can happen in winter. We answer that in can you replace a rooflight in winter, and give the practical side of timing in how long it takes to replace a rooflight.

In a cob or stone cottage, the lime plaster needs lime to be made good, because gypsum does not bond to it well and will crack away from it.

The inside is where a replacement can look careless. Old rooflights are often found with a lining that has been painted over several times, a plaster edge that has chipped where the old frame was removed, and a crack running from the corner of the opening into the ceiling. Where the rooflight sits in a lath and plaster ceiling, which is common in Georgian and Victorian houses in St Leonards, Southernhay and Topsham, the plaster is held on timber laths with a few millimetres of key behind it, and careless work can shake a whole section loose. In a cob or stone cottage, the lime plaster needs lime to be made good, because gypsum does not bond to it well and will crack away from it.

For those reasons we sheet the floor, seal the doorway to the rest of the house and protect the ceiling around the opening before we start. We form the lining back to a clean edge, make good with the right plaster for the ceiling and leave the opening ready to paint, or painted where the schedule says so. Our guide to replacing a rooflight without damaging the ceiling goes through the order of work. Dust is managed and the room is left clean.

We have more on how we finish an opening in our guide to how a rooflight is finished inside.

A new unit is made with fresh seals and a new desiccant, and a good one lasts for many years.

You can, for a while. It still keeps the weather out, and a misted unit is not dangerous. But it will not clear, it will usually get worse, and it loses more heat than a sound unit. If the room is a kitchen, a bedroom or a landing you use daily, most people find the cloudiness bothers them within a season or two.