Why does condensation form inside a roof lantern?

Every kitchen or bathroom below a lantern breathes moisture into the room all day, and that moist air rises and cools as soon as it reaches the glass. A kettle, a hob, a dishwasher opening at the end of a cycle and a shower two doors down all send water vapour upward, and a lantern over a kitchen extension in a new-town township such as Orton or Hampton often collects more of it than the rest of the ceiling, simply because warm air rises straight to the highest, coldest point in the room.

The air itself is rarely the problem. It is what happens when that air meets a surface cold enough to bring it to its dew point, the temperature at which water vapour turns back into liquid. On a still, cold night, the outer face of a lantern's glass can sit several degrees below the room's air temperature, and the aluminium of a glazing bar cools faster still, which is why misting almost always starts at the bars before it reaches the open pane.

A glazing bar has less glass depth to insulate it than the open pane either side, so its inner face cools quickest and gathers the first film of moisture, right where the bar sits in the eyeline from the table below. The ridge bar running the length of the lantern behaves the same way, and the corners where two bars meet are usually the coldest points of all.

Slimmer aluminium sections with a thermally broken core, the kind used in slim rafter profiles, hold their inner face closer to room temperature than a bulkier bar with a solid core, so they mist later and clear faster once the extractor or the heating comes on. The improvement sits in the metal, not only in the glass next to it.

Kitchens produce more water vapour in an evening than almost any other room in the house: steam off pans, a kettle boiled several times, a dishwasher and washing machine both venting as they run. Planning where the lantern sits over the table also means planning where that moisture rises to, and a cooker hood ducted outside rather than recirculating through a filter removes far more of it at the source than any change to the glass can.

• A ducted hood clears steam before it reaches the ceiling; a recirculating filter only moves it around the room.
• An island under the lantern often sits furthest from any wall vent, so an opening vent in the glass itself does more work there.
• A closed kitchen door at night traps the same moisture under the lantern until morning, rather than letting it spread through the house.

The single biggest lever is what sits between the panes. A sealed double-glazed unit with a low-emissivity coating and a warm-edge spacer bar, rather than an older aluminium spacer that conducts cold straight to the edge of the glass, keeps the inner pane's surface temperature closer to the room's, so it takes a colder night to bring it to dew point. Triple glazing narrows that gap further again, at the cost of a heavier unit that the frame and kerb must be built to carry.

Solar control coatings chosen for a south-facing lantern do not by themselves change condensation risk. The coating manages heat gain and glare, while the spacer bar and gas fill between the panes manage the surface temperature that condensation depends on, so the two specifications are chosen together rather than swapped for each other.

A lantern sits on a kerb, or upstand, raised above the flat roof, and if that kerb is timber with no insulation of its own, cold bridges straight up through it to the frame sitting on top, cooling the whole perimeter of the glass from below. An insulated kerb breaks that path, and current thermal requirements under Building Regulations generally call for one on a new lantern.

The flat roof itself needs its own fall away from the kerb, typically a fall of 1:80, so standing water never sits against the upstand and finds its way into the timber inside. A kerb that stays dry on the outside stays warmer on the inside face too, and a wet, cold kerb is often the reason a lantern mists along one edge and not the others.

Glass and kerb specification only slow condensation down; the room still needs a way to shed the moisture it makes. An extractor over the hob, ducted outside rather than filtered and recirculated, removes cooking steam before it reaches the ceiling, and a bathroom fan on a humidistat does the same job through the evening. An opening vent built into the lantern itself lets warm, damp air out at the highest point in the room, which is exactly where it collects.

None of this works against keeping the room warm. Heat loss through a lantern and condensation on it are governed by the same surface temperature, so a well-specified unit that holds its heat is also the one least likely to mist.

Surface condensation on the inside face clears once the room warms up or the extractor runs. A haze sitting between the panes, one that never clears whatever the room does, means the sealed unit's seal has failed and the gas fill between the panes has been lost or replaced by damp air. That calls for the unit to be renewed, not cleaned, because cleaning cannot reach the inside of a sealed cavity.

None of these choices show up in a photograph, but they decide whether the sky stays clear from the table below or turns to a grey smear by the second winter. A sightline sketch drawn before the roof is cut sets out the eye position, the bar spacing and the reveal, and the same conversation covers glass, kerb and ventilation together rather than leaving condensation to be solved after the fact. See the sightline sketch for how that drawing works, and what affects the cost of a rooflight for how glass and kerb specification change the numbers. For the fuller picture on shape, size and siting, the roof lantern installation page covers the whole job.

Does opening a window help more than a vent in the lantern?

A window lower in the room brings in cooler, drier air to replace the warm, moist air rising out through a vent at the top, so the two work together. A vent low down on its own does not have the same effect, because warm air does not sink towards it.

Will condensation damage the frame over time?

An aluminium frame does not rot the way timber can, but water sitting on a sill or in a corner for long periods can find its way into fixings and linings below, so a lantern that mists heavily and often is worth having looked at rather than left.

Does a bigger lantern mist more than a small one?

Size on its own is not the deciding factor. A large lantern with warm-edge spacers and a good kerb can perform better than a small one with an old aluminium spacer and a cold timber upstand, because bar count and spacer specification matter more than the overall footprint.

Is condensation worse on a north-facing lantern?

Orientation changes solar gain rather than condensation risk directly, though a room that stays cooler for longer on a north-facing extension gives moist air less chance to warm back up and evaporate off the glass, so ventilation matters slightly more there.