The short answer
Yes, a lantern can mist up on the inside, and it is usually the kitchen's fault, not the glass's. Warm, steamy air touches the coldest surface in the room, which is often the lantern, and the moisture settles as droplets. A good extractor, a warm-edge double unit and a thermally broken frame all keep it clear. Fogging between the panes is a different fault.
What is happening on the glass
Air holds water vapour, and warm air holds more of it than cold. When that warm air meets a surface cold enough, it can no longer hold all its moisture, and the excess turns to liquid on the surface.
The temperature at which this starts is called the dew point. Boil a pan in a kitchen and the air fills with vapour. The steam rises, because warm air does, and it gathers under the ceiling. If the lantern's inner pane is colder than the dew point of that air, droplets form. They run down the glass and pool at the bars.
That is why a lantern is the classic place for it. A lantern is the highest surface in the room, where the warm damp air arrives first, and it is also made of glass, which is always colder than a plastered wall. Nothing is wrong with the lantern when it happens on a cold morning after cooking. The room is simply damper than the glass can tolerate.
A single plain pane would fog badly. Modern sealed double glazing is far better, though the inner surface still runs cooler than the walls. Our page on whether roof lanterns are energy efficient enough for Part L explains how the glass is rated for heat loss, and the same qualities that save heat also keep the inner surface warmer.
Three places condensation can appear
Where the water shows tells you what it means. Sorting the three cases saves a great deal of guessing.
| Where | What it means | What to do |
|---|---|---|
| Inner face of glass | Room air is too damp for the glass | Ventilate, warm the room |
| Outer face of glass | Glass is well insulated, sky is clear | Nothing, it clears by mid-morning |
| Between the panes | The unit's seal has failed | Replace the unit |
The outer case surprises people. On a still, clear night the outside of a highly insulated pane can drop below the dew point of the outside air, because the glass radiates heat to the sky and little warmth arrives from the room. A dewy lantern at dawn is, oddly, evidence that the glazing is holding its heat in.
The third case is a fault. If moisture sits inside the sealed gap, the edge seal of that unit has failed and the gas fill has escaped. No wiping will clear it. The sealed unit is replaced, or the lantern is renewed if the whole set is tired. Our misted and failed rooflight replacement page covers how that is done.
The extractor matters more than the glass
You can fit the best glass made and still have a wet lantern if the steam has nowhere else to go. Ventilation is the first line of defence.
The ventilation guidance in Building Regulations Part F expects a kitchen extractor capable of moving air at a set rate, set in the guidance as 30 litres per second for a cooker hood that extracts outside and 60 litres per second for any other kitchen fan. That is a guide, and the installation in your house depends on the layout. What matters in practice is that the fan exists, that it vents outside rather than recirculating, and that it runs while you cook and for a few minutes afterwards.
A fan that is irritatingly loud tends to be switched off. That is where the quiet brief comes in. A quieter extractor gets used, and a used extractor keeps the lantern clear. We note the fan position in the quiet spec when a kitchen is involved, so that the noise you hear and the damp you avoid are planned together.
Other small habits help too. Lids on pans, a window or the lantern's own vent opened a crack, and a door closed between kitchen and hall so the steam does not drift to colder rooms. Our page on opening vents on a roof lantern shows when a vent is worth adding, and the electric and opening rooflight installation hub covers the powered kind.
Glass and frame choices that keep it clear
The build of the lantern decides how cold the inner pane gets. Four features do the work.
- A low-emissivity coating, a thin invisible layer that reflects heat back into the room and keeps the inner pane warmer.
- A warm-edge spacer, the strip that separates the two panes. Metal spacers conduct cold to the glass edge, and warm-edge versions do not, so the edge stays clear of droplets longer.
- A gas-filled cavity, usually argon, which slows heat moving from room to outside.
- A thermal break in the frame, a strip of insulating material inside the aluminium bars so they do not carry cold straight through.
Weak points show up first at the bars and the base, where aluminium and timber meet glass. That is where droplets appear first in a cold spell. The aluminium roof lanterns page explains how frames are built, and our answer on the best glass for a roof lantern compares the glazing options.
Blinds, heating and small changes at home
A blind can cause the problem it is meant to soften. A blind drawn across the lantern traps a pocket of warm air against the glass, but it also shields the glass from the room's warmth, leaving the pane colder behind it. Steam reaches the glass and sits there unseen until the blind is raised.
Leaving a blind open while cooking, or choosing one that stands clear of the glass, helps. See lighting and blinds around a roof lantern for the options.
The room's heating matters as well. A kitchen kept at a steady, modest temperature shows less condensation than one that swings from cold to hot at mealtimes. Underfloor heating warms the room from below and raises the air temperature, though it does not warm the glass itself. Where a new build has just been plastered, the building is still drying out and condensation can be persistent for the first winter. That passes as the walls dry.
If the lantern is sound and the ventilation is right, a few beads of water in winter are normal, and they wipe away in seconds. Our guide to cleaning a roof lantern shows how to do it safely.
Condensation questions, answered
Does condensation mean my lantern was badly fitted?
Not by itself. Inner-face droplets point to room humidity and glass temperature. A fitting fault shows instead as staining or damp at the upstand, which is a separate matter and is judged at a survey.
Will a new lantern stop condensation altogether?
A modern build will reduce it a great deal, but no glass can defeat a steamy room. Ventilation still does half the job.
Is a flat rooflight better than a lantern for condensation?
Neither is better by shape. The glazing build and the room's ventilation decide. A flat unit is covered on our flat roof rooflight installation page, and our kitchen extension lantern page covers the room planning.
Can a skylight be specified to resist it?
Yes. The coating, spacer, gas fill and frame are all choices. Get a quote and the choices go into the written spec for your room. The service itself is described under roof lantern installation.