Roof lantern condensation in a kitchen

Where the water comes from before it reaches the glass

A pan of pasta at a rolling boil. A dishwasher door opened at the end of its cycle. A kettle four or five times a day. A tumble dryer venting into the room rather than through the wall. Laundry on an airer in the corner because the garden is wet. And the people, who each add water to the air simply by being there.

Cooking is the visible source and it is not the biggest one. The steady overnight load matters more, because a closed kitchen with a dishwasher on a delayed cycle and clothes drying quietly on a rack keeps adding vapour through the small hours, which is exactly when the roof is at its coldest and nobody is opening a door.

Why the glazing bar wets before the pane does

On a modern lantern the sealed unit is often the warmest part of the whole assembly. Two panes with a low emissivity coating, an argon fill and a warm edge spacer hold an internal surface temperature within a couple of degrees of the room. The metal carrying that unit is a different proposition.

A glazing bar runs from the outside face to the inside face, so unless it is interrupted it conducts cold straight into the room. Where the polyamide break in the section is generous, the bar stays close to room temperature. Where the section is older, slimmer or specified without a proper break, it can sit several degrees below the glass, and beads collect along the bars while the panes either side stay perfectly clear.

Thermal breaks, and why a kitchen tests them hardest

The polyamide strip inside an aluminium section is the reason a modern lantern behaves at all. It separates the outer shell from the inner shell so heat cannot walk across the metal, and its depth is the number that decides how warm the inner face stays on a cold night.

A kitchen finds the limit of a shallow break faster than any other room because it combines high humidity with high internal temperature. A bedroom at eighteen degrees and moderate humidity will forgive a mediocre section. A kitchen at twenty two degrees with a pan boiling will find every millimetre of it, which is why the frame specification deserves as much attention on a kitchen lantern as the glass does.

The perimeter of the pane, and the spacer inside it

Every sealed unit has a spacer bar holding the two panes apart around the edge, and that spacer is a thermal shortcut between them. Traditional spacers are aluminium, which conducts freely. Warm edge spacers use stainless steel foil or a composite and cut the transfer substantially.

The visible difference is a narrow band of water around the edge of each pane while the middle stays dry. It looks like a fault in the glass and it is a property of the spacer. On a kitchen lantern, asking for warm edge spacer at order costs very little and removes the one pattern people find most irritating, because it appears at eye level in a room where the ceiling is the feature.

The reveal, the plaster line and the mark that appears in January

Sometimes there is no water on the glass at all and there is a damp band on the plaster where the reveal meets the ceiling. That is not the lantern. That is the kerb underneath it, and it means the insulation in the roof does not meet the insulation in the box without a gap.

The symptom is seasonal, appearing in the coldest fortnight and drying out in March, and it often carries a faint grey bloom of mould in the corners where the air is stillest. It is a build-up problem rather than a glazing one, and it is dealt with by making the insulation line continuous around the box, which is covered in more detail on the page about lantern kerbs.

Where the water goes once it has formed

A properly designed lantern expects a certain amount of surface water and gives it somewhere to go. The inner face of each bar has a shallow channel running down the slope towards the eaves frame, and the eaves frame has a drainage path out to the external face through weep slots.

That system works while it is clear. It stops working when the slots are painted over during decoration, blocked by mastic applied by somebody being helpful, or filled with the fine grease film that a kitchen deposits on everything over five years. Water that cannot drain sits in the frame corner and eventually finds the plaster, which produces a mark that reads exactly like a leak and is not one.

The extract hood, and the difference recirculating makes

A recirculating hood filters grease and pushes the same air back into the room. It does nothing at all about water vapour. Every gram of moisture from the hob stays in the kitchen and ends up on the coldest surface, which is the glass overhead.

A ducted hood takes the vapour outside. The difference in a kitchen with a lantern is dramatic and it is usually the single most effective change available, more so than any glazing upgrade. Where a hood sits over an island, ducting it means running through the ceiling void or the roof, which is a decision worth taking at the same time as the lantern rather than two years later.

Where a hood duct should terminate on a flat roof

A duct that ends in the roof void does not solve anything. It moves the moisture from the kitchen into the build-up, where it condenses on the underside of the deck and shows up later as staining on the ceiling some distance from the lantern.

The termination belongs outside the covering, through a purpose made roof cowl with a back-draught flap, set clear of the lantern kerb so warm wet air is not discharged straight onto cold glass. Rigid ducting rather than flexible, kept as short and as straight as the layout allows, because every bend in a flexible run reduces the flow and every metre of unlagged duct in a cold void condenses inside itself.

Moving air across the underside of the glass

Still air against a cold surface is what allows a film of water to build. Air that is moving, even slowly, both warms the surface slightly and carries vapour away before it settles.

In an open plan kitchen the layout usually does this by itself, because the room is large and there is circulation through it. In a narrow side return infill with a lantern at the closed end, air can sit almost motionless above the units all evening. A ridge vent left on a night setting, or a trickle vent in the rear doors, is often enough to break that stillness, and the operation of those vents is covered under lantern ridge vents.

Winter cooking, closed doors and the week it shows up

People report kitchen lantern condensation in a narrow window of the year. Late November through early February, on clear still nights, when the sky is open and the glass radiates heat away to it faster than a cloudy night allows.

Those are also the nights nobody opens the bifolds. The room is sealed, the cooking is heavier because it is winter, and the outside face of the glass may be at or below freezing. A lantern that has behaved impeccably for nine months will show a full sheet of water on the third clear frosty night in a row, then behave again for another year. That pattern is diagnostic in itself.

Underfloor heating changes the picture in a way that surprises people. It produces a very comfortable room with warm air held low down and relatively little convective movement, which is pleasant to stand in and unhelpful for the ceiling.

A radiator sets up a rising plume that stirs the room and pushes warm air across the ceiling plane. A screed floor does not. The result is a warm kitchen where the glass overhead is still cold, the air near it is calm, and vapour arriving from the hob has nothing to disturb it. Where a kitchen has underfloor heating and a lantern, the ventilation strategy matters more than usual.

What a hygrometer tells you, and where to put one

A small digital hygrometer is the cheapest diagnostic instrument in this whole subject. It reads relative humidity and temperature, and a week of watching it turns an argument into a measurement.

Put it on a wall away from the hob and away from the door, at about worktop height, and read it at the times water appears rather than at midday. A kitchen that sits comfortably in the forties and fifties during the day and climbs steeply in the evening has a source and a ventilation issue, not a glazing one. A kitchen that never comes down is holding moisture somewhere, and the drying laundry is usually the culprit.

Water between the panes, which is a different diagnosis

Everything above concerns the room side of the inner pane, where water can be wiped off. Water sitting between the two panes cannot be wiped off, and it is telling a different story entirely.

It means the edge seal of the sealed unit has failed and the desiccant inside the spacer has taken up all the moisture it can hold. No amount of ventilation or heating will clear it. That unit has reached the end of its working life, and on a lantern individual panes can usually be changed within the existing frame rather than the whole structure coming off, which is discussed under unit replacement.

Kitchens in older Essex stock, and the two patterns seen most

Two situations come up repeatedly across Chelmsford, Braintree and the Blackwater villages. The first is a Victorian or Edwardian house with a side return infilled and a lantern over the new kitchen. Solid brick walls, a small volume, one external elevation, and a room that gets humid quickly because there is not much air in it to dilute anything.

The second is a large rear extension on an interwar semi or a newer estate house at Great Notley or Beaulieu Park, with bifolds, an island and a big lantern. Plenty of volume, but the doors stay shut from November and the island hood recirculates. Both produce water on the glass and each wants a different first move: ventilation in the small room, ducted extract in the large one.

Specifying a lantern for a kitchen from the start

Four things asked for at order remove most of this before it happens. A frame section with a deep polyamide thermal break. Warm edge spacers in the sealed units. A soft coat low emissivity glass with a genuinely low U-value, which keeps the inner pane closer to room temperature. And an opening vent at the ridge, even if the intention is to use it rarely.

Alongside that, the kerb wants insulating continuously with the roof and the hood wants ducting outside. Sunspire fits new lanterns and replaces units that have reached the end of their life, and in a kitchen the conversation about moisture is worth having before the order rather than after the first cold snap, because every one of those choices is made at order and none can be added later.

What a survey looks at in a kitchen that already has one

The survey starts inside and works outwards. Where the water sits, and whether it is on bars, panes, spacer lines or plaster. Whether the frame drainage slots are clear. What the hood does with the air it collects. Where the laundry dries. What the trickle vents in the doors are set to, and whether they have ever been opened.

Then it goes into the roof if there is access, to see where the insulation meets the kerb and whether anything is discharging into the void. That sequence usually identifies the cause without anything being taken apart, and in a good number of kitchens the answer is a change to how the room is ventilated rather than anything to do with the glass at all.