Roof lantern condensation in a kitchen

The four surfaces water shows up on

Get this right first, because it determines everything that follows.

• Inner face of the glass. Room air meeting cold glass. Normal in small amounts on cold mornings, a problem if it runs.
• Inner face of the frame or the glazing bars. More serious. The frame should be warmer than the glass, so water here points at a thermal break that is inadequate or absent.
• The plaster reveal below the frame. Not the lantern at all. This is a cold bridge at the kerb where the insulation stopped.
• The outer face of the glass, on the outside. Water on the wrong side of the building. This is a compliment, not a fault, and we explain why below.

A fifth possibility, water between the panes, is a failed sealed unit and is dealt with separately.

The kitchen is the wettest room you own

A pan of pasta at a rolling boil puts out roughly a litre of water an hour. A dishwasher venting at the end of its cycle releases a burst of steam into the room. A tumble dryer that is not condensing or ducted puts out two to three litres a load. Add breathing, kettles, washing up and a drying rack, and a family kitchen generates something in the order of three to six litres of water vapour a day.

That water has to go somewhere. It either leaves the building through ventilation, or it condenses on the coldest surface available. Before the extension, that surface was a window and a cold external wall. Now it is four square metres of glass overhead.

Dew point, in actual numbers

Air at 21 degrees and 50% relative humidity has a dew point of about 10 degrees. Any surface colder than 10 degrees will collect water. Raise the humidity to 65%, which is easy to do while cooking, and the dew point climbs to about 14 degrees. At 75% it is around 16 degrees.

Now compare that with what the glass is doing. On a night when it is 0 degrees outside, the inner surface of a good triple glazed unit sits around 17 to 18 degrees. A decent double glazed unit with a warm edge spacer sits around 15 to 16 degrees. An older double glazed unit with an aluminium spacer might be 12 to 13 degrees, and at the very edge of the pane, right against the spacer, several degrees colder still.

Set those two sets of numbers against each other and the pattern is obvious. Good glass in a normally ventilated kitchen stays clear. The same glass in a kitchen at 70% humidity will mist, and no glazing specification available will stop it.

The edge of the pane is the cold part

Condensation almost always starts as a band 30mm to 60mm in from the edge of each pane, not in the middle. That band is where the spacer bar holding the two panes apart conducts heat around the perimeter.

A traditional aluminium spacer is a good conductor and drops the edge temperature noticeably. A warm edge spacer, made from stainless steel foil or a structural foam, cuts that loss and typically lifts the edge temperature by 2 to 3 degrees. That sounds trivial. Against the dew point figures above, 2 degrees is often the entire difference between a clear pane and a wet one.

Reading a quotation for a thermal break

If water is appearing on the aluminium bars rather than the glass, the frame is colder than the glass, which should not happen in a properly built lantern.

The quotation should state a frame U-value or a whole unit U-value, not only a centre pane glass figure. A centre pane figure of 1.0 tells you about a piece of glass, not about the product. Ask for the polyamide thermal break width, which in a decent lantern runs from 14mm to 34mm. Ask whether the ridge and the eaves member are broken as well as the rafters, because some systems break the rafters and leave the head and sill continuous, and those are exactly the members that run the full length of the lantern.

The kerb is where the cold bridge hides

Water on the plaster reveal below the lantern, or a grey line of mould along the top edge of the reveal, is not a glazing problem. It is the insulation on the outside face of the kerb, either missing, discontinuous, or stopping short of the frame.

The kerb is a timber box standing in outside air. Unless the roof insulation continues up its outer face and meets the underside of the lantern, the timber and the plasterboard on its inner face run cold, and the reveal becomes the first place water forms. It is invisible once built, which is why we set the specification out in detail on our page about kerbs and who builds them.

The extraction rate the regulations expect

Part F of the Building Regulations asks for intermittent mechanical extract in a kitchen at 30 litres per second where the extract is adjacent to the hob, or 60 litres per second elsewhere in the room. A continuous system runs at a lower rate all the time.

Those are not aspirational figures, they are the minimum for a room of ordinary size, and a large open plan kitchen diner with a lantern over it is not a room of ordinary size. The volume is bigger, the moisture sources are the same or greater, and the cooking is often done on an island where the extract has the least help from surrounding walls.

Where a lantern condenses persistently, the extract is the first thing to measure and by a wide margin the cheapest thing to put right.

Recirculating hoods keep every gram of it

A great many island extractors, and most of the handsome ones, recirculate. Air is drawn through a grease filter and a charcoal filter and blown straight back into the room. Smell is reduced. Water vapour is untouched. Every gram of steam from every pan stays in the kitchen and finds the lantern.

If the hood is recirculating and the lantern is misting, that is the answer. Ducting to outside is the fix, and on an island in a room with a lantern above, the duct route usually has to be planned when the floor or ceiling is open rather than afterwards. It is worth raising with the builder at the same time as the lantern.

A moisture budget across a day

Add the column up for a household drying laundry indoors and the total passes six litres. No amount of glazing specification absorbs that. Ventilation does.

The first winter is not the normal winter

A new extension is full of water. Screed, plaster, brickwork and adhesive all carry construction moisture, and a floor screed alone can hold a hundred litres or more that has to leave the building through the air in the room.

The result is a first winter in which humidity runs high whatever the household does, and a lantern that mists in its first January may be perfectly dry the following one. Drying takes months rather than weeks, and it goes faster with the heating on and the windows cracked than with the house sealed up.

So if the extension was finished in autumn, hold your judgement until the second heating season before concluding anything about the glazing. Keep a note of what you are seeing and when, because a pattern over two winters is worth far more than an impression from one cold week.

Blinds make it worse, not better

A blind fitted under a lantern traps a layer of still air against the glass. That layer cools, the glass gets colder still because the room’s warmth is no longer reaching it, and the moisture that does get past the blind condenses more readily than it did before.

Closed blinds overnight in winter are a common cause of a lantern that used to be dry and now is not. If blinds are wanted, integrated ones running within the frame with a small ventilated gap behave better than a tight fabric panel below the frame, and either way opening them in the morning matters. Our page on blinds in a roof lantern covers the options.

Mist on the outside is good news

On clear autumn mornings you may find the outer face of the glass fogged from outside, sometimes so thoroughly that the view disappears until mid morning.

This is external condensation, and it happens because the outer pane is well insulated from the warmth of the room, radiates its heat to a clear night sky, and drops below the dew point of the outside air. Only a genuinely efficient unit does it. It is evidence that the lantern is keeping heat inside where it belongs. It clears as the sun reaches the glass, and there is nothing to do about it and no reason to want to.

Water between the panes

Misting inside the sealed unit, which cannot be wiped off from either side, means the perimeter seal has failed and the desiccant in the spacer is saturated. Nothing can be done to the unit; the pane is replaced.

On a lantern, sealed units are usually held by a capping and gaskets, so an individual facet can be taken out and a new unit fitted without disturbing the frame. Where the lantern is under a manufacturer’s product warranty, this is a claim worth making before paying for anything, and warranty periods on sealed units commonly run to five or ten years.

Winter habits that clear most cases

In order of effect. Run the extract while cooking and for ten to fifteen minutes afterwards, not just during. Leave trickle vents open through the winter rather than closing them for warmth. Dry laundry somewhere other than under the lantern. Keep the room’s temperature steady rather than letting it swing, because a room that cools to 15 degrees overnight and is heated hard in the morning drives water onto every cold surface it can find. Open the blinds in the morning.

Give those a fortnight before concluding anything is wrong with the lantern. A room’s moisture behaviour changes slowly.

When it genuinely is a build problem

Four patterns tell you the fabric is at fault rather than the household. Water on the aluminium bars rather than the glass. A wet mark or mould line on the reveal that returns every winter. Condensation that persists all day in a room with working extraction. And water appearing at one corner or one edge only, consistently, when the rest of the lantern is clear.

The last of those points at a specific cold bridge or a gap in the insulation at that corner, and it is worth investigating properly with a thermal camera on a cold morning rather than guessing. None of these are things a homeowner can be expected to diagnose from the ground.

What we settle before a lantern is ordered

At survey we ask about the extract and whether it ducts outside, we look at where laundry gets dried, we check what the kerb detail is going to be and who is building it, and we specify warm edge spacers and a properly broken frame as standard rather than as an upgrade. Those four decisions cost very little at order stage and cannot be revisited afterwards.

That is also why the workmanship guarantee runs to ten years: the details that keep a lantern dry and warm are buried in the perimeter where nobody can check them later. If your existing lantern is misting and you want an opinion on which of the four surfaces it is on, call 01865 704245 and describe what you are seeing.