A south facing roof lantern is the brightest thing you can put on an extension, and on a July afternoon it is also the hottest. The same glass that makes a kitchen feel open in February can make it uncomfortable in August if nobody thought about where the sun sits. Orientation is not a fine detail to sort out after the lantern is chosen. It decides how much solar control the glass needs, where the blinds go, how the room is ventilated and how the ceiling looks at four in the afternoon. This guide follows the sun across a lantern skylight through the day and the year, then turns that into choices you can write into a specification: the glass coating, the pane build, the blind, the vent and the position on the roof.
How the sun crosses a lantern in a Sussex summer
At our latitude the sun is high for only part of the year. A lantern is a pitched glass roof, so it catches that high sun far more directly than any window in the wall beneath it.
Around midsummer the sun climbs to roughly 62 degrees above the horizon at midday, and in December it manages little more than 15. A vertical window sees the summer sun at a glancing angle and the winter sun nearly face on. A lantern is the other way about. Its glass is tilted at around 20 to 25 degrees, which means the summer sun, high in the sky, strikes it closer to square on. That is why a lantern can gather heat in June in a way a patio door does not.
The sun also travels a wide arc. In midsummer it rises in the north east and sets in the north west, so even a north facing slope of a lantern gets a little direct sun at each end of the day. At midday it stands due south. The afternoon is the critical time, because the room has already been warming since breakfast. By four o'clock the floor, the worktops and the walls have stored heat, the air is warm, and the sun is sliding round to the west with the lowest, strongest beam of the day.
A lantern has four roof slopes, sometimes more. That is a real advantage. Whichever way the ridge runs, some glass always looks towards the sun, and some looks away from it. The question is how much of the glass is on the sunny side, how large the whole lantern is, and what the glass is made of.
What each compass direction does to the room
The answer depends on which way the lantern faces, but also on what stands around it: a neighbour's gable, a tall hedge, a mature oak at the end of the garden. Treat the table as a starting point and the site as the final word.
| Facing | July afternoon | Winter and spring | Glass leaning |
|---|---|---|---|
| South | Strong, high sun all day | Generous, low, welcome warmth | Solar control, plus a blind |
| West | Low, hot, glaring sun late on | Bright evenings | Solar control, strong blind |
| East | Bright and warm early, cool by lunch | Gentle morning light | Clear glass suits most rooms |
| North | Soft, even, little direct sun | Steady but cool light | Clear glass, best insulation |
Two points sit behind the table. First, a west facing lantern can be harder to live with than a south facing one. Midday sun is high and can be shaded by a modest overhang, but late sun is low and arrives straight through the glass at head height. Second, a north facing lantern is not a dim one. It throws no harsh patches, and kitchens and studios can work better under that even light.
Heat gain, and why glass behaves the way it does
Sunlight passes through clear glass as short wave energy, strikes the floor and worktops, and turns into heat that the glass then hinders from leaving. This is the greenhouse effect, and a lantern is a small greenhouse on the roof.
The figure that describes how much of the sun's energy gets through a pane is the g-value, sometimes called the solar factor. A high g-value lets in more of the sun's energy, which is good on a cold January morning and poor on a hot afternoon. A low g-value turns more of it away. Standard modern double glazing with a low emissivity coating is tuned to hold heat in, so it usually lets a lot of solar energy through. Solar control glass is tuned differently. It carries a coating that reflects or absorbs a larger share of the sun's heat before it reaches the room.
There is a trade, and it is worth knowing before you order. Solar control coatings reduce the heat, and they also reduce the amount of visible light. They tend to give glass a slight tint, often a soft blue, green or grey, and they can change how a sunny garden looks from the kitchen table. Modern coatings are far more neutral than the tinted glass of a generation ago, but the difference can be seen when a sample is held up beside clear glass. We always ask people to look at a sample in the light of their own room before settling.
Glass also works with the Building Regulations. The limiting value for rooflights in Approved Document L is about heat loss, the U-value, and sits alongside the overall glazing area limit for an extension. A solar control coating is a separate property and sits on top of that. The two do not conflict, and a well specified unit can meet both. Our answer on whether roof lanterns are energy efficient enough for Part L covers the insulation side.
When solar control glass earns its place
Solar control is not automatic on every lantern. It adds to the specification, it changes the light, and on the wrong roof it is simply unnecessary. The test is the combination of direction, size and what the room is used for.
It usually earns its place when several of these are true at once:
- The lantern faces south or west, or has no shade from trees or neighbouring buildings.
- The glazed area is large compared with the floor area beneath it.
- The room is a kitchen or dining space that is also warmed by an oven, a hob and people.
- The extension has large glazed doors on the same side, so heat is arriving from two directions.
- The room is used in the afternoon and evening in summer rather than just at breakfast.
It is usually worth leaving out, or choosing a lighter coating, on a north facing lantern, on a lantern shaded by a tall tree, or on a small lantern over a room that is mostly used in winter and in the morning. In those rooms clear glass gives the most natural daylight and the best winter warmth from the sun that does arrive.
The decision is easier to make at the survey than from a drawing. We look at the garden, the neighbours' roofs, the trees and the size of the opening, and note which way the ridge will run. That goes into the written quiet spec alongside the acoustic choices, so the glass build for light, heat and sound is chosen together. The same inner pane that helps with the sound of rain sits in the same sealed unit as the solar control coating, so the two decisions share one build. Our page on the quiet spec explains how that document is set out.
Layers of the pane that do the work
A lantern pane is not one sheet. It is a sealed unit with an outer pane, a cavity and an inner pane, and each layer can be given a job. Choosing where the solar control sits and what is laminated is a joined-up decision.
The outer pane faces the weather and the sun. It can carry the solar control coating, which reflects heat before it crosses the cavity. Some outer panes are toughened as well, to cope with thermal stress. The cavity is filled with gas and sealed at the edge with a spacer. A warm edge spacer keeps the edge of the unit from becoming a cold line where condensation forms on winter mornings. The inner pane is commonly laminated, which holds broken glass together and adds a layer that damps sound. It is the pane people stand under, so it is the one that matters for safety.
Thicker glass makes a difference to the feel of the room in two ways. It damps the noise of heavy rain, and it slows how quickly the inner surface picks up heat. A thicker outer pane with a laminated inner pane will be calmer under a summer downpour than a thin pair, and steadier in a heatwave as well. We cover that side of the build in our answer on the best glass for a roof lantern and in the page on why the inner pane is laminated.
Self-cleaning coatings are a further layer, and they sit on the outside of the outer pane. They do not alter the heat, but they do interact with the sun, because the coating uses daylight to break down dirt. The details are on the page about self-cleaning glass on roof lanterns. A dirty lantern on a south facing roof looks worse and lets in less light, so it is worth thinking about together with the heat.
Blinds, vents and the other half of the answer
Glass sets the baseline, but nothing beats stopping the sun before it enters the room or letting hot air out at the top. Good summer comfort under a lantern comes from glass, a blind and ventilation working together.
A blind fitted inside the lantern stops the light at the glass, but the glass has already been warmed, so the heat stays in the room. A blind fitted on the outside intercepts the sun before it reaches the glass and is much more effective, though it has to be made to cope with wind and rain and is less common on lanterns. Internal blinds work best when the fabric is light coloured and reflective on the side facing the glass. Dark fabric absorbs heat and releases it into the room. A separate page covers lighting and blinds around a roof lantern, and another answers whether a roof lantern can have blackout blinds.
Hot air rises, and a lantern is the highest point of the room. That makes it the natural place for an exit. An opening vent in the lantern lets warm air leave while cooler air is drawn in at a low level, for example through a door or a window on the garden side. Even a small opening, used with a door cracked open at the other end, produces a steady flow. It costs nothing to run, and it also helps with the moisture that builds up in a kitchen. Our answer on whether a roof lantern can have opening vents explains how they are built, and our service page on electric and opening rooflight installation covers motorised options that can close themselves when it rains.
Shade you already have, and shade you can choose
Before adding to the glass specification, look at what the site is doing for free. Trees, neighbouring roofs, a parapet or an overhanging eave can take the sharpest sun out of the day.
A deciduous tree on the south or west side is a good friend to a lantern. It shades the glass in summer when the leaves are out and lets the low winter sun through when the branches are bare. A tall evergreen does the same job all year, which may be more shade than you want in February. A neighbour's two storey gable to the west can cut the late afternoon sun before it reaches the room, and that changes the answer on glass entirely. We check the sun path at the survey rather than guessing from the drawing.
You can also choose where on the roof the lantern sits. A lantern pushed towards the house wall, in the shadow of the first floor, sees less of the afternoon sun than one placed at the far end of the extension in open garden. The position that suits the kitchen layout is not always the position that suits the sun, and it is better to talk about that with the plan in front of you. The measurements that set the footprint are in our roof lantern size guide, and the way a lantern sits over a cooking space is in the page on planning a roof lantern over a kitchen extension.
Finally, the shape of the lantern matters. A lantern with a high ridge and steeper pitch shows more of its glass to the morning and evening sun. A shallower pitch looks more like a flat roof to the midday sun. Neither is better in the abstract. It is a matter of which part of the day you want to soften.
Winter and spring: the reason not to overdo it
Solar control is a summer solution to a summer problem, and it has a cost in the colder half of the year. Sun that would overheat the room in July is welcome in March.
A south facing lantern can lift a kitchen on a bright, cold day without the heating doing anything. That free warmth is lost if the glass is chosen only for the hottest afternoon. The balance is a judgement: enough control to keep the room usable in a heatwave, not so much that the room feels grey and cool in early spring. This is why we do not put the strongest coating on every south facing lantern. A moderate coating, a good internal blind and a vent can give a better year round result than the darkest glass available.
Glass that insulates well helps here too. A good low emissivity coating, a warm edge spacer and a gas filled cavity keep the heat in at night when the sun has gone. The performance of the whole lantern, frame included, is what counts, and our service page on energy efficient rooflights explains how those parts combine. Our answer on why a roof lantern gets condensation is also linked to orientation, because a north facing lantern stays cold longest on the inside and tends to show it first.
Is the room too hot? Questions that decide the spec
These are the questions that come up when a homeowner has a south or west facing extension and a lantern on the drawings. The answers are general. The right specification depends on the roof and is written down for your address.
Will a south facing roof lantern make my kitchen too hot?
It can, if it is large, unshaded and fitted with clear glass and no blind. It does not have to. A solar control coating, an internal blind and an opening vent together keep most rooms comfortable. Our answer on whether a roof lantern makes a kitchen too hot in summer goes further into the causes.
Can solar control glass be added later?
The coating is part of the sealed unit, so it cannot be applied afterwards in any lasting way. Adding it later means renewing the glass units, and sometimes the frame has to come out to do it. It is far easier to decide at the start. If a lantern already overheats the room, we treat it as a replacement job, and our page on misted and failed rooflight replacement describes how we approach that.
Does solar control glass make the room darker?
A little, yes. It reduces visible light as well as heat, though the change is much smaller than people expect from old tinted glass. A sample held up in the room is the honest test, and we would rather you chose with the glass in your hand.
Does the orientation change the quiet side of the spec?
Indirectly. A room that needs the vent open for heat may also be letting in more noise, and a thicker laminated inner pane helps both the comfort and the sound of rain. Our page on how loud a roof lantern is in heavy rain goes through that build.
When you are ready to choose, send us the plan and the compass direction of the garden through the quote form, or message us on WhatsApp. The full service is on the roof lantern installation page.