Roof lantern glazing options and solar control

A roof lantern is a raised, faceted glazed structure that sits on a flat or very shallow roof and lifts the daylight up above the ceiling line. Picture a shallow glass pyramid or a long glass ridge tent bolted to a kerb. The classic shape has four sloping sides meeting at a central ridge, though hipped, gable and lean-to variants are all common over the single-storey extensions of south Essex. Each sloping face is a separate pane, or a run of panes divided by slim bars, and the whole thing is finished with a ridge along the top.

Two features set it apart from a flat rooflight or a wall window. The first is that a lantern is almost always the centrepiece of the brightest room in the house, usually a knocked-through kitchen and dining space across the back of the building. Nobody puts a lantern over a utility room. It goes where people cook, eat and sit, and it stays in use from breakfast to late evening. The second is the shape itself. Because the sides slope and face outwards, a single lantern presents glass to more than one point of the compass at the same time. A flat rooflight faces straight up and deals with one patch of sky. A lantern’s east face catches the morning, its west face catches the evening, and its south face catches the lot.

That geometry is the reason a lantern rewards careful glazing and punishes lazy glazing. Get it right and you have a room that stays bright and even-tempered through a July heatwave. Get it wrong and you have built a very expensive greenhouse over the one room the household cannot avoid.

The word “glazing” hides a stack of decisions. What you are actually choosing is a sealed unit, and every layer in it changes how the finished lantern behaves.

Number of panes. Standard is double glazing: two panes with a sealed cavity between them. Triple glazing adds a third pane and a second cavity, which lowers heat loss further but adds weight and cost. On a large lantern the extra weight matters, because it loads the bars and the kerb, so triple is a considered choice rather than an automatic upgrade.

Cavity fill. The gap between panes is usually filled with argon, a dense inert gas that conducts heat less readily than air. It is a cheap improvement that shaves the U-value without changing anything you can see.

Low-emissivity coating. A microscopically thin metallic layer on one of the internal glass faces, invisible to the eye, that reflects long-wave heat back into the room in winter. Nearly every modern sealed unit has one. It is what makes the glass insulate rather than merely separate inside from outside.

Solar-control coating. A separate coated layer, usually on the outer pane, tuned to reject the sun’s near-infrared before it enters the cavity. This is the layer that governs summer behaviour, and it is the one most often left off a lantern that has otherwise been specified perfectly well for winter.

Laminated inner pane. Two thin sheets of glass bonded around a plastic interlayer, so the pane holds together if it breaks. Overhead glazing has to be safe if it fails, and a laminated inner pane is the usual way of meeting that requirement. The interlayer also blocks almost all ultraviolet, which is why a well-specified lantern keeps flooring and furniture from fading.

Optional extras. A self-cleaning outer coating uses daylight and rain to break down and wash off surface dirt, which earns its keep on a lantern you are never going to reach with a cloth. An acoustic interlayer takes the edge off rain noise and traffic, worth considering near a main road. Neither changes the thermal picture, so treat them as comfort choices layered on top of the coatings that do.

The single most useful thing to understand about lantern glazing is the difference between a solar-control coating and a body tint, because they look similar on a brochure and behave nothing alike.

A body tint is glass with colour mixed through it, grey, bronze or a blue-green. It cuts solar gain by absorbing energy inside the glass. The trouble is that absorbed energy does not vanish. The pane heats up and then radiates a good share of that warmth inward, so the room still gets a chunk of the heat, just with a delay and a dimmer view. Tinted glass also takes your daylight down along with the heat, in roughly equal measure, so you trade brightness for a fairly modest cooling effect. On a lantern that exists to bring light into a deep-plan room, that is usually the wrong bargain.

A solar-control coating works by reflection, not absorption. It is a selective filter: it reflects the invisible near-infrared that carries most of the sun’s heat while letting the visible daylight through. The result is a pane that stays comparatively cool because it is bouncing the heat away before it can get in, and stays bright because it is not touching much of the visible spectrum. Modern coatings are close to colour-neutral, so the room reads as clean daylight rather than the brown cast an old tinted unit gives.

The number that captures this is the G-value: the fraction of the sun’s energy that gets through the glass, expressed between 0 and 1. Clear double glazing sits around 0.6, meaning three fifths of the solar energy comes indoors. A good solar-control unit brings that down to roughly 0.3, halving the heat load without halving the light. If you want the physics of that figure in more depth, our guide on solar-control glazing and the G-value takes it apart in detail. For a lantern the headline is simple: ask for the coating, and judge it by its G-value, not by how dark it looks.

Here is where a lantern departs from every other piece of glazing in the house. Because it is faceted, its sides do not share an aspect. On a hipped four-sided lantern sitting square to the building, one face looks roughly north, one south, one east and one west. The sun never treats those four faces the same way on the same afternoon.

The south face is the one under pressure. On a clear midsummer day it meets the sun at a favourable angle for hours and pours energy into the room below. The west face takes over in the late afternoon, when the low sun strikes it almost square-on and the house has already been warming all day. The east face gets a sharp burst first thing and then falls quiet. The north face sees no direct beam at all and simply delivers steady, glare-free daylight.

That opens up a choice most people are never offered. You do not have to glaze all four faces identically. On a lantern with a punishing southern aspect, the sensible specification puts high-performance solar-control glass in the south and west faces and clearer, higher-transmittance glass in the north face, so you knock down the heat where it arrives without dimming the whole structure. Whether mixing panes is worth it depends on the size of the lantern and the room beneath it, and it is exactly the sort of decision worth raising during a bespoke lantern design rather than after the glass is ordered.

This is not a detail you can settle from a catalogue. It needs someone to stand on the roof, work out which way each face points and glaze accordingly. A lantern quoted with one glass specification for the whole unit has been priced without anyone checking the compass.

These are typical published figures for the glazing you will be offered on a domestic roof lantern. Exact values shift with the manufacturer, the coating and whether a number is quoted for the centre of the pane or the whole sealed unit, so read them as the shape of the market rather than a fixed price list. The pattern is what matters.

Run your eye down the two middle columns. The U-value moves from 1.3 to 0.8, better by around a third. The G-value moves from 0.60 to 0.27, better by more than half, and it is governing a quantity of energy that on a bright day is many times larger than the trickle the U-value controls. If a lantern is overheating a room, the answer lives in the G-value column, not the U-value column.

Notice the body-tinted line as well. Its G-value of 0.43 is only a mild improvement on clear glass, yet its light transmittance has fallen to 0.42, worse than any other option on the list. It removes almost as much of your daylight as it does of your heat. That is the opposite of what a lantern is for, and it is why a proper solar-control coating beats a tint on both counts.

A lantern is not only glazing. It is glass held in a frame, divided by bars, sitting on a kerb, and every one of those elements changes the finished performance.

Frame material. Most modern lanterns use a thermally broken aluminium frame: aluminium inside and out with an insulating barrier sandwiched in the middle to stop cold tracking straight through the metal. That break is what lets a slim aluminium frame achieve a respectable whole-unit U-value. Timber frames are still available and suit period properties, and some budget lanterns use uPVC, which needs bulkier sections to stay rigid. The slimmer the frame and bars, the more of the aperture is glass and the more light reaches the room.

Sightlines and bars. Sightline is the width of frame you can see. A contemporary aluminium lantern can hold its ridge and rafter bars down to something in the region of 40 to 60mm, where an older design might be twice that. Slimmer bars mean more glass, but they also mean the glass is doing more of the thermal work, so slim sightlines and good glazing belong together.

The upstand, or kerb. A lantern does not sit on the flat roof directly. It sits on a raised kerb, usually a timber upstand around 150mm high, which lifts the glass above standing water and gives the flashing something to turn up against. That upstand is a weak point for heat unless it is insulated, because it is a continuous ring of structure around the whole lantern. A well-built lantern has an insulated, correctly detailed upstand. It is worth asking how the kerb is built, because the finest glass in the world loses its advantage if it is sitting on an uninsulated timber box.

None of this is glazing, but all of it decides whether the glazing you paid for actually performs. When we install a lantern the survey covers the kerb and the structural opening as carefully as the glass order, because the two only work as a set.

Southend faces south across the Thames Estuary, which is unusual for an English seaside city and is the reason solar control matters more here than the brochures assume. Most of the lantern work across south Essex goes over the rear extensions of the 1930s semis that fill this patch, single-storey additions across a south-facing garden, glazed precisely so the family can sit under the sky. A lantern on a roof like that, with an open southern horizon and the estuary bouncing extra light up from below, is collecting sun through more hours than an inland roof would.

So the specification follows the room and the aspect together. A south-facing kitchen lantern over a hard tiled floor, with an oven and hob already adding heat, wants the lowest practical G-value, somewhere around 0.29 to 0.34, in its southern and western faces. A north-facing lantern over a room used mainly in winter can take clear glass and all the daylight it will give. A lantern over a home office wants solar control regardless of aspect, because glare on a screen ruins an afternoon well before the temperature does.

Glass settles what arrives. Getting rid of what has arrived is a separate job, and it is why an opening element earns its place on a lantern over a tall room. Hot air stratifies and collects at the highest point, which on a lantern is exactly where the ridge sits, so an opening vent or an electric opening unit at the top clears trapped heat far faster than any window at head height. On a large or high lantern, where reaching a manual opener twice a day is never going to happen, the electric option is the sensible one. Roof lanterns are one part of a wider family of overhead glazing, and if a full pyramid is more than the room needs, a run of flat-roof skylights can do a similar job with a simpler build-up.

You can settle a lantern specification in one conversation. These are the questions that separate a considered quote from a catalogue order.

1. “Which way does each face point, and are you glazing them the same?” On a faceted lantern this is the first question, not an afterthought. If the answer treats all four sides as identical, orientation has not been considered.
2. “What is the G-value of the solar-control glass?” A figure should come back. “It is energy efficient” and “it is A-rated” are not answers, because window energy ratings in this country are weighted towards winter and say nothing about summer heat.
3. “Is the U-value centre-pane or whole-unit?” Centre-pane numbers ignore the bars and the frame and always flatter the glass. Whole-unit values are the honest comparison, and the ones Building Control works to.
4. “What is the light transmittance?” Read it next to the G-value. You want a low G with the light transmittance still up near 0.6, which is a coating doing its job, not a tint darkening the room.
5. “How is the kerb built and insulated?” The upstand is a continuous thermal path around the whole lantern. Ask how it is detailed, because it can quietly undo good glazing.
6. “Who notifies Building Control?” A new lantern is notifiable under the Building Regulations. Part L covers thermal performance and Part K covers the safety glazing overhead. We make that notification to Southend-on-Sea City Council, or the relevant local authority for your address, as part of the installation.

One point worth holding onto. The regulations set a ceiling on U-values, but they do not set a floor on G-value for a lantern going onto a house you already own. Nothing in the rulebook forces anyone to think about summer solar gain on an ordinary domestic extension. That is exactly why so many quotes skip it, and why a lantern full of fully compliant glass can still leave you with a room nobody wants to sit in on a July afternoon. If you want the glazing worked out properly for your own roof, face by face, our roof lantern installation service starts every job with the orientation survey, and you can ask for a specification with the G-value written on it for each face before you commit to anything.