Roof Lantern Glass Options

Chapter 1

What sits between the glazing bars

Every roof lantern glass option is a variation on one sealed unit: two or three panes, a gas-filled gap, and coatings on particular faces. Knowing which face does what makes the choices far easier to compare.

Start from the outside. The outer pane of a lantern unit is toughened glass, heat treated so that if it breaks it crumbles into small blunt pieces. Any self-cleaning coating goes on its outer face, where rain and daylight can reach it. Solar control coatings usually sit on the inner face of that outer pane, tucked inside the sealed cavity where weather cannot wear them. The cavity itself is filled with argon, which slows heat moving across the gap better than air, and it is held apart by a warm edge spacer rather than a plain aluminium strip, so the edges of the glass stay warmer in winter.

The inner pane, the one above your head, is laminated: two sheets of glass bonded to a plastic interlayer. If it cracks, the interlayer holds the pieces in place instead of letting them fall into the kitchen. That is a safety requirement for overhead glazing under Approved Document K, and it is covered in more detail in is roof lantern glass toughened? The low-emissivity coating, which reflects warmth back into the room, normally sits on a cavity face of the inner pane.

A triple-glazed unit adds a middle pane and a second cavity. Nothing else about the logic changes: toughened outside, laminated inside, coatings on the faces where they work best and last longest.

So when a quote says "solar control, self-cleaning, double glazed", it is describing three separate decisions. The coating that handles heat, the coating that handles dirt and the number of panes are chosen independently, and you can mix them. A lantern skylight glass specification can be clear and self-cleaning, or blue solar control without self-cleaning, or neutral solar control in triple glazing. The rest of this guide takes the decisions one at a time, then puts them together by orientation.

Chapter 2

Two numbers that describe any glass

Light and heat arrive together through a roof lantern, but glass can be made to treat them differently. Two figures on the specification sheet tell you how a given unit strikes that balance.

The first is light transmittance, often written LT or TL. It is the share of visible daylight that passes through the unit, so a figure of 0.70 means 70 per cent of the light falling on the glass reaches the room. This is the number that tells you how bright the space will be.

The second is the g-value, the total solar energy transmittance. It counts the visible light as well as the infrared you feel as warmth, plus the heat the glass absorbs and then passes inward. A g-value of 0.60 means 60 per cent of the sun's energy striking the glass ends up in the room. This is the number that tells you how warm the room will get on a bright afternoon.

Dividing the first by the second gives what the glass trade calls selectivity. Clear low-E glass has a ratio a little above one: it lets light and heat through in roughly equal measure. A good neutral solar control unit can approach two, meaning it passes nearly twice as much light as heat. That ratio is the most useful single idea when choosing lantern glass, because the aim is almost always the same: keep the daylight, lose as much of the heat as you can.

The figures below are typical ranges for double-glazed lantern units, as a guide. Every manufacturer's coatings differ, so the sheet for the actual unit is what counts.

A self-cleaning coating trims a few points off the light figure on any of these rows. Laying the numbers side by side shows why the choice is rarely just "clear or tinted". A neutral coating gives up a modest slice of light and removes nearly half the heat. A blue coating removes more heat again, but it also takes a much larger share of the light, which matters under a lantern whose whole job is daylight.

Chapter 3

Clear low-E glass and when it earns its place

Clear glass with a low-emissivity coating is the brightest option and gives the truest colour, which makes it the right choice wherever summer heat is not the problem.

The low-E coating is almost invisible. It is a thin metallic layer that lets short-wave daylight in and reflects long-wave warmth from the room back downward, which is why it matters as much in January as in July. Every lantern unit we fit carries one. What makes a unit "clear" is simply the absence of a solar control layer on top.

Where does that suit a Cambridge home? A lantern on a north-facing rear extension is the obvious case. The glass faces the open sky rather than the sun for most of the day, so it collects soft, even light and relatively little direct heat. Cutting the light by a third with a solar control coating would buy protection against a problem the room does not have. The same logic applies to a lantern over a deep room in a Romsey or Petersfield terrace, where the side-return extension sits between two party walls and a neighbour's rear wing shades the roof for much of the afternoon. Our answer to will a roof lantern work on a north facing extension? goes into that steady, glare-free light in more depth.

Clear glass is harder to justify on a lantern that sees the sun from late morning until evening. Remember that roof glass lies almost flat, so it meets the high summer sun close to head-on. In late June the Cambridge sun climbs to around 61 degrees at midday, and a lantern with clear glass on an open, south-facing roof can take the room several degrees above comfort by mid-afternoon, even with a vent open. For the full picture of that heat, see do roof lanterns make a room hot?

There is one more point in clear glass's favour. It keeps the view honest. Looking up through clear glass, the sky is the colour of the sky, cloud edges are crisp, and at dusk the light fades naturally rather than through a tint. For people who choose a lantern to feel connected to the weather overhead, that counts.

Chapter 4

Neutral and blue solar control compared

Both neutral and blue solar control glass cut the heat that comes through a lantern. The difference is what each does to the colour and quantity of the light that is left.

A solar control coating works by reflecting and absorbing much of the infrared part of sunlight before it reaches the room. Modern neutral coatings are spectrally selective: they are tuned to let most of the visible band through while turning back a large part of the invisible heat. The result looks, from inside, very close to clear glass. White walls stay white, the timber of a kitchen worktop keeps its colour, and the room reads as daylit rather than tinted. From the garden, a neutral unit has a faint silver or grey sheen, noticeable mainly when the sky is bright.

Blue glass combines a solar control coating with a tint, either in the glass body or in the coating itself. The tint absorbs more energy, so the g-value drops further than a neutral unit can manage. The cost is in the light. Less of it gets through, and what does arrive carries a cool cast. Under a blue lantern, pale walls pick up a slight blue tone, skin tones look cooler and food on the table under the glass loses some warmth. From outside, the lantern reads plainly as blue, which some owners like on a contemporary extension and others find at odds with a gault brick house or a period cottage.

Which suits which room? Neutral solar control is our default wherever a lantern faces south or west and heat needs managing, because it keeps the thing a lantern exists for. Blue has a case where heat is the overriding concern and colour matters less: a very large lantern over a fully glazed garden room, for example, where the walls already let in plenty of light and the roof glass is there as much for the view of the sky. Grey and bronze tints exist too, with similar trade-offs.

Heat control does not stop at the glass. Vents near the ridge let warm air escape, and blinds catch the sun that the coating lets through. Our guide to controlling heat and glare under a roof lantern ranks those measures by effect.

Chapter 5

Self-cleaning glass and what it can and cannot do

Self-cleaning glass does not stay spotless on its own. It carries a coating that breaks down and loosens dirt, so that rain can carry it away, and it only works with daylight and water to help it.

The coating is a very thin layer, usually based on titanium dioxide, on the outermost face of the lantern. It does two things. Ultraviolet light activates it, and in that state it slowly breaks down organic dirt such as pollen, leaf residue and the film left by traffic and chimney smoke. It is also hydrophilic, meaning water spreads across it in a sheet rather than beading into droplets. When it rains, that sheet runs down the pane and takes the loosened dirt with it, and there are fewer dried droplet marks afterwards.

A lantern is a good home for the coating. Its panes are pitched, commonly somewhere around 20 to 25 degrees, so water runs off rather than pooling, and the glass faces open sky, so it gets both rain and daylight. It is less effective in a few situations that are common in Cambridge gardens:

• Under trees. Limes and sycamores drop sticky honeydew in summer, and heavy sap or a bird dropping can outpace the coating.
• On a north-facing hip. That face gets less ultraviolet, so the coating works more slowly there than on the south side.
• Under a sheltering wall. A lantern tucked against a two-storey rear wall may see less rain on the side nearest the house.
• After a long dry spell. The coating needs a shower to finish the job, so dust can build up through a dry summer.

The coating does nothing for the inside face of the glass, which still needs a wipe from time to time, especially over a hob. It also needs gentle care outside: plain water or a mild cleaner and a soft cloth, never abrasive pads, and never products containing silicone, which can coat the surface and stop it working. How do you clean a roof lantern? covers safe access and a sensible routine. Self-cleaning glass reduces how often a lantern needs attention; it does not remove the need entirely.

Chapter 6

Double or triple glazing for a lantern

Triple glazing keeps more heat in and makes the glass feel warmer from below, but it is heavier, lets through a little less light and asks more of the frame. Double glazing meets the regulations comfortably with the right coatings.

Part L of the Building Regulations sets a limiting U-value of 2.2 W/m²K for rooflights, assessed in the horizontal plane. A good aluminium lantern with thermally broken bars and argon-filled, low-E double glazing comes in below that. Triple glazing lowers the figure further, and the gain shows up in three ways you can feel. There is less heat loss through the roof on winter nights. The inner pane stays closer to room temperature, so there is less of the cool air that slides off cold glass and settles on the table below. And condensation on the inside face becomes less likely, because the surface is warmer than the dew point for longer. That last point matters in a kitchen with a lot of steam; our answer on whether roof lanterns get condensation explains the mechanism.

The costs of a third pane are real, though. Each extra layer of glass removes a few per cent of the light, and on a lantern the difference between double and triple can be noticeable on a dull February day. The unit is heavier, which may call for deeper glazing bars and a stiffer ridge, and on a large lantern that extra depth can change the look from inside. The weight also reaches the upstand and the roof structure, so it has to be part of the structural thinking from the start; how heavy is a roof lantern? gives the ranges.

A fair rule for a Cambridge home: double glazing with a good low-E coating suits a typical kitchen extension and keeps the frame slim. Triple glazing earns its place on a very large lantern, over a room kept warm for long hours, in a well-insulated newer home where the lantern would otherwise be the coldest surface in the envelope, or where condensation is a known worry. Homes at Eddington or Trumpington Meadows, built to high insulation standards, are the kind of house where a double-glazed lantern can feel noticeably colder than the rest of the room. More on that is in do roof lanterns make a room cold?

Chapter 7

Matching the glass to the way the lantern faces

A lantern is not one orientation but four: a hipped lantern has faces pointing in different directions, and the roof it sits on faces the sun in its own way. The glass choice should follow the light, not the brochure.

The simplest approach is one specification across every pane, chosen for the hardest-working face. On a lantern with an open aspect to the south or west, that means neutral solar control throughout. It keeps the colour of the light consistent across the whole lantern, which matters more than it sounds: two coatings side by side are visible from below as panes of slightly different tone, and from the garden as panes of different reflection.

On the flat Cambridge skyline, with almost nothing on the horizon to shade a roof, the sun reaches a lantern for a long day in summer. A rough guide to where each glass tends to fit:

1. South-facing or open roof, room used through the afternoon: neutral solar control, self-cleaning, double glazed.
2. West-facing kitchen used at teatime: neutral solar control, with an opening vent and a blind in mind from the start.
3. East-facing breakfast room: neutral solar control on a large lantern, clear low-E on a small one, since morning sun is lower and gentler on heat.
4. North-facing or heavily shaded: clear low-E, where every point of light transmittance counts.
5. Very large lantern over a glazed garden room: neutral or blue solar control, and triple glazing if the room is heated in winter.

The direction the ridge runs changes this too, because it decides which faces are the big side panes and which are the small hipped ends. A ridge running east to west puts a long face towards the south, while a ridge running north to south spreads the sun more evenly across the day. Our guide to roof lantern orientation and the sun sets out how that plays out.

Chapter 8

Reading a quote for lantern glass

Two quotes that both say "solar control glass" can describe very different units. A few specific questions will show whether the glass has been chosen for your room or picked from a default line.

Ask for the light transmittance and g-value of the actual unit, not of the glass range in general, and check both against the table above. Ask whether the solar control coating is neutral or tinted, and if possible look at a sample in daylight. Confirm that the inner pane is laminated and the outer pane toughened. Ask whether the self-cleaning coating is on every pane or only some. Check whether the U-value quoted is for the centre of the glass or for the whole lantern including frame, and whether it is assessed in the horizontal plane as Part L now requires; a centre-pane figure will always look better than the whole-unit value. Finally, ask who confirms that the lantern meets Building Regulations and handles the Building Control notification where the work is notifiable.

Glass choice also affects cost. Solar control and self-cleaning coatings and a third pane each add to the price of the units, and a heavier unit can add to the frame and the structure. General market ranges, and what drives them, are on our skylight and rooflight costs page. The way we weigh glass against orientation, room use and season is explained in how we plan the light, and the lantern service as a whole is on our roof lantern installation in Cambridge page. When you are ready, book a light survey and we will bring samples to the room itself.

Chapter 9

Questions people ask

Can I have different glass on different faces of the lantern?

Yes, units can be ordered with different coatings per pane. It is usually better to use one specification throughout, because mixed coatings show as panes of slightly different tone from inside and outside. Mixing makes most sense where one face is shaded all year.

Does solar control glass make a room darker in winter?

A neutral solar control unit passes a little less light than clear glass all year round, which is noticeable mainly on dull days. Blue glass takes away more. On a north-facing or shaded lantern, clear low-E glass keeps the winter light better.

Is self-cleaning glass worth having on a lantern?

On an open roof that gets rain and daylight, it reduces how often the outside needs cleaning and leaves fewer water marks. Under trees or against a sheltering wall it helps less, and it never cleans the inside face.

Can the glass be changed later if the room gets too hot?

Only by replacing the sealed units, which is possible on many lanterns but not cheap. It is far better to choose the glass at the start, using the orientation and how the room is used, and to add a vent or blinds where the heat needs more control.