Guide · Roof Lantern Installation
Roof lantern glazing and solar control
A roof lantern points at the whole sky, which is the reason it is worth having and the reason its glass has to work harder than any other glass in the house. A vertical window facing south takes the sun at a shallow angle for part of the day. A lantern at 25 degrees takes the June sun almost square on at midday, when the solar altitude over Essex is around sixty one degrees and the energy arriving on a horizontal surface is at its annual peak.
So the glass specification is not a detail to be settled at the end. It is the difference between a room that is bright and steady all year and one that is unusable from eleven in the morning through most of the summer. This page sets out what is in a lantern sealed unit, what each number on the spec sheet does, and how the specification is chosen for a particular roof.
The three numbers that describe a lantern glass
Every sealed unit is described by three figures, and they do not move together. The U-value is heat loss in watts per square metre per kelvin: lower is better and it matters in February. The g-value, sometimes written as the solar factor or g, is the fraction of solar energy that gets through: lower means a cooler room in July. Light transmission, written as LT or VLT, is the fraction of visible light that gets through: higher means a brighter room.
The tension is between the second and the third. Anything that blocks solar heat blocks some light too, and the whole craft of a modern coating is widening the gap between them. The ratio of light transmission to g-value is called the selectivity, and a figure above 1.7 is a genuinely good solar control glass. Below about 1.3 you are buying a tint.
What a lantern sealed unit is actually built from
Read from outside in, a typical lantern unit is a 6mm toughened outer pane, a 16mm cavity filled with argon and closed with a warm edge spacer bar, and a 6.8mm laminated inner pane made from two sheets of 3mm glass bonded to a 0.76mm interlayer. The coatings sit on the internal faces of the cavity, never on an exposed surface.
The convention for numbering those surfaces runs from the outside. Surface one faces the sky, surface two faces the cavity from the outer pane, surface three faces the cavity from the inner pane, and surface four faces the room. Which surface a coating occupies changes what it does, which is worth understanding before comparing two quotes.
Low-E coatings, and the surface they belong on
A low emissivity coating is a microscopically thin layer of silver and metal oxides that reflects long wave infrared, which is the wavelength radiated by warm objects. It is what makes modern double glazing perform like the triple glazing of thirty years ago.
In a British climate the low-E coating goes on surface three, the room side of the cavity. From there it reflects the heat radiating off your radiators back into the room instead of letting it cross the gap. Put the same coating on surface two and it starts rejecting incoming solar heat instead, which is a solar control function rather than an insulating one. A good lantern unit does both, with a soft coat solar control layer on surface two and a low-E layer on surface three.
The g-value, and what number to aim at
Clear double glazing with a standard low-E coating runs a g-value around 0.6, meaning sixty per cent of the solar energy hitting it arrives in the room. Over three square metres of horizontal glass in July that is a substantial amount of heat, and it is why an unspecified lantern gets hot.
Solar control glass for a lantern typically lands between 0.28 and 0.40. The useful benchmarks are these: 0.5 and above is effectively unmanaged and only appropriate on a north facing or heavily shaded roof. Between 0.35 and 0.45 is a moderate specification suitable for most orientations in a room that is not heavily used at midday. Below 0.35 is what a south or west facing lantern over a kitchen wants, and below 0.30 is worth considering where the lantern is large relative to the room.
If a quote names a U-value but no g-value, the summer half of the specification has not been made.
Light transmission, and what a low g-value costs you
The trade is real but it is smaller than people fear. A clear low-E unit transmits around seventy to seventy five per cent of visible light. A good neutral solar control unit at a g-value of 0.33 still transmits sixty to sixty five per cent. The eye adapts logarithmically, so a ten per cent reduction in transmitted light is very close to imperceptible in a room that is being lit from above.
What is perceptible is the difference between sixty per cent and forty per cent, which is where older or cheaper solar control products sit. Those darken the room noticeably on a grey December afternoon, and they are the reason solar control has an undeserved reputation for making rooms gloomy. A current generation product does not do this.
Neutral, blue and the colour of the light
Solar control coatings are metallic and they have a colour. Older and cheaper products read distinctly blue or green, both from inside looking up and from the garden looking at the roof. On a bright sky that blue cast is obvious and it makes the daylight in the room feel cold.
The premium products are described as neutral and they are, near enough. The number to ask for is the general colour rendering index for transmitted light, written Ra. Above 95 the light in the room is effectively unaltered. Around 90 is acceptable. Below 85 the glass is putting its own colour into your kitchen every day of the year, which is a large consequence for a small saving.
Solar control options compared
| Specification | Typical g-value | Light transmission | Where it belongs |
|---|---|---|---|
| Clear with low-E only | 0.60 | 75% | North facing, or shaded by a tall building |
| Light solar control | 0.42 | 68% | East facing, or a small lantern in a large room |
| Neutral solar control | 0.33 | 62% | The general case for a flat roof in Essex |
| High performance solar control | 0.26 | 52% | South or west facing, or a large glazed area |
| Body tinted, grey or bronze | 0.30 | 40% | Where a visible tint is wanted for its own sake |
Laminated glass, and why it is not optional overhead
The inner pane of any glass that sits over your head should be laminated. Toughened glass is strong, but when it eventually fails it fails completely and instantly, breaking into a shower of small cubes. Directly over a dining table that is unacceptable, and Building Regulations Part K addresses the risk of falling glass in overhead glazing.
Laminated glass holds together on its interlayer. A break leaves a cracked pane hanging in the frame rather than a floor covered in glass, and it can then be replaced in an orderly way. The standard specification is a toughened outer, which takes hail and thermal stress, and a laminated inner, which takes the safety duty. Any lantern quoted with toughened glass on both panes is quoted below the sensible standard.
Thermal fracture, and why the outer pane is toughened
Glass breaks from uneven heating. Where the centre of a pane is in full sun and the edges are shaded by capping and cold, the middle expands and the edges do not, and the tensile stress at the edge finds any flaw left by cutting. This is thermal fracture, and it produces a characteristic crack running perpendicular to the edge.
Solar control glass absorbs more energy than clear glass, which makes it hotter, which makes thermal fracture more likely. That is precisely why the outer pane is toughened: the process puts the surface into compression and raises the tolerable temperature differential from around 35 degrees to well over 200. On a lantern with high performance solar control, toughening the outer pane is not an upgrade, it is the correct specification.
The cavity, and whether argon is worth it
The gap between the panes carries most of the insulation, and its width matters. Below 12mm the gas conducts too readily. Above about 18mm it starts to convect, with warm gas rising up the inside of the cavity and cold gas falling down the outside, which undoes the benefit. Sixteen millimetres is the sweet spot and it is what nearly every good unit uses.
Argon is denser and less conductive than air and it improves a unit’s U-value by roughly 0.2 W/m²K for very little money. It is standard. Krypton performs better again and is used where the cavity has to be narrow, but it is expensive and it makes no sense in a lantern where there is room for a proper 16mm gap.
Warm edge spacers, and the perimeter of the pane
The spacer bar holds the panes apart, contains the desiccant that keeps the cavity dry, and carries the seal. Traditional spacers were aluminium, which is an excellent conductor, so the perimeter of every pane was a cold line running around the glass. That line is where condensation appears first.
A warm edge spacer is made of stainless steel, a structural foam or a composite, and it cuts the linear thermal transmittance at the glass edge substantially. On a lantern this matters more than on a wall window, because the pane is horizontal and any water that forms at the cold edge runs down into the frame junction rather than harmlessly down a vertical face. Warm edge is worth specifying every time.
Double or triple glazing in a lantern
Triple glazing improves the U-value, from around 1.2 to around 0.9 for the glass alone, and it is common in northern Europe. In a British lantern it is rarely the right call, for three reasons.
The weight is roughly fifty per cent higher, which pushes the bars deeper and the sightlines thicker. The third pane cuts light transmission by another eight to ten per cent, which fights the point of the lantern. And the additional coating surfaces make a neutral appearance harder to achieve. Where a house is being built to a genuinely low energy standard the calculation can favour it. In an ordinary extension, a good double glazed unit with the right coatings is the better product.
Self-cleaning outer coatings
A self-cleaning coating is a permanent layer of titanium dioxide fired into the outer face of the glass at manufacture. It does two things. Ultraviolet light triggers a photocatalytic reaction that breaks down organic dirt, and the surface is hydrophilic, so rain sheets across it rather than beading and carries the loosened dirt off.
On a lantern this is worth having, because the glass is at an angle where dirt lands and out of reach where it stays. It reduces cleaning frequency substantially rather than eliminating it, and it does very little for inorganic deposits like the mineral marks left by hard water running off a wall above. It adds a small amount to the unit price and it can be combined with solar control on the same pane.
Rain noise, and what the glass can do about it
Rain on a horizontal or near horizontal pane is louder than rain on tile, and directly over a quiet room it is noticed. Two glass decisions reduce it meaningfully.
The first is laminate. A polyvinyl butyral interlayer damps the vibration that a single monolithic pane transmits straight through, and since the inner pane should be laminated anyway for safety, this comes free. The second is asymmetry. A unit built with a 6mm outer and a 8.8mm laminated inner performs better acoustically than one built 6mm and 6.8mm, because two panes of matching thickness resonate at the same frequency and reinforce each other. Both choices are made at order and neither can be added afterwards.
Switchable and electrochromic glass
Electrochromic glass changes its tint under a small applied voltage, moving from a g-value around 0.40 down to near 0.09 in a few minutes, and it can be controlled by a sensor, a timer or a phone. On a west facing lantern over a room used in the evening it does something no static specification can, because it lets the glass be clear in December and dark at four on an August afternoon.
The cost is a multiple of standard glass, the units want a cable run to the frame, and the tint at full state is a distinct blue. It is a real product rather than a curiosity, and it makes most sense on a large lantern in a room with a genuine overheating problem where shading is impractical.
How orientation and pitch feed into the specification
The angle of the glass changes how much energy arrives on it, and by a lot. At a shallow lantern pitch of 15 degrees the surface is nearly horizontal and it catches the summer sun almost square on. At 35 degrees the same lantern sheds a meaningful fraction of the midday June sun and gathers rather more of the low December sun, which is often the more useful trade.
So pitch and glass are chosen together. A shallow lantern on a south facing roof needs the strongest solar control available. A steeper lantern on the same roof can take a softer specification and give up less light. The pitch decision in its own right is set out on the pitch page, and the general question of energy performance across all rooflight types is covered under energy efficient rooflights.
Ventilation is part of the solar strategy
Glass rejects heat. It does not remove heat that is already in the room, and a lantern over a kitchen is generating its own from the hob, the oven and the people. The glass specification and the opening strategy are two halves of one answer.
A vent at the ridge of a lantern sits at the highest point in the room, which is where the hottest air collects, and opening it lets that air leave while cooler air is drawn in through doors and windows lower down. On a summer evening that moves more heat out of a room than any glass coating keeps out. How those vents work and how they are driven is set out on the ridge vent page.
Part L, and what the calculation actually needs
Building Regulations Part L sets a maximum U-value for rooflights in new work in England, and rooflights are assessed against a value referenced to the vertical plane, which is a technicality worth knowing because it explains why a lantern’s quoted number and its compliance number can differ.
The building control submission wants the glass U-value, the whole-product U-value including the frame, the g-value, and the total glazed area against the new floor area. All of those come off the manufacturer’s data sheet, and Sunspire supplies them with the notification as part of the job rather than leaving the homeowner to chase the maker for a PDF.
What a glazing line on a quote should say
A specification worth comparing names the make and product name of the coating, not just the phrase solar control. It gives the build-up in millimetres, outer to inner, including the interlayer. It states the U-value for the glass and for the whole product separately. It gives the g-value, the light transmission and ideally the colour rendering index. It says whether the spacer is warm edge and whether the cavity is argon filled.
That is eight or nine figures and any competent supplier has them to hand. A quote that reads “double glazed solar control unit” and stops has told you almost nothing, because that phrase covers everything from a g-value of 0.5 to a g-value of 0.25 and a price difference to match. The wider process, from survey to handover, is covered on the roof lantern installation page.
Keep reading
More on roof lantern installation
Aluminium and timber roof lanterns compared
Read the guide →Structural openings and steel for a roof lantern
Read the guide →Roof lantern ridge vents and how they are operated
Read the guide →Roof lantern pitch and how steep it should be
Read the guide →Roof lantern kerbs and who builds them
Read the guide →
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