What glass is used in a roof lantern?

Each pane in a lantern is not a single sheet of glass. It is a sealed unit: two panes for a double-glazed lantern, three for a triple, separated by a spacer bar around the edge and filled with argon gas. The spacer holds the panes apart at a fixed distance, the sealant round the perimeter keeps the gas in and moisture out, and the gap does most of the insulating work. Argon is denser than air and conducts heat more slowly, so the same cavity performs better filled with it.

The panes themselves are float glass, the ordinary flat glass that nearly all windows use, but processed in ways that matter for a roof. Over your head, glass has to be safety glass by law, and it has to shrug off heat, wind and the odd bit of hail without a problem. That is why lantern units are specified pane by pane rather than bought off a shelf, and why two lanterns that look identical from the garden can behave completely differently in a heatwave.

A typical modern lantern over a kitchen or dining space runs somewhere between about 24 and 44 millimetres thick across the whole unit, depending on whether it is double or triple and how thick each pane is. The frame bars carry that weight down to the ring beam, so the glass specification and the structure are tied together and cannot be chosen in isolation.

Glazing in a roof sits above people, so Part K of the Building Regulations treats it as a place where safety glazing is required. In practice that means toughened glass, laminated glass, or a combination of the two, and for overhead glazing the sensible build-up combines both.

Toughened glass is heat-treated so that if it breaks it crumbles into small blunt granules rather than long shards. It is roughly four to five times stronger than untreated glass of the same thickness. The catch is that when it does break, the whole pane goes at once. On its own, over your head, that is a lot of glass to come down.

Laminated glass is two sheets bonded around a plastic interlayer. If it breaks, the fragments stick to the interlayer and the pane stays roughly in place rather than falling into the room. That is exactly the behaviour you want from the inner pane of anything glazed above head height. So the standard specification for a lantern is toughened on the outside, where the strength is needed to take wind and impact, and laminated on the inside, so that nothing can drop through. The laminate interlayer also cuts a useful amount of noise and blocks nearly all the ultraviolet that fades furniture and flooring.

The glass is only half the story. The performance comes from microscopically thin metal-oxide coatings applied to the panes, and there are two you should know about.

The first is a low-emissivity coating, usually shortened to low-E. It sits on an inner face of the cavity and reflects heat back towards the room in winter instead of letting it escape through the glass. Almost every lantern unit sold now carries one, because it is the main reason the U-value comes down far enough to satisfy Part L. On its own, low-E glass is a winter measure: it keeps warmth in.

The second is a solar-control coating, and this is the one that decides whether a lantern is comfortable in July. It reflects a portion of the sun’s energy before it can pass through the glass and turn into heat in the room. The measure for it is the G-value, the fraction of solar energy that gets through, running from 0 to 1. Clear double glazing sits around 0.6, so roughly sixty per cent of the sun’s energy comes through as heat. A solar-control unit typically sits near 0.3 or below. The two coatings are not alternatives. A well-specified lantern for a sunny roof carries both: low-E for the winter and solar control for the summer.

This is where the answer stops being generic. A lantern is horizontal glazing, so it faces the sky rather than a compass point, and in high summer the sun is almost square on to it for hours. Orientation barely rescues you: a flat roof glazed with clear units bakes the room below whichever way the house is turned.

Southend-on-Sea faces south across the Thames Estuary, with an open horizon and light bouncing off the water, so a rear lantern here collects sun from mid-morning until the sun swings west with very little to shade it. Put a large expanse of clear glass over a kitchen, the one room that already makes its own heat from cooking, and the room becomes a greenhouse by early afternoon. That is the job we are built around, and the fix is in the glass. For a south-facing or flat-roof lantern, ask for solar-control glazing with a G-value around 0.3 or below rather than the clear units a supplier will quote by default. You can read the reasoning in full on our energy-efficient skylight installation pages, and the same thinking runs through every roof lantern installation we specify.

A lower G-value does cost you a little daylight and adds a faint tint to the glass, usually a blue or grey cast that is barely readable from inside. It also blocks the same sun in winter that it blocks in summer, which is why a genuinely north-facing lantern is the one case where clear glass is the better call and the free warmth is worth having. Everywhere the sun actually reaches, the trade is worth making.

When a quote comes in, ask for the glass specification sheet rather than a phrase. “Solar control” is printed on units sitting at 0.5 as well as ones at 0.28, so the words alone tell you nothing. The sheet should give you three figures for the same build-up: the G-value for summer heat, the light transmittance for how bright the room stays, and the U-value for winter warmth. Judge all three together rather than one in isolation.

• Safety. Confirm toughened outer and laminated inner, so the unit meets Part K over your head.
• Summer. Ask the G-value. For a flat or south-facing lantern, look for around 0.3 or lower.
• Daylight. Ask the light transmittance alongside it, so you know what you are trading for the heat control.
• Winter. Check the U-value meets current Part L for a new or replacement rooflight.

If a tired lantern is coming out, that is the moment to change the glass, because the opening and the structure are already there and the only decision left is what goes back in. Work out the orientation and the rough size first, then let the numbers on the sheet, not the sales phrase, settle it. If you would rather hand that part over, tell us which way the roof faces and roughly how big the opening is on 01702 898232 or request a quote, and we will come back with a glass specification and a price for the whole roof rather than just the frame.