The short answer
Neither is better in every case. A roof lantern set into an insulated solid roof keeps the extension warmer in winter, cooler in summer and calmer in rain, while still giving a generous column of daylight. A fully glazed roof gives the most light and the most exposure to heat, cold and weather noise. The choice depends on the room and its aspect.
What you are actually comparing
A lantern is a glazed feature sitting on a solid, insulated roof. A fully glazed roof is glass from wall to wall, carried on a frame of its own. The two behave very differently once the weather turns.
With a lantern, most of the roof is ordinary construction: structural deck, insulation, a waterproof layer and a ceiling below. The glass covers only the middle, usually a fraction of the room's floor area. That solid part does the quiet, unremarkable work of keeping heat in and weather out, and the lantern skylight supplies the daylight.
A glazed roof swaps almost all of that construction for glass. Extension roofs of this kind are often sold in a conservatory style, or as an orangery with a glass top. Every square metre of roof is a window, so there is nothing solid left to share the load of insulating the room.
Both are legitimate. We specify and install lanterns, and our page on roof lantern installation in Crawley explains what that involves. A fully glazed roof is a different kind of structure, and a fair comparison has to say where it wins as well as where it struggles.
Winter warmth and the regulations
Glass loses heat faster than an insulated roof, however good the glass. That single fact explains most of the winter difference between the two designs.
Even the best double or triple glazed unit is a thinner barrier than a roof build-up packed with insulation. Swap most of a roof for glass and the room has a large cold surface above it. You feel it as a radiant chill when you sit under it on a January evening, and the heating works harder to compensate. Condensation gathers on cold inner surfaces too, and our answer on why a roof lantern gets condensation shows the same physics on a smaller scale.
Building Regulations care about this. Part L sets limits on heat loss for new extensions, and roof glazing is held to its own allowances. A lantern is easy to fit within those allowances, because the solid roof around it carries most of the load. A large glazed roof can need a far higher specification of glass and frame to pass, and the amount of glazing may be limited in the thermal calculation. The detail is covered in are roof lanterns energy efficient enough for Part L.
A lantern also responds well to better glass. The coatings, gas fill and warm-edge spacers that improve a glazed unit cost less when they cover a lantern than when they cover an entire roof.
Summer heat and daylight
A glass roof gathers sun all day. A lantern gathers it over a much smaller area, which is easier to control.
On a bright July afternoon the sun comes through roof glass at full strength. A glazed roof on a south or west facing extension can turn the room into a greenhouse, and the answer is usually blinds, solar control coatings, or both. Those add cost, and blinds dim the light you built the roof to gain. Building Regulations now also ask new homes to limit overheating risk, and the same thinking is sensible for an extension, which in practice makes large areas of unshaded roof glass harder to justify.
A lantern brings in less solar heat because there is less glass, and what it does bring in is high above head height. Hot air collects at the top, and a lantern with opening vents lets it leave. Our page on opening vents for a roof lantern explains how that works, and does a roof lantern make a kitchen too hot in summer goes through the heat question in full.
On light, the glazed roof does win. You get daylight across the whole ceiling, including the edges of the room. A lantern puts most of its light in the middle, which suits a kitchen island or a dining table, and a wall of bi-fold doors fills the sides. For many extensions the combination gives a balanced room, with light at the centre and at the garden edge.
Rain, hail and the sound overhead
The quietest roof is the one with the most mass in it. That favours the lantern again.
Rain on a large glazed roof has a great deal of hard surface to strike, and the sound reaches the room directly, with no insulation layer to absorb it. A lantern has the same acoustic weak point, but only over its own footprint, and the solid roof around it stays mute. With either design, the glass build decides the rest. A laminated acoustic inner pane damps the drumming, and a thicker outer pane moves less under rain.
That specification goes into the quiet spec that comes with each quote, whichever roof you choose. For a detailed comparison of the glass itself, see lantern glazing builds for a quieter kitchen.
Snow sits differently on each. A lantern has sloped panes that shed it reasonably fast, while a shallow glazed roof can hold a blanket that darkens the room for a day or two. The load on either roof is worked out by the structural designer, and our note on roof lantern snow load shows what is checked.
| Question | Roof lantern | Fully glazed roof |
|---|---|---|
| Winter warmth | Solid roof does most of the work | Large cold surface overhead |
| Summer heat | Smaller glazed area, vents help | Needs solar control and blinds |
| Daylight spread | Strong in the middle | Even across the room |
| Rain on glass | Noise only under the lantern | Noise across the whole ceiling |
| Part L | Simpler to satisfy | Tighter glass specification |
Cost, structure and choosing well
Price and structure follow the same pattern as comfort: the more glass, the more the roof has to be engineered.
A lantern needs an insulated upstand and a structural opening, but the roof around it is conventional. A fully glazed roof is a structure in its own right, with frames, rafters or beams sized for the glass, and its cost rises with area. We cannot give a figure without knowing the room, and our page on what affects the cost of a roof lantern lists what moves it.
Choose the lantern when the extension is a kitchen, a dining room or any room you want to be comfortable year round. Consider a fully glazed roof only when the garden view is the room's point, the aspect is benign and you accept the cost of heating, shading and a tighter glass specification. A middle route exists: a larger lantern, or two lanterns, over a solid roof. Our roof lantern size guide shows how large a lantern can go before it starts to behave like a glass roof.
Whatever you choose, get the quote in writing with the glass build named. The quote we give covers it, and the survey is where we listen to the room first.
Questions people ask before choosing
Will a lantern make the room feel as bright as a glass roof?
Not across the whole ceiling, but the brightness at the centre is high, and wall glazing lights the edges. Most rooms feel bright rather than dim.
Can a fully glazed roof be quiet in rain?
It can be improved with laminated acoustic glass and a thicker outer pane, but the area is large and the sound has nowhere to be absorbed. A lantern limits the exposure.
Is planning permission different for the two?
The rules depend on height, position and the property, and a glazed roof that changes the extension's form can be treated differently. Our page on whether a roof lantern needs planning permission covers the lantern side.
Can I add a lantern to an existing flat roof?
Often yes, with a new upstand and structural opening. The survey checks the existing deck and falls first.