A rooflight brings more light than a sun tunnel of the same roof footprint, because it is a window: a pane of glass open to the whole sky, with a lining that spreads what comes in. A sun tunnel brings less light but can reach places a rooflight cannot, because its tube can run around rafters, through a loft and into a room with no outside wall. So the usual rule is simple. Choose a rooflight where the room sits directly under the roof and you want the most daylight. Choose a sun tunnel where the room is buried in the middle of the house or the roof above it is hard to open up.
The rest of this guide sets the two side by side, room by room and roof by roof, so you can see which one your house is asking for.
What each one actually is
A rooflight, whether it is a roof window on a slope, a flat rooflight on a kerb or a roof lantern, is a glazed opening in the roof. The glass is large, usually from a few hundred millimetres across to well over a metre, and the light travels straight down through a short lining into the room. Nothing sits between the sky and the ceiling opening except the glass and the lining.
A sun tunnel is a daylight pipe. A small dome on the roof collects light, a tube with a highly reflective inner surface carries it down by bouncing it from wall to wall, and a diffuser in the ceiling spreads it out. The dome is typically a few hundred millimetres across, so the cut in the roof is small, and the ceiling opening is a neat round disc.
That gives you the whole difference in one line. A rooflight is a window that happens to be in the roof. A sun tunnel is a light source that happens to be fed by the roof. Neither is better in the abstract. They do different jobs, and the same house often suits both in different rooms.
Which brings more light
On a like-for-like roof, the rooflight wins on quantity, and the reason is area. Glass area decides how much daylight gets in. A roof window may admit light through a pane several times the size of a sun tunnel dome, and a roof lantern or large flat rooflight more still. A tunnel makes up some of the difference by being efficient, because its lining reflects most of the light it collects, but it cannot match a big pane.
Quality of light differs as well. A rooflight gives direct daylight that moves across the room through the day, with bright sun on a clear morning and a soft glow on an overcast afternoon. A sun tunnel gives an even, diffused light that changes less. That steadiness suits a hallway or a bathroom, where you want a room to feel bright without any drama, and is less suited to a kitchen or living room where you want daylight with some life to it.
The length of the tunnel counts for a lot. A short, straight, wide tunnel through a shallow roof delivers well. A long run with bends loses light at each turn and each metre. Our answer to how much light does a sun tunnel give goes through those factors, and rigid or flexible sun tunnels explains why a straight rigid tube usually carries more than a flexible one.
Where a sun tunnel is the better answer
The tunnel is the answer in the rooms a rooflight cannot reach. These are the common cases in Devon homes.
- A landing or stairwell in a 1930s semi or a Victorian terrace, where the nearest outside wall is a room away.
- An internal bathroom, cloakroom or en suite with a loft space or bedroom above it.
- A bungalow hall or corridor, where the roof is close above the ceiling and the run is short.
- A utility room or walk-in wardrobe in the middle of the plan.
A tunnel is also the lighter touch on the roof. It needs a small opening that often fits between rafters without cutting one, which makes the structural work simpler and the roof work shorter. Where a roof is complicated, with valleys, chimneys, a dormer or an awkward ridge, a small dome can often go where a large frame cannot.
If the room is a bathroom or hallway, our guide to sun tunnels for bathrooms, landings and hallways covers the detail of those rooms.
Where a rooflight is the better answer
A rooflight is the better choice when the room is directly beneath the roof and the aim is real daylight, not just a lift in a dark space. A loft bedroom, a kitchen extension, a stair hall under the roof slope and a living room with a sloping ceiling all fall in this group.
It is also the better choice when you want ventilation. Many rooflights open, and an opening rooflight can contribute purge ventilation, which matters in a kitchen, a bathroom or a loft room. A sun tunnel does not open, although some models carry a small ventilation fan in the tube. And where a loft room needs an escape window, a roof window can serve if it meets the size and position rules in Approved Document B, which no sun tunnel will.
Finally, a rooflight is the choice when you want something to be seen. A roof lantern over a kitchen island or a big fixed rooflight over a stair hall becomes a feature of the room. A sun tunnel is deliberately modest, and its diffuser is a plain disc.
| Point | Sun tunnel | Rooflight |
|---|---|---|
| Amount of daylight | Modest, even and soft | Greater, direct and changing |
| Rooms it suits | Hallways, landings, bathrooms, cloakrooms | Loft rooms, kitchens, living rooms |
| Reaches a room with no outside wall | Yes, through a tube | Only where the room is under the roof |
| Opening in the roof | Small, often between rafters | Larger, usually with trimmed rafters |
| Ventilation | None as standard | Available on opening models |
| Opening at the ceiling | A round diffuser disc | A lined opening, straight or splayed |
The roof decides some of this
The roof over the room is often the practical limit. A pitched roof with a short loft and a straight drop suits a tunnel well, and a long run across a deep loft or through a hip does not. On a flat roof, a dome on a collar or kerb and a short tube is straightforward, and a flat rooflight is equally so. Sun tunnels on flat and pitched roofs covers how the flashing and the tube change with the roof.
The covering matters too. A tunnel on slate needs a flashing to suit the thickness of the slate, and a tunnel in plain tiles needs one to suit the tile. Rooflights meet the same coverings with their own flashing kits. In either case the weathering is fitted to the roof you have, and we check the covering at survey rather than assuming it.
Where the loft above the room is cold, a tunnel passes through it, and the tube needs insulation to stop the cold surface causing condensation. A rooflight in the same position needs a lined shaft, also insulated. Both are workable, and both are part of the plan we write down.
Warmth, heat and the rest of the day
Daylight is only one part of what a roof opening does. Both options also affect how warm the room feels in winter and how it behaves on a hot afternoon, and the two differ in a way that is easy to miss.
A rooflight is a large area of glazing, so the glass specification counts. Double glazing with a warm-edge spacer is the baseline, triple glazing is available, and a laminated inner pane is good practice overhead. Solar control coatings can take the edge off a south-facing loft room in summer. A sun tunnel has a much smaller glazed area, so it gains and loses less heat through the opening. That is one reason a tunnel suits a bathroom: it adds daylight without adding a large cold surface.
The tunnel has its own cold spot, which is the tube running through the loft. If it is not insulated, warm damp air from the room can meet the cold tube wall and condense. That is handled with insulation around the tube and a sealed joint at the ceiling, and our answer to do sun tunnels cause condensation explains it. A rooflight in a cold loft has the same issue in its lightwell, which is why the lining is insulated as well as plastered.
After dark, the picture changes. A rooflight shows the night sky and gives no light of its own, unless you add ceiling lights around it. Many sun tunnels take a lamp kit in the diffuser, so one opening serves the room by day and by night. In a small room, that can replace the ceiling light altogether.
The ceiling you look up at
Whichever you choose, you will see the opening every day, so the finish deserves a decision of its own. A sun tunnel leaves a round diffuser in a trim ring. It looks right when the ring sits flat against the ceiling, the plaster around it has been made good and the paint line is clean. On an old lath and plaster ceiling, the round hole is cut so that the plaster does not crack, and the edge is made good in lime where the ceiling is lime.
A rooflight leaves a lined opening, and the shape of the lining is a choice. A straight lining is simple and a splayed one spreads light into the room. We explain the options in how a rooflight is finished inside.
Both go into the finish schedule attached to each quote. It sets down the room, what sits above the ceiling, how the opening is formed and finished, and the plan for dust. If you are weighing the two options for one room, we can price each and show the finish for both in writing.
Cost, planning and Building Regulations
A sun tunnel is usually the smaller job. The unit is smaller, the roof work is shorter and there is often no structural trimming, so it usually costs less than a rooflight for the same room. Rooflight prices vary widely with the type, size, glazing and roof, and what sets them is covered on the costs page.
On planning, a rooflight or sun tunnel dome on a house generally falls within the Class C right (Part 1, Schedule 2, General Permitted Development Order), as long as it stands out less than 150mm from the roof plane and stays under the roof's top line. A conservation area keeps Class C unless an Article 4 direction removes it, a listed building requires listed building consent for outside changes, and a flat has no householder permitted development. Where it varies, the planning authority can confirm.
On Building Regulations, cutting a new rooflight into a roof that is already built counts as building work needing Building Control approval, because rafters are trimmed and the unit must meet Part L. A sun tunnel is a smaller intervention, and we confirm the position for your roof at survey. Either way, a notifiable job is notified by us.
Making the choice, room by room
Take the house as a whole before choosing for one room. A dark landing and a gloomy bathroom beside it may share one answer, and a loft room above them may want another. Walking through with the roof plan in mind usually shows which rooms sit under open roof and which are buried.
If you are standing in the room now, a few questions settle most cases.
- Is the room directly under the roof? If yes, a rooflight is possible. If not, a tunnel may be the only way in.
- Do you want daylight with character or a steady brightness? A rooflight for the first, a tunnel for the second.
- Does the room need ventilation or an escape route? Then a rooflight.
- Is the roof complicated or the budget tight? A tunnel is the lighter option.
Some houses use both: a rooflight over the stair hall and a tunnel in the bathroom beside it. If you are unsure, send us photographs of the room and the roof through the quote form, and we will survey, measure, and write down both options with the finish for each. The detail of the tunnel itself is on our sun tunnel installation page, and the work carries our 10-year workmanship guarantee.