The answer
A sun tunnel gives a soft, even pool of daylight, enough to make a windowless landing, hall or bathroom feel naturally lit by day. The brightness depends on tube diameter, tube length, how many bends there are, which way the roof faces and how large the room is. Shorter, straighter runs deliver more.
What kind of light to expect
A sun tunnel does not behave like a bulb. Daylight enters the dome from the whole sky, runs down the tube and leaves through a frosted diffuser, giving the room a gentle, shadow-soft glow that shifts as the weather and the hour change. On a bright day it can be remarkable. On a heavy grey one it is quieter, just as every window is.
That is the honest answer to brightness: it is daylight, so it moves. What it gives you is a room that is lit from the sky for most of the working day, with no switch needed to see where you are going.
Five things that set sun tunnel brightness
Two sun tunnels can look alike on the roof and perform differently in the room. These are the factors that decide it.
- Tube diameter. A wider tube gathers more sky and passes more light, and it lights a larger floor.
- Tube length. Light loses a little each time it bounces. A brief drop between roof and ceiling delivers more than a long climb.
- Bends. Every elbow costs some light. A straight drop is the brightest route.
- Tube finish. A mirror-polished rigid tube carries more than a ribbed flexible one.
- Roof direction and shade. A roof that sees open sky for long hours gives the dome more to collect than one under a tall tree or beside a taller building.
Matching the tunnel to the room
A small room needs less. A single tunnel can light a landing, a cloakroom or a small bathroom comfortably, and the whole floor feels bright. A larger room, such as a long hallway or a wide kitchen corner, may call for a wider tube or two tunnels spaced along the ceiling so the light spreads without dark ends.
Ceiling height matters too. In a tall stairwell the light has further to travel before it reaches the floor, so the position of the diffuser and the size of the tube both count. We look at all of this at the survey, because a plan drawn for the room beats a rule of thumb.
A sun tunnel is not a roof window
If a room can take a full skylight, a larger pane of glass collects more daylight than a small dome ever can, and it gives a view of the sky as well. A tunnel earns its place where that bigger opening is not possible, or where the room is far from the roof and needs a light source that can travel. Our answer on sun tunnels and roof windows sets the two side by side.
The same logic applies to a rooflight in a flat roof. Where there is space for one, it will outshine a tube. Where the room sits beneath a loft, the tube can be the better fit.
Morning, noon and evening
Daylight shifts through the day, and a tunnel follows it. In the morning the dome catches low light from the east side of the sky, and by midday it collects from overhead, which is when the tube is at its best because the light drops straight down the shaft. Late in the day the glow softens and fades with the sky.
Season matters in the same gentle way. Winter light comes in low and weak, so the room is quieter than in June. A tunnel still lifts a dark space from gloom to something readable, which is the point. If you want a room that holds its brightness all year, the survey is the moment to talk about tube width and the angle of the roof.
Shade is the thing that changes the day most. A chimney, a dormer or a neighbouring gable can throw a shadow over the dome for part of the day. We check where the shadows fall before suggesting a position on the roof.
Getting the best from the light
A few small choices lift the result. Pale ceilings and walls bounce the light further into the room. A clean dome keeps the intake bright, so a wipe from the roof edge or a rinse in the rain helps. Keeping the diffuser clear of shelves and hanging lights stops the glow being blocked at the last step.
A sun tunnel can also be fitted with an electric light kit in the diffuser, so the same ceiling point serves day and night. For the full picture of options and what shapes the quote, see the sun tunnels hub, or read what a sun tunnel is if you are new to the idea.
Check these before you choose
Think about the room before you think about the product. These questions shape the answer.
Before you book a survey
- How far is the room from the roof?
- Is the route straight, or does it have to bend?
- Does the roof see open sky or a tall tree?
- How big is the floor you want to light?
- Would a full skylight fit instead?
Brightness, answered briefly
Will a sun tunnel work on a cloudy day?
Yes, because it gathers light from the whole sky, not only direct sun. The room is dimmer under thick cloud, as it would be with any window, but it stays naturally lit.
Can I dim a sun tunnel?
Some designs include a damper or a closing mechanism in the tube so you can reduce the light. Ask at the survey if the room is a bedroom or a place where you want darkness at times.
Does a sun tunnel let in heat?
The dome is a sealed, insulated unit and the opening is small, so a sun tunnel is a modest heat path compared with a large pane of glass. Your surveyor will talk through the roof direction with you.
