In brief
A sun tunnel gives a soft, even wash of daylight, enough to light a small windowless room in daytime, but the amount varies. It depends on the tube diameter, whether the tube is rigid or flexible, how long the run is, how many bends it takes, which way the roof faces and what the weather is doing. Makers publish guidance by model, and we match the size to the room.
- Diameter: a wider tube collects and delivers more daylight
- Tube type: a smooth rigid tube passes more light than a ribbed flexible one
- Length and bends: every extra metre and every angle costs some output
- Roof aspect and shading: a south-facing slope with open sky collects the most
- Weather and season: output follows the daylight outside, so dull days give less
What a sun tunnel actually does
A sun tunnel does not make light. It collects daylight through a clear dome on the roof and carries it down a mirror-finished tube to a diffuser in the ceiling, where it spreads into the room. What comes out is a share of what went in, so everything that affects the daylight at the dome, or the loss along the tube, changes what you see below.
That also explains the character of the light. It is diffuse, with no hard shadows and no view, rather like a large pale ceiling panel. It brightens a hallway, landing, bathroom or utility room well, but it does not turn a room into a conservatory. If you want a view or a lot of sky, sun tunnel or skylight explains which suits the room.
Tube diameter
The wider the tube, the larger the dome that gathers daylight and the larger the diffuser that releases it. Doubling the width gathers considerably more than twice the light, since the area grows faster than the diameter. For a cupboard-sized cloakroom a small tube may be right. A hallway, a landing, or a kitchen corner with no window usually does better with a larger one.
We do not quote a figure for the lumens or the area a given size will light, because the maker publishes that by model and the figures differ from one product to the next. At the survey we compare the maker's guidance with the size of your room and its other windows, and put the chosen model in the written quote.
Room size matters as much as tube size. A tunnel in a narrow landing lights the whole space, while the same tunnel in a wide kitchen lights the area under the diffuser and fades towards the walls. Pale walls and a light floor return more of the light into the room, and dark finishes absorb it. Two smaller tunnels sometimes suit a long hallway better than one large one, and we can price both arrangements side by side.
Rigid tube, flexible tube and bends
Light bounces down the tube, losing a little at each reflection. A rigid tube has a smooth mirror surface, so it loses less over a straight run. A flexible tube has ridges, and each ridge scatters some light, so it delivers less over the same distance. In return it can curve around a rafter or a tank, where a rigid tube would need an angled elbow.
- Keep the run as straight and as short as the loft allows
- Use angled rigid elbows where a bend is needed, not a kink in a flexible tube
- Pull flexible tube tight, since slack folds trap shadow and cut the light
- Fit the tube so the joints are taped and clean inside, because dust and gaps both lower the output
The trade-offs are set out in rigid or flexible sun tunnel.
Roof position and orientation
The dome collects what falls on it, so its position matters. A slope facing south, or close to it, with open sky above, receives the most direct sun through the day. A north-facing slope still brings light, but at lower strength. Trees, a taller neighbouring house or a chimney stack can shade a dome for part of the day, and a flat roof gives a clear view of the sky but needs the right flashing, covered in sun tunnels in flat roofs.
Pitch has a small effect too. On a steep roof the dome looks sideways more than up, which suits low winter sun. On a shallow or flat roof it looks upward and collects more from the high summer sky. We check the slope and its surroundings on the survey and tell you what to expect.
Weather, season and time of day
Output follows the daylight outside. On a bright day a sun tunnel makes a hallway feel properly lit. On an overcast afternoon it gives a gentler glow, and at night it gives nothing at all unless a light kit has been fitted. That is not a fault of the tunnel. It is what the product does, and the room should be judged against a typical daytime, not the brightest hour of summer.
The question of short winter days has its own page, do sun tunnels work in winter. In short, the tunnel carries what daylight there is, and a room that was dark before will be lighter at any season.
Checking before you order
Before choosing a size, look at three things in your own house. First, where the tube would run: go into the loft and look for ducting, pipes, tanks and joists between roof and ceiling. Second, what the room is used for, because a bathroom lit mainly in the morning and a hallway used all day need different thinking. Third, what else lights the space, such as a glazed front door or a window on the landing.
Our survey is free. We measure the run, check the roof covering and aspect, and write down the model, tube type, expected lead time and any work needed from a builder or roofer. Cost depends on the same factors, and how much a sun tunnel costs explains how the quote is built. The full range is on the sun tunnels page.
Related questions
- Is a sun tunnel brighter than a small window?
- It can be in a room with no outside wall, because it brings daylight from the roof. A window gives a view and ventilation that a tunnel cannot, so the two do different jobs.
