The short answer
A sun tunnel gives roughly the daylight of a modest ceiling light fitting during the day, enough to read by in a small bathroom, landing or hall, but far less than a rooflight of the same width. A tube carries light down from a small dome, and every metre of tube and every bend takes some away. The output depends on the tube diameter, the length, the number of bends, the roof and the sky, so it cannot be a single figure.
What sets the output
Five things matter, roughly in this order.
- Diameter
- The wider the tube, the more sky it gathers. Common sizes run from about 250mm to 350mm across. A 350mm tube collects almost twice the area of a 250mm one, so the step up in brightness is real and worth paying for in a room with no other window.
- Length
- Light is reflected down the tube by a mirror finish, and each reflection loses a little. A short tube through a thin ceiling void loses very little. A long one, through a deep loft, loses noticeably more.
- Bends
- A straight tube is the best performer. Each angled elbow reduces output, so a tube that has to dodge a joist or a chimney will be dimmer than the same length run straight.
- Tube type
- Rigid tubes with a polished mirror surface are generally brighter than flexible ones, which have ridges that scatter light. We compare them in rigid or flexible sun tunnels.
- Roof and sky
- A dome on an unshaded slope sees more sky than one under a tree or a neighbour's gable. Pitch and direction also change when the best light arrives.
What that means in a real room
Used in the right place, a sun tunnel makes a surprising difference to a dark space. A windowless bathroom, a downstairs WC, a landing or a hallway between rooms is usually transformed from needing a light on in the morning to needing none. The light is soft and even, because the diffuser spreads it across the ceiling rather than casting a patch.
Think of it as a lighting source for a small space, not a way to flood a living room. A tunnel is not a substitute for a window in a room you use all day, and in a large kitchen you would need a different product. If you are weighing the two, our guide on sun tunnel or rooflight sets them side by side.
Cloudy days, mornings and evenings
A sun tunnel works on daylight, not only direct sun, so it keeps working under cloud, though it does give less. On a bright, clear day it will be strongest around midday, and in a grey Devon winter it will be dimmer and shorter-lived at either end. That is true of any daylight source, windows included. We cover this in do sun tunnels work on cloudy days.
After dark there is no daylight to carry, so a tunnel gives nothing unless a light has been fitted inside the diffuser. That option exists, and it is dealt with in can a sun tunnel have a light fitted.
Getting more from the one you fit
Several choices help. Pick the widest tube the roof and ceiling will take. Keep the run short and straight if the layout allows. Choose a roof position with a clear view of the sky, and avoid the shaded side of a chimney. A pale ceiling around the diffuser helps the light spread, which is why a neat, white finish matters here.
That finish is the part we pay attention to. A diffuser sits flat on the ceiling, and the plaster or paint around it needs a clean edge so the fitting looks deliberate, not stuck on. The finish schedule we write for every quote records the ceiling type, how the collar is made good and whether it is left ready to paint.
The diffuser and the tube behind it
The ceiling end matters as much as the dome. A frosted or prismatic diffuser spreads the light across the ceiling so the room looks evenly lit, not spotlit. Some diffusers are slightly domed, others flat, and a flat one sits closer to the ceiling for a tidier look. Whichever you choose, the opening is cut through the plasterboard or plaster and a collar fixed behind it, so the way that ceiling is made good decides how neat the finished edge looks.
The tube can be fitted with an extra dome layer on top that cuts heat loss, and it can include a damper to dim the light. Both help in a bathroom, where you might want to cut brightness or keep warmth in, and neither changes how much light the tunnel collects.
Bathrooms, halls and loft routes
The rooms that suit a tunnel best are the ones a window cannot reach. In a bathroom, the diffuser sits above the shower or bath, where it also helps with the look of the ceiling. In a hall or landing, the light from one tunnel runs along the ceiling and picks out the stair. We go through the room-by-room choices in sun tunnels for bathrooms, landings and hallways.
Where the tunnel passes through a loft, the route is part of the answer. A tube that drops straight between two joists is bright. One that must wander round a water tank or a purlin loses light at every bend. Our answer on whether a sun tunnel can go through a loft covers how the route is planned and insulated.
Is a sun tunnel enough for your room?
For a bathroom of ordinary size, a WC, a cupboard-sized utility or a short hallway, one tunnel is usually plenty by day. For a longer landing or a bigger bathroom, two can be better than one large one. For a living space, look at a roof window or a flat rooflight instead.
The only reliable way to judge is to look at the roof, the loft and the room together. When you ask for a quote, we survey the route the tube would take, measure the length and bends and tell you what size to expect to work. For the general picture, see sun tunnel installation.