How much light does a sun tunnel give?

By diameter

250mm: roughly the light of a modest ceiling fitting on an overcast day, and considerably more in sun. Enough that you stop reaching for the switch in a hallway.

350mm: about double, because the collector area is about twice as large.

What reduces it

Run length. Every metre costs some light. Under two metres and straight is excellent, two to four metres is good, beyond four with bends it is worth a frank conversation about whether it is worth doing.

Bends. Each elbow costs more than a metre of straight tube.

Flexible tube. The corrugations scatter light at every ridge. A long flexible run can deliver less than half what a rigid one would.

Which way the roof faces. A south facing collector gathers more than a north facing one, though the difference is smaller than for a rooflight because the collector is domed and gathers from a wide angle.

Through the day

It is daylight, so it varies. Bright on a sunny afternoon, softer on an overcast morning, and nothing after dark. That variation is part of the appeal in a space that otherwise has no connection to outside at all.

The comparison that matters

Not against a rooflight, which will always win. Against the room as it is now: a windowless landing where the light is on all day. Against that, the difference is substantial.

Enough not to reach for the switch

That is the honest measure, and for a windowless bathroom or hallway it is a good one. A well specified rigid sun tunnel gives soft, even daylight through the middle of the day that makes a room usable without artificial light.

What it does not give is the brightness of a rooflight. A 350mm tunnel has an aperture of under a tenth of a square metre, and no amount of reflective efficiency turns that into a window.

What decides the output

Diameter, above everything. Output goes with area rather than width, so a 350mm tube passes roughly twice the light of a 250mm one, not forty percent more, and a 530mm doubles it again.

Getting this wrong is the commonest disappointment with sun tunnels. A 250mm tube fitted where 350mm was needed produces a faint glow, and the cost difference between the two is small.

Rigid or flexible. A rigid mirrored tube reflects 98 percent or better at each bounce. A flexible foil concertina reflects perhaps 80 percent and scatters light at every fold, delivering roughly a third as much over a typical run.

Run length and bends. Light loss compounds along the tube. A short straight drop in a bungalow delivers far more than a long run through a loft and a first floor void, and every bend costs more than a metre of straight tube.

Obstruction above the dome. A chimney, a tree canopy or a taller neighbouring building costs more than orientation does.

What it looks like in the room

A disc of soft daylight in the ceiling, brighter at midday and dimmer at each end of the day, with no glare and no shadows cast. It reads as ambient light rather than as a beam.

Two smaller tunnels spaced apart give a better result than one larger one, because two pools read as daylight while one reads as a spotlight in the middle of the ceiling.

After dark

Nothing. A sun tunnel tracks the day and there is no light in it at night. Most units take an optional light fitting inside the diffuser so the same ceiling opening does both jobs, and on a bathroom or hallway that is worth specifying at the outset rather than adding later.

A realistic expectation

In an internal WC, a 250mm tunnel is enough to walk in and see. In a family bathroom, a 350mm makes the room feel daylit. In a hallway or a kitchen, 530mm or two smaller tunnels gives genuine working light.

Nobody has ever told us their sun tunnel was too bright.

How to judge it before committing

Look at what the room needs. If you want to walk in and see, 250mm in a small internal WC is enough. If you want the room to feel daylit, 350mm. If you want genuine working light in a kitchen or a hallway, 530mm or two smaller tunnels.

Then look at the route in the loft, because a short straight run at 350mm will beat a long bent run at 530mm.

And be clear about what it is not. There is no view, no ventilation, no sense of sky and nothing after dusk. What there is, on a dull afternoon in a room that has never had daylight, is enough light not to reach for the switch.

For the money that is a good proposition, and it is why we recommend a tunnel over a rooflight on a fair number of jobs.

Set expectations honestly

A sun tunnel is not a small rooflight. It delivers considerably less light than a rooflight of equivalent cost, and it is not trying to.

What it delivers is a genuine, daylight quality illumination in a place that currently has none at all. In a windowless hallway, that is the difference between a light burning all day and a space that reads as part of the house.

What determines the output

Diameter, which is the biggest single factor and the cheapest to change. Going up a size costs little and delivers noticeably more.

Bends, which cost more than length. A single bend forces many more reflections than several metres of straight tube. Two bends can halve what arrives.

Length, because every metre loses a little.

Tube type. A rigid tube with a highly reflective internal finish delivers substantially more than a flexible one over the same run, because the corrugated inner surface of a flexible tube scatters light at every ridge.

Roof orientation, which matters less than people expect, because the dome collects light from the whole sky rather than only from direct sun.

The rule that matters

Short and straight beats long and clever, every time. Moving the ceiling position by a reasonable amount to achieve a straight run is worth doing.

A hallway or landing directly under the roof is the ideal case and gives the best results.

What it feels like

It varies with the weather and the time of day, which is the point. It is daylight rather than a lamp, and the space changes through the day in a way electric light does not reproduce.