What is a sun tunnel?

What it is for

Internal hallways, landings, windowless bathrooms, WCs and utility rooms. Places where a rooflight cannot reach because there is a floor or a roof void between the room and the sky.

What it is not

A substitute for a rooflight. Where a rooflight fits, it delivers far more light and it is the better answer. The sun tunnel exists for the places a rooflight cannot go.

It is also not ventilation. It is a sealed unit and it moves no air, so it does not replace an extractor in a bathroom.

How much light

A 250mm tunnel gives roughly the light of a modest ceiling fitting on an overcast day and a good deal more in sun. A 350mm gives about double.

The right comparison is not a rooflight but the dark hallway you currently switch a light on to cross in the afternoon. Detail on the sun tunnel page.

A mirrored tube that carries daylight

A sun tunnel is a small dome in the roof, a highly reflective tube running down through the roof space, and a diffuser in the ceiling of the room below. Daylight enters the dome, bounces down the tube and comes out of the diffuser as soft, even light.

It exists to solve one problem: a room with no external wall and no available roof slope for a proper rooflight. Internal bathrooms, WCs, hallways, landings, utility rooms and walk in wardrobes. In a house like that a sun tunnel is frequently the only daylight option there is, and it costs a fraction of anything else.

Rigid against flexible

The single most important specification decision, and the cheaper option is usually the wrong one.

A rigid tube is a run of mirrored metal sections. It reflects 98 percent or better at each bounce, so light travels a long way down it with modest loss.

A flexible tube is a concertina of reflective foil. Every fold scatters light rather than passing it along, and the material reflects perhaps 80 percent per bounce. Over a three metre run that difference compounds into most of the light.

The practical result is a flexible tunnel delivering something like a third of what a rigid one of the same diameter and length gives. Where the route genuinely will not take a rigid tube, a flexible one is better than nothing and we will fit one having told you what to expect.

Sizing

Diameter matters more than people expect, because light output goes with area rather than width. A 350mm tube passes roughly twice the light of a 250mm one, not forty percent more.

As a rough guide: 250mm suits an internal WC, an ensuite or a small hallway. 350mm suits a landing, a decent bathroom or a small kitchen. 530mm is for a room you actually live in.

Two smaller tunnels frequently beat one large one, because two pools of light read as ambient daylight while one reads as a spotlight in the middle of the ceiling.

What a tunnel will not do

Not a rooflight. There is no view, no ventilation and no sense of sky, and after dusk there is nothing at all. Most units take an optional light fitting inside the diffuser so the same ceiling opening does both jobs.

What it is, on a dull afternoon in a windowless bathroom, is enough soft daylight to walk in without reaching for the switch. For the money that is a good proposition, and it is why we recommend it over a rooflight on a fair number of jobs.

The detail that prevents trouble

A metal tube passing through a cold loft collects condensation, and water running down it arrives at the diffuser looking exactly like a leak.

Three details prevent it and all three get skipped on cheap installations: the tube insulated where it crosses unheated space, the ceiling ring sealed airtight so room air cannot get into the tube, and, where the tunnel serves a bathroom, an extract fan actually doing its job. A tunnel does not ventilate.