What is a sun tunnel?

The short answer

A sun tunnel is a reflective tube that carries daylight from a small dome on the roof, down through the roof void, to a flush diffuser set in the ceiling of the room below. There is no glazed opening in the room and no view of the sky. What arrives is even, shadowless daylight that changes through the day with the weather, from a fitting that looks like a recessed light and is often mistaken for one until somebody notices there is no switch. It exists to light rooms that a rooflight cannot reach: internal bathrooms, windowless landings, hallways with no external wall, and box rooms buried in the middle of a plan. Installation detail and specification sit on the sun tunnel installation page. This one explains what the thing is and how it works.

The parts, from the dome down to the diffuser

A sun tunnel is a kit of three assemblies, and knowing the names makes every other conversation about it easier.

The roof collector. A clear acrylic dome or a flat glass unit sitting on a flashing that is matched to the covering, exactly as a roof window’s flashing kit is. This is the aperture that gathers light.
The tube. Rigid or flexible, with a highly reflective inner surface, running from the collector through the roof void to the ceiling. This is the part that does the work and the part that most affects the result.
The ceiling diffuser. A translucent panel in a trim ring, set flush with the plasterboard, which spreads the light coming out of the tube across the room rather than dropping it as a bright circle on the floor.

There is nothing else. No frame in the ceiling, no plastered reveal, no opening sash, no glazing bars, and on the standard product no moving parts at all.

Why a mirrored tube behaves nothing like a plain shaft

The whole product turns on one number: the reflectance of the tube lining. A plain painted builder’s shaft between the roof and a ceiling loses most of its light within a metre, because matt plaster reflects perhaps 80 per cent of what hits it and the light bounces many times on the way down.

A sun tunnel tube uses a specular reflective film with a reflectance in the high nineties. Each bounce loses one or two per cent instead of twenty. After a dozen bounces a plain shaft has lost almost everything and a mirrored tube has lost very little, which is why a 350mm tube running two and a half metres can light a bathroom that has no window in it.

Two consequences follow directly from that arithmetic. Length matters, because more distance means more bounces. Bends matter more, because a change of direction forces extra reflections at a steep angle, and each bend costs noticeably more light than the equivalent length of straight tube.

Rigid tube against flexible tube, and what each is for

Rigid tube comes in polished sections that push together, holds its shape, and has the higher reflectance of the two by a clear margin. It needs a route it can actually take, with elbows at defined angles, which means the roof aperture and the ceiling aperture have to be reasonably close to plumb or connected by a route that can be set out.

Flexible tube is a concertina with a reflective liner, and it will thread past a purlin, a cistern or a soil vent pipe that a rigid run could not get round. It is far more forgiving to install and it delivers less light, partly because the reflectance is lower and partly because the corrugations scatter the beam. Where the run is short and the obstacles are many, that trade is usually worth taking. Where the run is long, rigid tube is the only sensible answer.

Diameter, and the floor area a tube will light

Sun tunnels come in a small number of diameters, and the diameter sets the ceiling of what is possible before anything else is considered.

Nominal diameter	Typical room	What to expect
250mm	Small en suite, WC, cupboard, short hallway	Enough to move around without a switch on a normal day. Fits comfortably between joists at 400mm centres
350mm	Bathroom, landing, box room, kitchen corner	The most commonly specified size and the best balance of light against a straightforward ceiling opening
450mm and above	Larger hallway, stairwell, kitchen, open landing	Substantially brighter, and usually needs a joist trimmed or the run planned around the ceiling structure
Two tubes	Long hallway, wide landing	Two moderate tubes spread light far more evenly than one large one in the middle

Doubling the diameter roughly quadruples the collecting area, so the step from 250mm to 450mm is much larger than it sounds. What limits it is the ceiling: a diffuser wider than the joist spacing means cutting and trimming a joist, which turns a half-day job into a longer one.

What the room actually gets, through a day and through a year

The honest description is useful daylight rather than dramatic daylight. A 350mm tunnel into a small bathroom on an overcast Gloucestershire morning gives roughly the light of a decent ceiling fitting, spread evenly and with no shadow. On a bright day with sun on the dome it is considerably more than that, and the room reads as properly lit.

What changes through the year is the sky, not the unit. In midwinter, first light and dusk both fall inside the working day and a tunnel goes dim at the same moments the rest of the house does. Through the summer, a north-facing collector on a Gloucester terrace still delivers steady diffuse light from ten in the morning until well into the evening. Many units are available with a light fitting mounted inside the tube, so the same diffuser does both jobs and the ceiling has only one thing in it.

Daylight without a view, and why that is the point

A sun tunnel gives light and nothing else. No sky, no opening, no ventilation unless a ventilation kit is specified as an extra. For a room with a window that is a poor substitute. For a room with no external wall it is the only option there is, and the comparison is with a light switch rather than with a rooflight.

That framing also settles the regulations question in most cases. Habitable rooms have daylight and ventilation expectations that a tunnel alone does not satisfy, so a bedroom or a living room needs a glazed opening. Bathrooms, WCs, hallways, landings and utility rooms have no daylight requirement at all, which is exactly why they are the rooms that end up windowless, and exactly why a tunnel suits them.

The building work is much smaller than a rooflight

This is the practical difference that decides most jobs. Putting a rooflight into a pitched roof means a structural opening, cut rafters, doubled trimmers, a flashing set, an insulated reveal, plasterboard, plaster and decoration, and the room below is disrupted for two days.

A sun tunnel takes a circular aperture through the covering, a circular aperture through the ceiling, and no structural alteration at all where the tube fits between existing rafters and existing joists, which at 400 or 600mm centres a 250mm or 350mm tube generally does. There is no plaster box to build, no reveal to insulate and no decoration to make good beyond the trim ring. That is why a straightforward tunnel is commonly a single day with two people and why the figure sits a long way below an opening in the same roof.

Condensation, insulation and the two seals that matter

A tube runs from a cold roof space into a warm room, which means it is a potential path for both heat and moisture, and the detail at each end is what keeps it behaving.

At the ceiling, the trim ring is sealed to the plasterboard and the vapour control layer so warm humid air from the bathroom below cannot rise into the loft around the outside of the tube. At the roof, the flashing is dressed into the covering exactly as any other roof penetration is, lapped in the direction of flow. In between, the tube itself is insulated where it passes through the cold void, and where 400mm of blown loft insulation is present the tube is sealed through it rather than left with a gap around it.

Get those details right and the inside of the tube stays clear. Get them wrong and you find condensation running down the inside of a mirrored tube into a bathroom ceiling, which is the commonest complaint about badly fitted units and is entirely a detailing matter rather than a fault in the product.

Gloucestershire rooms where a tunnel is the obvious answer

Certain plans recur here. The interwar semis across Longlevens, Hucclecote and Warden Hill nearly all have a bathroom over the stairs with one small window or none, directly under a hipped roof with a short easy run to the slope. Bungalows through Tuffley, Abbeydale and Churchdown have the shortest runs of all, with a shallow void and a tube length often under two metres. Gloucester’s Victorian terraces have long internal landings with no external wall in reach. And Cotswold cottages with thick stone walls and deep plans have interior rooms that no window will ever get to.

Where the choice is genuinely open between a tunnel and a rooflight, the comparison is set out on sun tunnels and where they beat a rooflight. The rest of the installation range, from roof windows to lanterns, is on the skylight and rooflight installation page.

Getting one specified for your own roof

What decides a sun tunnel is the route rather than the room. The survey drops a plumb line from where you want the diffuser, looks at what the tube would hit on the way up, checks the joist and rafter centres, measures the run, and looks at how much open sky the collector position can see once trees and neighbouring roofs are taken into account.

To get a view on yours, email info@severnskylightsgloucester.co.uk with a photograph of the ceiling where you want the light, a photograph of the roof slope above it from outside, a photograph inside the loft if you can safely take one, and the address. An indicative range comes back by email, then a fixed price after the survey.