How does a sun tunnel work? From dome to ceiling, one beam at a time

Tubular skylights look unlike a conventional rooflight, but the principle is ordinary optics. Light goes in at the top, reflects down a polished pipe and comes out at the bottom, where it is scattered into the room.

Think of it as a relay with three runners. The dome sits on the roof and gathers whatever daylight there is. The tube runs through the roof space, lined with a mirror finish that keeps the light moving instead of letting it soak into the walls of the pipe. The diffuser is the frosted or prismatic panel set into the ceiling of the room below. It is the only part you see from inside.

Between those three sit the quieter components that make the whole thing behave as a roof fitting: the flashing that seals the dome to the roof covering, the collar that joins dome to tube, and the ceiling ring that holds the diffuser. The route through a tiled or slate roof is covered in fitting a sun tunnel through a tiled or slate roof, and the flat roof version is in sun tunnels on flat roofs. Here we stay with the light.

Nothing in the system moves. There is no motor, no wiring and no switch. That is part of the appeal in a hallway or a cloakroom, and it is also why the choices made at the start, the diameter, the length and the finish, matter so much. They cannot be adjusted once the ceiling is plastered.

The dome is the collector. Its job is to accept light from as much of the sky as possible and pass it into the tube without losing much on the way.

Domes are moulded in clear or lightly tinted plastic, usually acrylic or polycarbonate, and some makers offer a glass option. A raised, curved shape is a deliberate choice. A flat pane would catch the sun well only when it was high overhead. A dome presents a surface to the sky at every angle, so the morning sun low in the east, the midday sun and the long evening light all find a way in. Many domes carry a prismatic or lens-like pattern moulded into the surface, which bends low-angle light downward toward the tube opening rather than letting it skim across the top.

Clear plastic also has to cope with weather. Sunlight over the years yellows and clouds cheap material, so a good dome is made from a grade that resists ultraviolet light, and some carry a coating that does the same job. The difference between an acrylic and a glass dome is worth its own discussion, and we set it out in which is better, a glass or acrylic sun tunnel dome.

The dome matters for sound as well as light. A thin plastic bubble is a small drum, and heavy rain finds it. We return to that below, because it is a specification choice and not a flaw in the idea.

Before the light reaches the tube it has to pass through a hole in the roof, and the hole has to stay dry. This is the job of the flashing, a moulded skirt that sits under the tiles or on the membrane and rises to meet the dome collar.

On a pitched roof the flashing is shaped to sit flat on the slope and shed water to either side, with the upper edge tucked beneath the tiles above and the lower apron lying over the tiles below. On a flat roof the dome sits on a short upstand, and the roof membrane turns up against it in the same way as it would around any rooflight. Either way, the flashing is the part that matters most for a long dry life, and the watertight detail is explained in how a sun tunnel is kept watertight on the roof.

Inside the roof space the collar continues down. There is usually a short section of rigid tube that joins to the flashing, so the first stretch of the light path is straight and well supported. That matters, because the joint between dome and tube is where light is most easily lost if the parts do not meet squarely.

The tube is the part people find hardest to believe, because daylight is not normally something you can pour down a pipe. It works because the inside of the tube is a mirror.

A beam of light that enters at an angle does not travel straight to the bottom. It strikes the wall of the tube, bounces off, strikes the opposite wall, bounces again and works its way down in a zigzag. Each bounce loses a small fraction of the light, because no mirror is perfect. The finish is therefore the whole point. The lining is a thin layer of highly reflective metal, often a silvered or aluminium-based film bonded to the tube wall, and the best finishes return nearly all of the light at every bounce. A cheap, dull or scratched lining loses more with each reflection, and because a long tube means many reflections, the loss adds up.

That is why tube length and diameter carry so much weight in any specification. A wide tube gives each beam fewer bounces over the same distance, because the walls are further apart. A short tube has fewer bounces in total. A narrow, long one sends the light through the most reflections and delivers the least at the bottom. The full set of trade-offs is in tunnel length, bends and how much light arrives, and the question of width has its own page in choosing a sun tunnel diameter.

Tubes come in two forms. A rigid tube is a set of straight, smooth-walled sections that join end to end, and an elbow can be added where the route must change direction. A flexible tube is a corrugated, stretchable pipe that can be pulled to a bend in a tight roof space. The rigid type has a smoother surface and fewer places for light to scatter, while the flexible type solves awkward routes. The comparison is worked through in rigid or flexible sun tunnels.

Every metre of tube and every elbow costs a little light. The table below shows the direction of each effect, not a measured result, because the amount depends on the make, the lining and the weather on the day.

A straight drop from dome to ceiling is the ideal. In a bungalow, where the roof space is shallow and the tube is short, that is often what you get. In a two-storey house the route may be longer, and a tunnel that has to pass through an upstairs room needs boxing in. The practical points are in whether a sun tunnel is easier to fit in a bungalow.

At a survey we measure the roof space before we talk about diameter. The height from ceiling to roof underside, the position of the rafters or trusses, the ceiling joists and any wiring or pipework in the way decide the route. The route decides the tube. The tube decides how bright the room can be.

At the bottom of the tube the light is still travelling in a fairly narrow column. If it simply came out of a hole, you would see a bright spot on the floor and a dark room around it. The diffuser prevents that.

A diffuser is a translucent panel that scatters the beam in all directions. It is often frosted, or patterned with tiny prisms, so that light leaves it across a wide angle and spreads over the ceiling and the upper walls. The ceiling then reflects it down into the room, which is why a pale, matt ceiling makes a tunnel look brighter. From below, the diffuser glows like a round ceiling light.

The finish you choose changes the feel of the room. A flat opal panel gives an even glow and hides the tube completely. A patterned one throws a little more light sideways. A version with a built-in lamp allows the same fitting to light the room after dark. The choices, and what each does to a ceiling, are in sun tunnel diffusers and ceiling finishes.

The diffuser also closes the ceiling. It carries a seal that stops warm air from the room reaching the cold tube, and it keeps dust and insects out. A badly fitted ring becomes a place for draughts, so the ceiling trim is part of the installation and not an afterthought.

A sun tunnel does not make light. It relays what the sky supplies, so the output rises and falls with the weather and the time of year. That is how every rooflight behaves, and it helps to set expectations accordingly.

On a bright day with the sun high, the dome collects a great deal, and the room below glows. In winter, with the sun low and the days short, the dome still collects light, but there is less of it to begin with. On an overcast day the light is soft and comes from the whole sky, which a dome is well suited to gather, so the room remains usefully lit, though less brightly than under direct sun. We deal with this directly in whether sun tunnels work on cloudy days.

Direction has less influence than people expect. A dome takes light from the whole hemisphere above it, so a roof that faces away from the sun is not a dead end. A south-facing slope gets the strongest midday gain, and a north-facing one gets a steadier, cooler light through the day. If your only available roof plane faces north, the tunnel is still a serious option.

Heat follows the same logic. The mirrored tube passes visible light far better than it passes warmth, and a good dome includes a filter against ultraviolet. Even so, summer warmth is a fair question, and it is answered in whether sun tunnels make a room hot in summer.

A tube is a cold pipe that passes through a warm house, so the design has to manage temperature. Warm, damp air meeting a cold surface leaves moisture behind, and in a bathroom or kitchen that air is plentiful.

Three details keep the system dry. First, the tube is sealed at both ends so that room air cannot drift up into the roof space. Second, a good unit has a double layer or a thermal break at the ceiling end, so the diffuser is not the coldest surface in the room. Third, the section of tube in the loft is wrapped or surrounded by insulation so that it does not become a cold bridge. In a loft with deep mineral wool, the tube needs to pass through the insulation with its own sleeve of warmth, and the gap around it should be filled. The interplay is covered in sun tunnels, loft insulation and heat loss and in preventing condensation in a sun tunnel.

Daylight is the point of a tunnel, and sound is the part nobody mentions. The dome is a thin shell in the open air, and the tube below it is a hollow channel that connects the roof to your ceiling. Both can carry noise into the room.

Rain on a thin dome can drum, and the tube can ring a little if the sections are loose or unsupported. In a town with aircraft overhead, the ceiling diffuser is a lightweight lid on a column that runs to the open sky, and it is reasonable to ask what reaches the room. Crawley sits beside Gatwick, so that is a question worth asking, and the specification is where the answer lives.

Quiet comes from choices, not luck:

• a dome with a thicker wall or a second skin, which calms rain impact;
• a tube that is fixed firmly so that it cannot vibrate against the joists;
• a sealed diffuser with a proper gasket, so that airborne sound is not finding gaps;
• a flashing bedded so that it does not tick or rattle in wind.

We write these choices down in the quiet spec that comes with every quote. The rain question has a page of its own in rain on the dome: keeping a sun tunnel quiet, and the aircraft question is answered in whether you can hear aircraft through a sun tunnel.

A sun tunnel is best described as a light source, not a window. It brings brightness to a room but shows you nothing outside, and it does not ventilate. That is a strength in some rooms and a limit in others.

It suits spaces that are deep inside the house: a landing with no window, a cloakroom, a utility room, a walk-in wardrobe, a corridor between two bedrooms. It suits a bathroom too, with the extractor fan working on its own line. It is a weaker choice for a room that needs a large area of glass, for instance a kitchen where you want to cook under open sky. There a rooflight of full size is the better tool, and the decision is laid out in sun tunnel or rooflight: choosing for a dark room.

A tunnel is also much smaller to fit. The hole in the roof is a round opening, typically a few hundred millimetres across, and it avoids the joist alterations that a full rooflight needs. The ceiling opening is similar in size. The work is contained, and the room below stays usable for most of it.

Two regulatory points belong here. Fitting a tunnel is a change to the roof covering and the ceiling, so it is worth asking how Building Regulations apply. Part L covers the thermal performance of the new opening and Part F the ventilation of the rooms affected. The position on consent is set out in whether a sun tunnel needs Building Regulations approval. Where the work is notifiable, we handle the Building Control notification and install to current Building Regulations.

Understanding the principle lets you read a quote properly. A sound specification answers each stage of the light's journey in plain words.

1. Dome: the material, the wall thickness, the coating against ultraviolet and whether there is an inner skin.
2. Flashing: the type for your roof covering, pitched or flat, and how it is sealed.
3. Tube: rigid or flexible, the diameter, the length and the number of elbows, along with the lining finish.
4. Insulation and sealing: how the tube is wrapped in the loft and how the ceiling end is sealed.
5. Diffuser: the pattern, the ceiling ring and whether a light is built in.

Each line answers a question about brightness, warmth or sound. If a quote names only a diameter and a price, most of those questions are unanswered. Ours sets them out in writing, along with what we heard at the survey, and the wider picture of what we fit is on the sun tunnel installation hub. When you are ready, get a quote, or message us on WhatsApp.

Is a sun tunnel a skylight?

It is a close cousin. A skylight or rooflight is a glazed panel set into the roof, and the light enters the room directly below it. A sun tunnel, sometimes called a tubular skylight, takes light from a small dome and carries it along a pipe, so it can serve a room that is some distance from the roof surface.

How does a sun tunnel carry light round a corner?

An elbow angles the tube, and the mirrored wall bounces the light onto its new path. Each bend costs some light, which is why a short, straight route gives the brightest room and why the number of elbows is written on the quote.

Will the room be bright enough to read in?

That depends on diameter, length and the day. A short, wide tunnel over a small room can feel like a generous ceiling light in daytime. A long narrow one over a large room will give a softer glow. We talk about the expected result at the survey, and how much light a sun tunnel gives goes into it further.

Does anything need switching on or maintaining?

The tunnel itself has no controls. The dome and the diffuser can be wiped clean when they gather dust or moss, and the method is in how you clean a sun tunnel.