Rigid or flexible sun tunnels: which tube carries the light

Daylight never travels down a sun tunnel in a straight beam. It bounces off the wall of the tube, again and again, and every bounce gives up a little of it.

The dome gathers light and sends it down the tube at all sorts of angles. Some of it runs straight to the diffuser. Much of it strikes the tube wall first, and a portion is absorbed at each strike. A smooth, highly reflective wall gives up very little per bounce. A wall with folds, ridges or a dull finish gives up more, and it also scatters the light in directions that send it back towards the dome. Over a short run the difference is modest. Over a long run, or one with several bends, it grows into the difference between a room that glows and a room that is merely less dark.

That is the whole argument for rigid against flexible, and everything else on this page follows from it. Reflectivity per bounce, and the number of bounces, decide the brightness. Our page on how a sun tunnel brings daylight into a dark room covers the dome, the tube and the diffuser as a whole, and how much light a sun tunnel gives puts the output in plain terms.

A rigid tunnel is a set of straight aluminium sections, lined inside with a mirror-finish reflective skin. The sections slide together, and each joint is taped so that no loft air can enter the light path.

The lining is the point. A polished, silvered surface turns light back down the tube with very little loss, so the dome at the top and the diffuser at the bottom can be a long way apart and the output stays useful. The walls are smooth, so light that strikes them at a shallow angle carries on towards the room instead of being thrown back up. Because the sections are stiff, the tube holds a true line. It does not sag between fixings, and it cannot collapse into a bulge that narrows the opening halfway down.

Rigid tubes come in straight lengths and in angled elbows. The elbows are usually adjustable, so a tube can leave the roof at the pitch of the tiles and rise or fall to reach the ceiling. A run with one elbow still behaves well. A run with three starts to lose light at each one, which is where tunnel length, bends and how much light arrives picks the subject up in detail.

Rigid is also the quieter build in use, and that matters here. A stiff tube with taped joints does not flap, rattle or crackle as the loft warms and cools through the day. It carries no ribs that tick when they expand. You hear the dome in heavy rain and little else, which is the subject of our page on keeping a sun tunnel quiet in the rain.

A flexible tunnel is a corrugated tube, like a large ducting hose, with a reflective surface on the inside. It is sold in a compact coil and stretched to length on site.

The ribs are what let it bend. You can curve it round a joist, swing it past a water tank or steer it under a purlin, and it will hold the curve you give it. That freedom is real and useful. The price of it is the same ribbed surface. Each ridge is a tiny step that catches light and throws some of it back up. The reflective skin is also usually thinner and less polished than a rigid lining. For a given length and diameter, a flexible light tube delivers less daylight to the room than a rigid one, and the gap widens the longer the run.

Flexible tubes also behave differently as objects. They need stretching to a good length, because a slack, bunched tube has more ridges per metre and so more loss. They need support, because a long span will sag under its own weight. And if they are pulled too tight, or bent too sharply, the ribs can kink and pinch the opening. Fitted well, a flexible tube is a sound working part. Fitted carelessly, it becomes the dimmest part of the system.

The table below compares the two on the points that decide a job. It describes the general behaviour of each build rather than the output of any one product.

Read down the third column and the pattern is clear. Flexible wins on routing and on speed, and on nothing else. Whenever a straight or gently angled line to the ceiling exists, the rigid tube is the better instrument.

Loss in a tunnel multiplies. A tube that loses a little light per metre does not lose it evenly. It loses a fraction of what is left at every bounce, so the damage compounds along the run.

Picture two tunnels, both 350 millimetres across, both running from a dome to the same landing ceiling. On a short, near-vertical drop through a one-storey roof, each of them delivers a bright, even circle of light, and you would struggle to tell them apart. Stretch the same two tunnels across a loft with a long, shallow angle and the picture changes. The light has to bounce many more times to reach the bottom, and the ribbed tube pays for each bounce a little more than the mirror tube does. The landing under the flexible tube is noticeably dimmer at noon, and far dimmer under a grey February sky.

That last point is the important one for a Sussex house. On an overcast day there is less light to spare. A tube with generous headroom still gives a pleasant room on a dull morning. A tube running close to its limit gives a room that only works in sunshine. Our answer on whether sun tunnels work on cloudy days explains why the dull-day output is the one to design for.

A flexible tube is the right answer when the loft itself refuses a straight line. There are a handful of situations where that happens, and in each the rigid tube would either not fit or would need so many elbows that it lost its advantage.

The common ones are these:

• A cold water tank or a hot water cylinder sits directly between the roof and the ceiling below.
• A trussed-rafter roof has a diagonal brace or a W-shaped web exactly where a straight tube would run.
• A chimney breast or a flue runs close to the route and needs clearance.
• The ceiling position is fixed by the room, but the roof position is fixed by the rafters, and the two do not line up.

In each of these a flexible length can take up the offset that a rigid tube would need two elbows to manage. Used that way, it is a short connecting piece, and most of the run can still be rigid. A good survey looks for exactly that split.

Trussed rafters became common from the mid 1960s, so the later New Town housing in Crawley makes the trussed-rafter case one to plan for, while earlier houses more often have cut roofs. Factory-made trusses are built as a net of slim timbers, each one carrying part of the roof load, and the loft between them is a forest of diagonals. Cutting a truss member to make room for a tube is not a decision to take lightly, because it is a structural alteration that needs an engineer's say-so. The better route is to find a path between the members, and sometimes that path is curved. The same thinking applies in older cut roofs with purlins and collar ties, where the timbers are heavier but fewer.

The best tunnels in awkward lofts are neither pure rigid nor pure flexible. They are a rigid run for the long straight part and a short flexible connector for the offset.

Think of the tube in three zones. At the top is the roof flashing and the first section, which must sit at the right angle to the roof surface. In the middle is the long drop through the loft. At the bottom is the ceiling collar that feeds the diffuser. The middle zone is where the light is lost, so the middle zone should be as smooth and as short as the loft allows. Any flexible piece belongs at the point where the obstruction forces a shift, and it should be stretched taut and kept as short as it can be.

A hybrid run keeps most of the mirror finish in the path and still clears the tank or the brace. It costs a little more thought at survey and a little more care on the day, and it returns a brighter room than a flexible tube all the way down. Where the obstruction is small, it is also possible to shift the roof position a hand's width along the rafter bay and lose the obstruction altogether. That option is worth checking before any bend is added.

You can learn a good deal about your roof in twenty minutes with a torch. Nothing in it needs you to move insulation or step off the joists.

Stand at the loft hatch and look along the line from the ceiling point you have in mind up to the underside of the roof. Ask what is in the way. A tank, a cylinder, a run of pipework, a wiring loom, a chimney, a diagonal timber. Then look at how deep the insulation lies, because the tube passes through it and will need boxing or wrapping where it does. Our page on sun tunnels, loft insulation and heat loss deals with that detail.

Then judge the line. If you could drop a weighted string from the roof to the ceiling point without touching anything, a rigid tube will go in on a near-straight path. If the string has to swerve, note where and by how much. A sideways swerve of a few centimetres can be absorbed by the angle of a rigid elbow. A swerve of half a metre or more, or a swerve in two directions, points towards a flexible piece or a move of the ceiling position.

A few facts make the survey faster if you have them to hand:

1. The roof covering: plain tile, interlocking tile, slate or flat roof membrane.
2. The approximate pitch of the roof.
3. The room below and the ceiling point you would like lit.
4. Whether the loft is boarded, converted or untouched.
5. Whether a loft conversion sits above or beside the route.

The last of those is worth a second look. A conversion changes what is above the ceiling, and the tube has to cross the conversion floor or the new stud wall to get there. Our answer on fitting a sun tunnel if you have a loft conversion sets out what that involves.

The tube you choose and the width you choose cannot be settled separately. A rigid tube of a modest diameter can out-perform a flexible tube one size larger, and a wider tube is harder to route past anything.

Common domestic diameters sit around 250 and 350 millimetres. The wider tube gathers more light at the dome and carries more of it down, and it needs a correspondingly larger hole in the ceiling, a larger flashing and more space between the rafters. On a trussed roof, the spacing of the timbers can make the decision for you before any light calculation begins. Our page on choosing a sun tunnel diameter walks through that trade.

The outside of the roof matters too, because the top section has to meet the covering cleanly. A tiled or slate roof takes a flashing that laps under and over the courses, and the tube top has to line up with the opening in the battens and the felt. That is a separate skill from the tube, and it is set out in our page on fitting a sun tunnel through a tiled or slate roof. On a flat roof the tube meets a short upstand instead, and sun tunnels on flat roofs covers that case. Either way, the tube type does not change the weatherproof detail at the roof surface. That detail belongs to the flashing and the dome.

Moisture is the quiet enemy of any tube that passes through a cold loft. Warm, damp air from the room below can meet a cold tube wall, and where it does, water forms.

Rigid tubes with taped joints keep the light path sealed from the loft. Warm room air still reaches the diffuser end, but the diffuser is sealed to the ceiling, so the damp has little route into the tube. Flexible tubes are made of thinner material and have more surface for droplets to gather on, and the ribs can hold a bead of water that a smooth wall would shed. The remedy is the same in both cases: a well-sealed ceiling collar, a tube wrapped or boxed through cold spaces where needed, and a sealed diffuser. Our page on preventing condensation in a sun tunnel covers the detail, and it matters most in a bathroom or a kitchen.

Dust is a smaller concern. Once the tube is sealed, the inside stays clean for years, because nothing enters it. The dome and the diffuser take the dirt, and both can be wiped from outside and inside.

When a tunnel reaches the end of its life, the tube itself rarely needs renewing. The mirror lining tends to outlast the dome and the flashing, so the usual job is a new dome and a fresh flashing on the same tube, where the old tube is sound. Where an old flexible tube has sagged or discoloured, we renew it with a rigid one if the loft allows.

A good survey works through a short sequence. Each step narrows the choice, and by the end the tube type is usually obvious.

First, the roof position is chosen for light, which means a spot on the roof that catches sun for much of the day and is not shaded by a chimney or a neighbouring gable. Second, the ceiling position is chosen for the room, which means the point where the light is most useful, such as over a basin or at the centre of a landing. Third, the line between the two is checked for obstructions. Fourth, the tube is chosen to suit the line: rigid where it is clear, hybrid where one obstruction forces a shift, and mostly flexible only where the loft leaves no other path.

Noise is checked alongside, because a tunnel is a hole in the roof like any other. The dome is the loudest part in rain, and the tube below it should not add to that. A rigid tube with taped joints and a snug ceiling collar keeps the sound where it belongs. The choices made for each room are written into the quiet spec that comes with every quote, along with the dome and the diffuser build.

If you want that survey done on your own loft, you can ask for a quote and we will look at the roof, the route and the room together. A message on WhatsApp with a photo of the loft hatch view is enough to start.

These are the points people raise once they see both types laid out. Each is answered for a typical Sussex house.

Is a flexible light tube ever the better choice?

Yes, when the loft leaves no clean line. A tank, a brace or a chimney can block a straight run, and a flexible section takes up the offset without cutting any timber. Even then, keep the flexible length short and stretched taut, and use rigid sections for the rest of the drop.

Will I see the difference between the two in my room?

On a short drop, barely. On a long or bent run, yes, especially under a grey sky. The mirror lining returns more of the daylight to the diffuser, so the circle of light in the ceiling looks brighter and more even. A tube that struggles shows a dim centre and a soft, weak edge.

Can I swap a flexible tube for a rigid one later?

It is possible where the route allows, because the roof flashing and the dome can stay in place while the tube is renewed. If the flexible tube was chosen to get round an obstruction, a rigid tube needs a new path or a hybrid run, and that is worth settling at survey.

Does the tube type change the planning position?

The tube type does not change the answer. Roof position, height above the roof and the loft structure decide it. Our page on whether a sun tunnel needs planning permission goes through each in turn, and where the work is notifiable we handle the Building Control notification.