Rigid against flexible tunnel runs

Rigid tube

Polished aluminium pipe with a highly specular internal surface, typically above 95 per cent reflectance and on the best systems above 98.

Light entering the collector bounces down it losing very little at each reflection. Because the surface is smooth, the reflections are specular: light keeps its direction rather than scattering.

Rigid needs a reasonably direct route, with elbows where the run has to shift. It gives by far the best output.

Flexible tube

A concertina of reflective material that bends around obstructions a rigid tube cannot pass.

The problem is the corrugation. Every ridge and valley scatters light in a different direction, so instead of a small number of clean reflections you get a great many diffuse ones, and the loss compounds with length.

A two metre flexible run can deliver noticeably less than half the light of a two metre rigid one.

Why reflectance matters so much

Because it compounds. Light bounces many times on the way down, and a small difference per bounce becomes a large difference over the run.

At 98 per cent reflectance over twenty bounces you retain about two thirds of the light. At 90 per cent over the same twenty bounces you retain about an eighth.

That is why a cheap tunnel and a good one are not close, even though the fittings look similar.

What we specify

Rigid by default, with elbows where the route requires them, and flexible only for short sections where there is genuinely no straight path.

If a supplier proposes a long flexible run, ask what output you are actually getting rather than what diameter you are buying.

The decision that decides whether a tunnel works

Of the three specification choices on a sun tunnel, diameter, route and tube type, this is the one where the cheaper option is most often the wrong one, and where the difference is largest.

A rigid mirrored tube delivers roughly three times the light of a flexible foil one of the same diameter over a typical run. That is not a marginal difference. It is the difference between a room that reads as daylit and one where somebody wonders whether the tunnel is working.

How each one is made

Rigid. Sections of tube, usually aluminium, with a specular reflective coating on the inner surface applied under factory conditions. The sections push or twist together and are taped at each joint. The surface is smooth, continuous and highly polished.

Flexible. A concertina of reflective foil over a wire helix, which can be bent and compressed to follow an irregular route. It arrives compressed and is extended to length on site.

Why the difference is so large

Light travels down a tunnel by reflection, and each reflection loses a percentage. Those losses compound, so a small difference per bounce becomes a large difference over a run.

Reflectivity. A rigid mirrored tube reflects 98 percent or better at each bounce. A flexible foil reflects perhaps 80 percent.

Over ten bounces, which is entirely ordinary for light entering at an angle, 98 percent per bounce leaves you with around 82 percent of the light. Eighty percent per bounce leaves you with around 11 percent.

That is the whole argument, and it is why the difference between the two grows with length rather than staying constant.

Scattering. The second effect, and it compounds the first. Every fold in a concertina is a surface angled away from the tube axis. Light striking it does not continue down the tube: it is deflected, frequently back toward the dome or into the tube wall at a shallow angle where it will take many more bounces to make progress.

A rigid tube passes light along; a flexible one scatters it and then hopes.

Sag. The third effect and the most avoidable. A flexible tube is hard to keep taut over any distance. Sag introduces exactly the shallow angle reflections that cost the most, and a tunnel that was adequate on the day of installation loses more as the tube settles.

What a flexible tube is genuinely for

Routes a rigid tube cannot take.

Where the roof position and the ceiling position cannot be lined up, where a structural member or a tank sits directly in the path and cannot be worked around, or where the tunnel has to negotiate a converted loft with no clear void, a flexible tube will go where a rigid one will not.

In those situations it is better than nothing, and nothing is the alternative. We will fit one having told you what to expect.

What we will not do is fit a flexible tube on a long run because it is quicker, and let you find out in November.

Compensating for a flexible run

If flexible is genuinely the only option, two things help.

Step up substantially in diameter. Output goes with area, so going from 350mm to 530mm roughly doubles the light entering. That does not recover a threefold loss, and it recovers a useful part of it.

Keep it as short and as taut as possible. Every metre matters more on a flexible run than a rigid one. Support it properly so it cannot sag, and resist the temptation to leave slack for convenience.

And be realistic about the result. On a run of three metres or more, a flexible tunnel delivers very little, and at that point it is worth asking whether a rooflight in a different room would serve the house better.

Improving the route instead

Almost always the better answer, and it costs nothing.

Move the ceiling position. Half a metre one way or the other frequently turns a route that needs a flexible tube into one that takes a rigid one. In a hallway, a landing or a bathroom the diffuser rarely needs to be dead centre.

Move the roof position. The other end of the same problem. A patch of roof a metre along may clear the purlin or the tank that was forcing the issue.

Use two shallow bends rather than one sharp one. Rigid systems include angled adaptors, typically 30 or 45 degrees, and two gentle changes of direction cost less light than one tight one.

Check whether the obstruction can move. A cold water tank can frequently be relocated. Cables can be rerouted. A soil pipe cannot.

A photograph of the loft space showing what sits between the roof and the intended ceiling position answers most of this without a visit, and it frequently identifies a better position that turns a compromised installation into a straightforward one.

The other differences

Longevity. A rigid mirrored tube has nothing to wear out. It is a sealed length of coated metal with no moving parts, and there is no reason a properly installed one should not still be working in thirty years with only the roof dome having been changed.

Foil is thinner, it can crease, and a crease is permanent damage to the reflective surface.

Condensation. Both need insulating where they cross unheated loft space, and both need an airtight seal at the ceiling ring. A flexible tube with its concertina profile is slightly harder to insulate neatly and slightly easier to get wrong.

Installation time. Flexible is quicker on site, which is part of why it is offered. That saving is a few hours against a permanent reduction in what the product delivers.

Our default specification

Rigid wherever the route allows it, which on a bungalow is essentially always and on a two storey house is usually.

Where the route genuinely will not take rigid, flexible with a larger diameter, properly supported, and an honest description of what to expect before you commit rather than afterwards.

And where neither will give a worthwhile result, we will say so. A sun tunnel is a good product within its limits and a disappointing one outside them, and it is better to hear that at the enquiry than after the ceiling has been cut.

The difference in one sentence

A rigid tube has a smooth, highly reflective inner surface that keeps light travelling in its original direction. A flexible tube has a corrugated inner wall that scatters light at every ridge, including backwards.

Over the same distance, the rigid tube delivers substantially more.

Why the corrugations cost so much

In a smooth tube a shallow angled ray travels a long distance between contacts with the wall, and each contact returns most of the energy in a predictable direction. The number of reflections over a given length is low.

In a corrugated tube, every ridge presents a surface at a different angle. A ray that was travelling cleanly along the tube is deflected, then deflected again, and some of it is turned back towards the dome. The effective path length increases and the number of interactions multiplies.

Small losses compound. That is why a flexible run of a few metres can deliver a fraction of what a rigid run of the same length does.

Where flexible tube is legitimate

Short lengths, used to get around a single obstruction that a rigid run genuinely cannot clear.

A metre of flexible tube linking two rigid sections around a purlin is a reasonable compromise. Using flexible tube for the whole route because it is easier to install is not.

The better answer for a bend

Rigid tube with a purpose made angled adaptor. Manufacturers supply elbow sections at set angles, and they are designed to turn the light with far less loss than a flexible section.

Two rigid runs joined by a proper elbow will comfortably outperform the same route in flexible tube.

Installation differences

Rigid tube comes in sections that have to be joined and taped, and the run has to be planned because the sections cannot be persuaded around anything.

Flexible tube can be pushed through and bent by hand, which is why it appears on jobs where nobody surveyed the loft first.

Both need insulating through unheated space, and rigid joints need taping so moist loft air cannot enter the tube.

The decision rule

Specify rigid, plan the route to be as straight as possible, and use manufacturer elbows where a turn is unavoidable.

Where a quotation offers flexible tube for a long run, ask what the delivered light will be, and consider whether moving the ceiling position by a reasonable amount would allow a straight rigid run instead. It almost always would.

Supporting the tube

A rigid run needs bracketing to the structure at intervals so it cannot sag or be knocked out of alignment, particularly where somebody may be moving about the loft.

A flexible run needs supporting more frequently still, because it will droop between fixings and every droop is an additional bend costing light.

A flexible tunnel that has been pushed into place and left unsupported will sag over years and the light output falls accordingly. That is a common cause of a tunnel that was acceptable at first and disappoints later.

Working around stored belongings

Lofts get used for storage, and a tunnel crossing a loft is something people will put boxes against.

Route it where that is unlikely, and where it must cross a storage area, box it or route it high. A crushed flexible tube is a real occurrence.

Joints and airtightness

Rigid sections need taping at every joint. It is not about rain; it stops moist loft air entering the tube, which condenses on the cold inner surface just as readily as air from the room.

Flexible tube has fewer joints, which is its one genuine advantage, and it loses far more light than the joints ever cost.