What is the difference between rigid and flexible sun tunnels?

Reflectance, and it is the single most consequential choice in specifying a sun tunnel. A rigid tube is a smooth mirror finished cylinder that loses very little at each reflection. A flexible tube is a corrugated concertina that loses considerably more and scatters light as well. On any run of length, rigid delivers a great deal more into the room.

What a rigid tube is

A straight mirror finished cylinder.

Sections of highly reflective tube are joined to make a straight run, with formed elbows where a change of direction is unavoidable.

What a flexible tube is

A reflective concertina. A corrugated tube with a reflective inner surface that bends by hand to follow an awkward route without any fittings.

Why reflectance matters so much

Because light bounces many times. A ray entering at an angle reflects repeatedly on its way down, so a small extra loss per bounce becomes a large loss over a run.

The mechanism is on how much light does a sun tunnel give.

What the corrugations do

Scatter as well as absorb. Each fold presents surfaces at varying angles, so light is redirected in unhelpful directions as well as being absorbed, and both cost output.

How much difference it makes

Substantial on any run beyond the shortest.

A rigid tube can carry usable light several times further than a flexible one of the same diameter, which is why manufacturers publish very different maximum lengths.

The limits are on how long can a sun tunnel run be.

Where flexible is the right answer

A short awkward route. Where the run is very short and something has to be dodged, a flexible tube gets round it without a fitting and the losses are tolerable.

Where it is not

Anything longer than a metre or two.

A long flexible run delivers a disappointing result on a grey day, and the difference in price against rigid is small compared with the difference in output.

What flexible costs

Less, and it is a false economy on most jobs.

The saving on the tube is modest against the access and the labour, so choosing flexible to save money rarely makes sense.

Where the money goes is on how much does a sun tunnel cost.

Installation speed

Where flexible genuinely wins. A flexible tube is quick to fit and needs no cutting or fitting of elbows, which on a difficult loft with poor access is a real advantage.

Rigid tube and bends

Handled with formed elbows.

Where a rigid run has to change direction, an adjustable elbow does it, and each one costs light in the same way a length of tube does.

That is on can a sun tunnel go round a corner.

Whether a rigid run has to be vertical

No, and it should be straight.

A rigid tube can run at an angle from a pitched roof to a ceiling position, and what matters is that it is straight rather than that it is vertical.

Cutting rigid tube to length

Part of the installation.

Sections are cut on site so the run finishes exactly at the ceiling ring, which is why a rigid installation looks deliberate inside the loft.

Sealing the joints

Done on both types.

Sections are joined and taped so the tube is closed along its length, keeping loft air and dust out of the light path.

Dust inside the tube

A long term appearance issue.

Dust settling on the inside of a diffuser is why a tunnel can dim over years, and a properly sealed tube keeps it out.

Rigidity in the loft

A practical difference.

A rigid tube is a self supporting element that stays where it was put. A flexible one can sag, be knocked out of line, or be crushed by somebody storing boxes against it.

What sag does

Adds effective length and bends.

A drooping flexible tube behaves as though it were longer and as though it had bends in it, so the losses are worse than the measured run suggests.

Insulating each type

Easier on rigid.

Purpose made insulated sections and sleeves are made for rigid tube, where a corrugated one is harder to wrap neatly and to keep wrapped.

Why it matters is on do sun tunnels cause condensation.

Mixing the two

Sometimes the practical answer.

A mostly rigid run with a short flexible section to negotiate one obstruction gives most of the performance with the flexibility where it is needed.

What a quotation should say

Which type, and how many bends.

Two quotations at the same diameter can differ enormously in output, and those two facts are what separate them.

Which we specify

Rigid, unless there is a specific reason.

The performance difference is large, the cost difference is small, and the room is judged on a grey afternoon rather than on a bright one.

What the inner surface is

A specialist reflective film or coating. Rigid tube uses a highly engineered reflective layer rather than simple polished metal, and the reflectance figures manufacturers publish are for that surface specifically.

Whether the surface degrades

Not within the life of the installation.

The reflective layer is inside a closed tube, protected from weather and from ultraviolet, so a sealed run performs the same in twenty years as on the day.

What does change is dust settling on the diffuser.

Diameter matching between types

Not interchangeable in output.

A flexible tube at one diameter does not match a rigid one at the same diameter, so comparing two quotations on size alone tells you very little.

How we quote it on a sun tunnel

We quote rigid tube by default and say explicitly where a short flexible section is being used and why, because the tube type is the specification decision that decides the result.

The whole picture is on the sun tunnel installation page.