Manufacturers state a maximum for each diameter and tube type, and the practical limit is usually shorter than the published one. Length costs light steadily, because every additional metre means more reflections, so a run that is technically within the maximum can still be disappointing in the room. Shorter and straighter beats longer and clever, every time.
Why length costs light
Because of the reflections.
Light entering the tube at an angle bounces its way down, and each bounce loses a small fraction, so the losses accumulate with distance.
The mechanism is on how much light does a sun tunnel give.
Why rigid tube goes further
Because it loses less per bounce.
A mirror finished rigid tube has a much higher reflectance than a flexible one, so a long rigid run delivers what a much shorter flexible one would.
The comparison is on what is the difference between rigid and flexible sun tunnels.
What a flexible tube manages
A short run, and no more.
The corrugations scatter as well as absorb, so flexible tube is intended for short awkward routes rather than for distance.
How diameter interacts with length
A wider tube goes further.
Light travelling down a wider tube strikes the walls less often for the same distance, so a larger diameter tolerates a longer run.
How to choose is on what size sun tunnel do I need.
What a typical house needs
Less than you would think.
A bungalow ceiling to a pitched roof is a very short run, and a first floor ceiling to a roof above it is usually only a couple of metres.
Where runs get long
A tall loft, or a ground floor room.
A steeply pitched roof with a high ridge, or a route down beside a stairwell to a ground floor ceiling, both produce runs at the limit.
Reaching a ground floor
Possible, and it is an undertaking.
A tube passing from the roof through a first floor space to a ground floor ceiling has to go through boxing or a cupboard, and it is a long run losing light all the way.
Where the tube passes through a room
Boxed in, and it is visible.
A run through a first floor room means a boxed column in the corner of that room, which is a decision about that room as well as about the one below.
Whether it is worth it
Frequently not.
A long run to a ground floor delivers modest light and costs a boxed duct through a bedroom, and a rooflight elsewhere or better electric lighting is often the better answer.
Bends and effective length
Each bend is worth extra metres. A bend costs light like a length of tube, so a run with two elbows behaves as though it were considerably longer than it measures.
That is on can a sun tunnel go round a corner.
How to keep a run short
Move the position. Shifting the diffuser a little within the ceiling to line up with a clear route to the nearest roof slope is usually possible and always worth doing.
Choosing the roof slope
The nearest one, not the sunniest. A shorter run on a north slope generally beats a longer one on a south slope, because the length costs more than the orientation does.
Why is on do sun tunnels work on a north facing roof.
What the survey measures
The actual route, not the vertical drop.
The run is measured along the path the tube will take, including any offset needed to clear obstructions, which is longer than the height between roof and ceiling.
What is in the way
The thing that lengthens runs. Purlins, water tanks, pipework and cables all force a route around them, and each detour adds length and often a bend.
What to look for is on can a sun tunnel go through a loft.
A flat roof run
Usually the shortest of all.
A flat roof over the room means a run of a few hundred millimetres, which is why flat roof tunnels perform so well.
That is on can a sun tunnel be fitted to a flat roof.
What the manufacturer publishes
A maximum per diameter and tube type.
Those figures are the limit at which the product still performs acceptably, and they are worth reading rather than assuming a tunnel will work at any length.
Where we would stop
Well inside the published maximum. A run near the limit delivers a disappointing result on a dull day, and somebody who was promised daylight notices.
What to do instead of a long run
Two shorter ones, or a different product.
Two tunnels serving two rooms from short runs is usually better than one long one, and where neither works a rooflight elsewhere may be the answer.
Extending an existing tunnel
Occasionally asked about, rarely useful. Adding length to an existing run because a ceiling has been altered means more losses on a system that was specified for the original distance.
It is usually better to reposition than to extend.
What a longer run looks like in the room
Dimmer, and duller in colour.
A long run does not simply reduce the quantity of light: repeated reflections shift its character slightly, so the light is flatter as well as weaker.
How we measure it on a sun tunnel
We measure the actual route and say when a run is too long to be worth doing, because a tunnel that disappoints on a grey afternoon is a poor result whatever the specification said.
The whole picture is on the sun tunnel installation page.
Thinking about this job? We survey across Worthing, West Sussex and the South Downs and give you a fixed price before any work starts, with a 10-year workmanship guarantee.