Run length, bends and light loss

Why losses compound

Not additive, multiplicative. At 98 per cent reflectance over twenty bounces you keep about two thirds of the light. At 90 per cent over the same twenty you keep about an eighth.

That is why the reflectance of the tube matters so much, and why a cheap tunnel and a good one are not close even though the fittings look similar.

Length

A longer run means more bounces before the light arrives.

• Under 2m, straight: excellent, close to the rated output
• 2 to 4m: good, noticeable loss, still a substantial improvement
• 4 to 6m: diminishing, worth a frank conversation
• Beyond 6m: we would generally advise looking for another solution

Bends cost more than length

A bend forces light into steeper angles of incidence, which means more bounces per metre, and some light is reflected straight back up the tube.

A single 45 degree bend is modest. Two 90 degree bends cost more than an extra metre of straight tube would.

So a slightly longer straight route generally beats a shorter one with two elbows.

Flexible tube

The corrugations scatter light at every ridge. On a short run the penalty is noticeable; on a four metre run it is the difference between a useful installation and a disappointing one.

Compensating

Stepping up from 250mm to 350mm roughly doubles the collected light, which offsets a long run. It costs more and it is usually the right call where the run cannot be shortened.

Planning the route to be straight is cheaper than compensating for a bent one.

How light travels down a tube

A sun tunnel works by reflection. Light enters the dome, strikes the inside of the tube, reflects, strikes again, and so on until it reaches the diffuser. Each reflection loses a small percentage, and those losses compound.

On a rigid mirrored tube, reflectivity is 98 percent or better per bounce. That sounds like almost nothing until you consider how many bounces occur over several metres, particularly for light entering at a shallow angle.

The practical consequence is that length costs light, and it costs progressively more the further you go.

What a bend does

More damage than a metre of straight tube, and for two reasons.

First, a bend forces additional reflections. Light that would have travelled a straight path with few bounces now strikes the tube wall several more times, each time losing a percentage.

Second, a bend scatters direction. Light arriving at a useful angle leaves the bend at a range of angles, some of which then take many more bounces to reach the diffuser.

The rule of thumb we use is that one bend costs roughly what an extra metre or two of straight tube costs. Two bends is a significant loss. Three is a route worth abandoning in favour of a different position.

Rigid against flexible, in this context

A flexible tube is a concertina of reflective foil. Every fold is a surface that scatters light rather than passing it along, and the material reflects perhaps 80 percent per bounce against 98 percent or better for rigid.

Over a short run the difference is noticeable. Over three metres it compounds into most of the light: a flexible tunnel delivers something like a third of what a rigid one of the same diameter and length gives.

A flexible tube is also harder to keep taut, and a sagging tube loses more again, because sag introduces exactly the shallow angle reflections that cost the most.

Where the route genuinely will not take a rigid tube, a flexible one is better than nothing and we will fit one having told you what to expect. What we will not do is fit a flexible tube on a long run and let you find out in November.

Typical runs and what to expect

A bungalow, roof to ceiling: frequently under a metre. This is where sun tunnels perform at their absolute best, and it is why they suit single storey property so well.

A first floor room in a house, through the loft: one to two metres. Straightforward and effective, and the standard case.

A ground floor room, through the loft and a first floor void: three to five metres. Workable, and this is where diameter has to compensate. On a run like that, 350mm should be the minimum and 530mm is worth considering.

Anything beyond five or six metres: approaching the manufacturer’s maximum, and the honest answer is that the result will be modest. At that point it is worth asking whether a rooflight in a different room would serve the house better.

How to improve a route

Move the ceiling position. The single most effective change. Half a metre one way or the other frequently turns a two bend route into a straight drop, and a straight four metre run beats a three metre run with two bends.

In a hallway or a landing there is usually latitude to do this. In a small bathroom with a fixed layout there may be less.

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

Route around the obstruction differently. Two shallow bends frequently cost less than one sharp one, and some obstructions can be passed on either side.

Check what is actually in the way. Water tanks can sometimes be moved. Cables can be rerouted. A soil pipe cannot.

What we do before quoting

Look at the actual route in the loft rather than assuming one. A photograph of the loft space showing what sits between the roof and the intended ceiling position answers most of it without a visit, and it frequently identifies a better position that halves the run.

That change costs nothing and it does more for the result than a diameter increase would.

The other losses, which are avoidable

A dirty dome. The whole product depends on collecting light, so a dome coated in algae or leaf debris costs output directly in a way a dirty window does not. Clean it when the gutters are done, and twice a year under trees.

An aged dome. Polycarbonate yellows and hazes under ultraviolet over fifteen to twenty years, at which point output drops noticeably. It is a replaceable part rather than a reason to change the installation, and a glass dome does not degrade at all.

A dusty diffuser. Dust collects on the upper face of the ceiling plate over years. Most twist off, and cleaning it is a five minute job that visibly improves the light.

Where the light goes

A sun tunnel works by reflection. Light entering the dome bounces along the inside of the tube until it reaches the diffuser, and every bounce loses a small percentage.

A highly reflective rigid tube might return well over ninety percent at each reflection, which sounds like almost nothing lost. The catch is the number of reflections. Over a straight run a ray may bounce a handful of times. Force it round a bend and it bounces many more.

Small losses compound. That is the whole arithmetic of this subject, and it is why the guidance is always the same: short and straight beats long and clever.

Why a bend costs more than length

In a straight tube a shallow angled ray travels a long way between contacts with the wall. Introduce a bend and the geometry changes: rays that were travelling nearly parallel to the tube are now striking the wall at steeper angles, more often, and some are reflected back up the tube towards the dome.

A single 45 degree bend costs meaningfully more than an additional metre of straight tube. Two bends in a run can halve what arrives at the ceiling.

Which means that a route which is a metre longer but straight will usually outperform a shorter one with a dogleg in it, and that is frequently a surprise to people planning the run.

Rigid against flexible

The single biggest specification decision after diameter.

A rigid tube has a smooth, highly reflective internal surface. Light reflects specularly, keeping its direction, and travels efficiently.

A flexible tube has a corrugated inner wall. Every ridge scatters light in all directions, including backwards, and the effective path length increases because rays no longer travel cleanly. Over the same distance a flexible tunnel delivers substantially less.

Flexible tube has a legitimate use: getting around an obstruction over a short distance where a rigid run genuinely cannot be made to work. It is a poor choice for a long run and a very poor choice for the whole route.

The best compromise, where a bend is unavoidable, is rigid tube with a purpose made angled adaptor rather than a length of flexible tube used as a joint.

Diameter, and why going up a size helps twice

A larger tube collects more at the dome, which is the obvious gain.

Less obviously, it loses proportionally less along the run. In a wider tube a ray travels further between wall contacts, so the number of reflections over a given length falls. The ratio of losses to distance improves.

That is why the advice on any long or bent run is to go up a diameter. It costs comparatively little and it addresses both halves of the problem at once.

What the numbers look like in practice

A short straight run of a metre or two into a hallway directly below the roof delivers a genuinely bright result, and it is the case sun tunnels are famous for.

A run of four or five metres, straight, still works well, particularly at a larger diameter.

The same length with two bends in flexible tube delivers a fraction of it, and it is the installation people describe as disappointing.

Designing the route

Start at the ceiling and look up rather than starting at the roof and looking down. Establish where the light needs to land, then find the straightest route from there to a workable roof position.

Moving the ceiling position by a reasonable amount to achieve a straight run is nearly always worth it, and moving the roof position along the slope frequently costs nothing at all.

The obstructions to plan around are water tanks, which sit precisely where hallways are, structural members, and in a converted loft, the finished room itself.

Two tunnels rather than one

In a long hallway, two smaller tunnels frequently give a better result than one large one, because they spread light along the space rather than concentrating it in a single pool with dark ends.

Each run is shorter and can be straighter, so the losses are lower on both.