How far can a sun tunnel run?

The practical bands

Under two metres, straight: excellent. Very little loss and the tunnel performs close to its rating.

Two to four metres: good. Noticeable loss, still a substantial improvement on a dark room.

Four to six metres, or shorter with several bends: diminishing. Worth a frank conversation about whether the result justifies the work.

Beyond six metres: we would generally advise against it and look for another solution.

Bends cost more than length

Every elbow scatters light and sends some back up the tube. A single 45 degree bend is modest; two 90 degree bends in a short run cost more than an extra metre of straight tube would.

A slightly longer straight route usually beats a shorter one with two elbows.

Rigid matters more on long runs

Because losses compound. On a short run the difference between rigid and flexible is noticeable; on a four metre run it is the difference between a useful installation and a disappointing one.

Diameter helps

On a longer run, stepping up from 250mm to 350mm compensates for some of the loss. It costs more and it is usually the right call where the run is unavoidably long.

Two storey runs

Bringing daylight from a roof down to a ground floor hallway means passing through a first floor, usually inside a boxed duct or a cupboard. It can be done and the output is much reduced.

Where somebody is set on it, we would specify the largest diameter, rigid tube, and the straightest possible route, and be honest about what to expect.

Six metres is a common maximum, and less is much better

Manufacturers of rigid systems typically quote a maximum run of around six metres, and some go further with additional extension sections. That is the technical limit.

The practical limit is shorter, because light loss compounds along the tube and the result at the diffuser is what matters rather than what the catalogue permits.

What actually costs light

Length. A rigid mirrored tube reflects 98 percent or better at each bounce, which sounds like almost nothing until you consider how many bounces there are over several metres. A two metre run delivers noticeably more than a five metre run of the same diameter.

Bends. Each one costs more than a metre of straight tube. Two bends in a run is a significant loss, and a route needing three is a route worth reconsidering.

Tube type. This dominates everything. A flexible foil concertina reflects perhaps 80 percent per bounce and scatters light at every fold, delivering roughly a third of what a rigid tube of the same diameter and length gives. On a long run a flexible tube delivers very little.

Typical situations

A bungalow, roof straight down into the ceiling below: often under a metre. This is where sun tunnels perform at their 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.

A ground floor room, through the loft and a first floor void: three to five metres, and this is where diameter matters. On a run like that, step up a size.

Compensating for a long run

Increase the diameter. Output goes with area rather than width, so a 350mm tube passes roughly twice the light of a 250mm one, and 530mm doubles it again.

On any run over about three metres, 350mm should be the starting point and 530mm is worth considering.

Keep it as straight as possible, even if that means moving the ceiling position slightly. A straight four metre run beats a three metre run with two bends.

Boxing in

Where a tube passes through a first floor room rather than a void, it has to be boxed in, which is a joinery job and a decorating job on top of the tunnel. That is worth pricing before committing, because at that point a rooflight in a different room may be better value.

What we do

Look at the actual route in the loft before recommending anything. A photograph of the loft space showing what sits between the roof and the ceiling position answers most of it without a visit, and it frequently identifies a better position a metre away that halves the run.

Length is not the only limit, and not the main one

Manufacturers publish a maximum run for each product, and on a rigid tunnel it is generous, comfortably enough for a two storey house.

But the published maximum is a structural and practical limit, not a performance one. Long before you reach it, the light has fallen away to the point where the tunnel is barely worth having.

What actually reduces the light

Length. Every metre of tube costs light, because each reflection loses a little.

Bends, far more than length. A single bend costs more than several metres of straight run, because it forces many more reflections. Two bends in a run can halve what arrives at the diffuser.

Tube type. A rigid tube with a highly reflective internal finish delivers substantially more light than a flexible one over the same distance, because the flexible tube has a corrugated inner surface that scatters light at every ridge.

Flexible tunnels are useful for getting around an obstruction in a short run. They are a poor choice for a long one.

The practical guidance

Straight and short beats long and clever, every time. It is worth moving the ceiling position by a reasonable amount to achieve a straight run.

A hallway or landing directly under the roof is the ideal case, and it is why sun tunnels perform so well there and so variably elsewhere.

Diameter

A larger diameter delivers more light and also tolerates a longer run better, because the ratio of reflections to travel is lower.

On any run that has to be long, go up a size. It is a small cost difference and it is the difference between a useful result and a disappointing one.