Diameter is the single decision with the largest effect on how much daylight a sun tunnel delivers, and it is the one most often economised on. This page is how it is chosen and why the answer is usually the larger one.
The two inputs
Room area and run length, and they pull in the same direction.
A larger room needs more light to feel lit, so it wants a larger diameter. A longer run loses more light along the way, so it also wants a larger diameter to start with more.
Which means a large room on a long run needs the largest tube available, and a small room on a short run is the case where a modest one genuinely suffices.
Why diameter does so much
Two effects working together, and the second is the one people miss.
A wider tube has more cross sectional area at the dome, so it collects more light in proportion to that area.
And a wider tube gives light a shallower average angle to the wall, so it travels further between bounces and bounces fewer times over the same run. Fewer bounces means less loss.
So going up a diameter increases both the light collected and the proportion of it that survives the journey. That is why the improvement is larger than the increase in area alone suggests.
The rough guide
| Situation | Diameter | Comment |
|---|---|---|
| Small internal room, short straight run | The smaller sizes | Cloakroom, small bathroom, landing |
| Bathroom or hallway, moderate run | Mid range | The common domestic case |
| Larger room, or a longer run | The larger sizes | Kitchen corner, long hall, deep roof space |
| Large room, long run, bends | Largest available, or two tunnels | Check it is within the system maximum |
Manufacturers publish figures for their own systems and those are what a specification should work from. The table is the shape of the decision rather than a substitute for them.
Going up a size
The recommendation this page exists to make.
The price difference between one diameter and the next is modest. The difference in delivered light is not, and it lasts for the life of the installation.
Which means on any marginal case the larger tube is the better buy, and the money is far better spent there than on almost anything else on the order. A tunnel that disappoints is almost always one that was specified at the smaller size.
What limits it
Four things, and they are all physical rather than commercial.
- Truss or rafter centres. The tube has to pass between them without cutting anything.
- Roof space depth. Particularly at the eaves or in a shallow pitched roof.
- The roof position. A larger dome needs more clear roof and a larger opening in the covering.
- The ceiling. A larger diffuser needs a larger opening between joists.
Truss centres, and the usual ceiling on size
The constraint that decides it most often on modern housing.
Truss centres are commonly around 600mm, and the clear space between is that less the truss timber. The largest tube that fits comfortably in that gap, with room to work, is what the roof will take.
Which is why sun tunnel ranges top out where they do, and why a trussed roof has a natural maximum. Going beyond it means a designed structural alteration, which for a daylight tube is rarely proportionate.
Ceiling joists at the other end
The same problem at the room end and it is easier to solve.
The diffuser needs an opening in the ceiling, and ceiling joists run at their own centres. A diameter fitting between joists needs no structural work; one that does not needs a joist trimmed, which is ordinary carpentry on a ceiling but is work.
Which occasionally makes the practical maximum a joist spacing rather than a truss spacing, and it is measured rather than assumed.
Room area, and what lit actually means
The input people find hardest to judge, and it depends on the room’s job.
A circulation space needs enough to feel lit and to read as connected to outside. A room where somebody works needs considerably more, and a room where somebody reads more again.
So the same floor area in a hallway and in a study wants different answers, and the useful question at survey is not how big the room is but what happens in it.
Where a tunnel is the wrong size of solution
Worth being honest about, because it is the boundary of the product.
A large room, used for detailed work, with no other daylight, is not a sun tunnel job however large the tube. The product delivers a useful amount of daylight into a modest space, and beyond a certain room size it produces a pleasant glow rather than a lit room.
Where a room is genuinely large and daylight is genuinely required, the answer is a rooflight, or several tunnels, or an acceptance that the room needs artificial light too.
Two tunnels against one larger one
The alternative worth considering once the largest single tube is reached.
Two tunnels distribute light across a room rather than concentrating it under one point, which in a long or awkwardly shaped space is a considerably better result than one larger tube.
They also give two shorter runs if the roof positions are well chosen, and two chances to find a straight route.
Against that, two roof penetrations, two ceiling openings and two lots of work. In a long hallway it is frequently right; in a small bathroom it is not.
The run length interaction
Worth spelling out because it is where a specification goes wrong quietly.
A diameter that is generous on a short run can be inadequate on a long one, and the run is frequently longer than anybody estimated because the offset and the bends were not counted.
Which means the diameter is chosen after the route is established, not before. A product selected from a room size alone, without a route, is a guess.
Bends count against the diameter
The same logic applied to the other loss.
A run with bends in it delivers less than a straight run of the same length, so a bent route needs a larger diameter to arrive at the same result.
Which occasionally reverses a decision: a straight route at a smaller diameter can outperform an offset route at a larger one, and where both are available that comparison is worth doing rather than assuming bigger wins.
What the manufacturer’s figures give you
The proper basis for the decision, and it is available.
Manufacturers publish performance data for each diameter over a range of run lengths, frequently with an allowance for bends. That is the figure to work from: this diameter, over this run, with this many bends, in this system.
A supplier working from that data can tell you what the installation will deliver. One quoting a diameter against a room size has skipped the part that matters.
The dome size follows
Worth knowing because it affects the roof.
A larger tube means a larger dome and a larger opening in the covering. On a plain tiled roof that is unremarkable. On a hand made tile or a stone slab roof, where every unit that comes off has to go back, a larger opening means more disturbance.
Which is one of the few situations where the covering argues for a smaller diameter, and it is worth weighing rather than defaulting either way.
The diffuser size follows too
At the room end, and it is a visual decision as well as a technical one.
A larger diffuser is a larger circle in the ceiling. In a small bathroom or a narrow hallway that is a noticeable object, and some people prefer a smaller one visually even at a cost in light.
That is a legitimate preference and it is worth making knowingly rather than by accident. Seeing a diffuser of each size, or marking the circles on the ceiling, settles it in a minute.
Cost, in shape
We publish no prices and the shape is worth describing because it is favourable.
The step from one diameter to the next is a modest proportion of the total job, because the work is largely the same: the same survey, the same roof opening, the same ceiling opening, the same labour.
Which means the marginal cost of more light is low, and it is the clearest example on this site of a specification decision where the larger option is straightforwardly better value.
What we would specify by default
A generous diameter, a rigid tube, and the straightest route the roof space allows.
Where the truss centres or the depth limit the diameter, we say so and specify the largest that fits properly rather than forcing one that does not. Where a room is too large for a single tunnel, we propose two rather than overselling one.
Checking it afterwards
Worth knowing that it can be verified.
A light meter reading on the floor at midday on a bright day, compared with the manufacturer’s figure for that diameter and run, establishes whether an installation is performing as specified.
Where it is not, the cause is generally the run rather than the tube: a flexible tube left slack, a bend nobody counted, or joints admitting dust. All three are findable.
Where the largest is not available
The honest position where the roof will not take what the room wants.
Where truss centres, joist centres or roof space depth cap the diameter below what the room and the run call for, the options are two smaller tunnels, accepting a lower light level, or a different product entirely.
What is not an option is forcing an oversized tube through a space that does not take it, which means cutting something that should not be cut or compressing insulation permanently. Both are refusals rather than compromises.
Existing tunnels that disappoint
A common enquiry, and diameter is frequently the answer.
Where somebody has a sun tunnel that does not deliver what they hoped, the causes in order of frequency are: too small a diameter for the run, a flexible tube left slack, bends nobody counted, and dust in an unsealed tube.
Three of those four are fixable only by replacing the run. Which means an underperforming tunnel is generally a re-installation rather than an adjustment, and it is worth getting the specification right the first time.
The one genuinely cheap improvement is the room’s decoration, which returns more of what does arrive.
What we confirm first on a sun tunnel
We establish the route before choosing a diameter, because the run length and the bend count are half the input. We work from the manufacturer’s performance figures for the actual installation rather than a general claim, and we recommend the larger size on any marginal case because the cost difference is small and the light difference is not.
Diameter is the lever with the largest effect and the smallest price. On a sun tunnel it is where the money should go.
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.