What size sun tunnel do I need?
250mm for a hallway or WC, 350mm for a bathroom or a longer run.
Part of our Sun Tunnel Installation guide
Two common domestic sizes, and the step between them is larger than the numbers suggest because light gathered goes with the square of the diameter.
250mm
The standard domestic size. Roughly the light of a modest ceiling fitting on an overcast day, and considerably more in sun.
Suits a hallway, a small landing, a WC, an en suite or a walk-in wardrobe.
350mm
About double the output, because the collector area is about twice as large.
Suits a family bathroom, a larger landing, a hallway with several doors off it, or any room where 250mm would be marginal.
What pushes you up a size
A long run. Over about three metres, step up to compensate for the loss.
Bends. Each elbow costs more than a metre of straight tube.
A north facing collector, marginally.
A larger room, obviously.
Two smaller against one larger
In a longer space such as a corridor, two 250mm tunnels spaced along it beat one 350mm in the middle, because light falls away from a single point and two pools cover more.
In a single small room, one larger tunnel is simpler and cheaper.
The honest comparison
Not against a rooflight, which wins wherever it fits. Against the room as it is: a windowless landing with the light on all day. Against that, even a 250mm is transformative.
Ceiling proportion
A 350mm diffuser in a small WC ceiling can look oversized. Worth considering the ceiling as well as the light.
Diameter decides everything
Light output goes with the area of the tube, not its width. That means a 350mm tunnel passes roughly twice the light of a 250mm one, not forty percent more, and a 530mm passes roughly twice what a 350mm does.
Getting this wrong is the commonest disappointment with sun tunnels: a 250mm tube fitted into a room that needed 350mm produces a faint glow rather than usable daylight, and the difference in cost between the two is small compared with the difference in result.
A working guide
250mm. An internal WC, a small ensuite, a short hallway, a walk in wardrobe. Rooms where you want enough light to see and orient rather than to occupy.
350mm. A landing, a decent family bathroom, a small kitchen, a utility room. The most commonly fitted size and the one that suits most situations.
530mm. A room you actually live in. A larger kitchen, a hallway you want genuinely bright, a stairwell.
What else changes the answer
Run length. Every metre of tube costs some light, and a long run through a loft and a first floor void loses more than a short drop in a bungalow. On a long run, step up a size.
Bends. Every bend costs light. A route that has to work around a purlin or a tank needs more diameter to compensate, or a different route.
Rigid or flexible. A flexible foil tube delivers roughly a third of what a rigid mirrored one of the same diameter gives. If flexible is genuinely the only option, size up substantially and expect less.
Roof orientation. Less than people think. The dome collects diffuse skylight as well as direct sun, so a north facing position works, just with less peak brightness on a sunny day.
Two smaller rather than one large
Worth considering seriously. Two 350mm tunnels spaced apart in a hallway or a kitchen read as ambient daylight. One 530mm in the middle reads as a spotlight, with the corners of the room still dark.
Where two are fitted in the same visit, the second costs meaningfully less than the first, because the access, protection and setting up are shared.
How to check before committing
Get into the loft and look at the route. A straight drop from a clear patch of roof to the ceiling position below is the ideal, and it is worth moving the ceiling position slightly to achieve it rather than accepting a bend to keep it central.
Then size for the room you want, not the tube that fits most easily.
If in doubt, go up a size
The cost difference between 250mm and 350mm is small. The light difference is roughly double. Nobody has ever told us their sun tunnel was too bright.
The one situation where the smaller size is right is a genuinely tiny internal WC, where a larger diffuser would look out of proportion in the ceiling.
Ask us
Send a photograph of the room, a photograph of the loft above it, and a rough measurement of the floor area. That is enough for us to recommend a diameter and a route without a visit.
The diffuser in the ceiling
Worth looking at as well as the tube. The diameter of the ceiling diffuser matches the tube, so a 530mm tunnel puts a substantial disc in the ceiling. In a small internal WC that can look out of proportion, which is the one situation where a smaller size is the right answer despite the lower output.
One last point
Nobody has ever told us their sun tunnel was too bright. The cost difference between one size and the next is small; the light difference is roughly double. If you are hesitating between two diameters, take the larger.
Diameter is the lever worth pulling
Manufacturers offer a small range of diameters, and the difference between them in delivered light is substantial while the difference in price is not.
A larger tube collects more at the dome and loses proportionally less along the run, because the ratio of reflections to travel is lower. So on any tunnel that has to be long, or that has a bend in it, go up a size.
Matching size to the space
A small tunnel suits a short straight run into a modest hallway, a cloakroom or a landing.
A larger one suits a bigger room, a longer run, or any route with a bend, and it is the right choice where the tunnel is the only daylight the space will get.
Two smaller tunnels frequently beat one large one in a long hallway, because they spread the light along the space rather than concentrating it in one pool.
The constraint
Joist spacing at the ceiling, and rafter spacing at the roof. The tube has to pass between structural members without cutting them, and that sets a practical maximum.
Which is why the survey looks at the loft before the size is chosen rather than after.
More on Sun Tunnel Installation
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