Guide
Rigid or flexible sun tunnels: what the tube changes
The tube between the dome and your ceiling does most of the work. Rigid and flexible tubes carry light in different ways, so the better one is whichever suits the loft above your room.
Two kinds of tube, built in different ways
A rigid tube is a hard metal pipe with a mirrored lining. A flexible tube is a ribbed, collapsible duct that stretches to length and bends round obstacles. The first is optical, the second is practical.
When you hear a salesperson praise one over the other, ask what the roof void looks like. The right tube is a response to the space it passes through.
The rigid tube
Rigid tubes are made of aluminium or similar sections with a highly reflective inner surface. They come in straight lengths and sometimes in angled elbows that fit together. Because the wall is smooth and unbroken, light bounces along it with little loss. They keep their shape, so what you fit is what you get.
The flexible tube
A flexible tube is a ribbed duct, often lined with a reflective foil or film. It arrives compressed and is pulled out to the length you need, then fixed in place. It can bend gently round a purlin, a pipe or a chimney breast, which a rigid pipe cannot do without elbows. The ribs scatter light a little more than a smooth wall does.
Why it matters
The tube decides how much light reaches the ceiling and how easily it can be fitted. Our sun tunnels page introduces the full system, and this guide concentrates on the middle part.
Which tube loses less light on the way down?
A rigid tube, run straight, loses the least. A flexible tube loses more, and loses more again with every bend or sag. The shorter and straighter either one is, the brighter the ceiling.
Light inside a tube hits the wall again and again, and each bounce takes a little away. The fewer the bounces and the better the surface, the more arrives at the diffuser.
Why smooth walls win
A polished rigid wall reflects strongly. On a straight drop, many rays go through with only a few bounces. A ribbed flexible wall reflects less cleanly, so more is absorbed or sent back up. For a long run, that difference becomes visible as a dimmer ceiling panel.
What bends do
Every change of direction sends light into the wall at a steeper angle, which costs brightness. A rigid tube with a proper elbow does this better than a flexible tube pulled round a corner, but the physics is the same. Pulling a flexible tube taut and keeping its bends wide and few keeps its performance close to the rigid kind. A saggy, tightly kinked tube is the worst case.
Putting it together
For a short, direct run, either tube works and the gap is hard to notice in the room. For a longer run, rigid sections make a measurable difference. We go through lengths and bends in our answer on how far a sun tunnel can run.
| Point | Rigid tube | Flexible tube |
|---|---|---|
| Light carried | Strongest, especially over a longer run | Good over a short run, weaker over a long one |
| Bends | Angled elbows, each with a small loss | Gentle curves, loses more if kinked |
| Loft space needed | A clear straight path | Can snake round beams and pipes |
| Fitting | Sections joined and sealed | Pulled to length and fixed |
| Suits | Direct drops, longer runs | Offsets, crowded roof voids |
What if the roof void is full of obstacles?
Then a flexible tube, or a rigid one with a small offset, is the answer. Water tanks, ductwork, wiring and framing in the loft often decide the tube type before light does.
A roof void is not an empty box. Between the roof covering and the ceiling lie rafters, purlins, ceiling joists, insulation, cables and, in some houses, tanks or ventilation ducting. The tunnel has to find a way through that lot.
Mapping the route
At the survey we look at where the dome could sit, where the diffuser should land and what stands in between. A straight rigid drop is the ideal, so we try that first. If a structural member sits in the line, we can often shift the dome sideways along the slope until the route clears. If shifting is not enough, we weigh an offset against a flexible tube.
Where flexible helps
A flexible tube suits a route that needs a gentle jog around a purlin or the edge of a storage platform. It is also forgiving where the ceiling and roof are not quite in line, such as in an older terrace with uneven timbers. The trade is some light, so we keep it as short as the route allows.
Terraces and semis
A Victorian or Edwardian terrace may have deep roof timbers and a floored loft, while an inter-war semi may have lighter framing. Newer estate houses can have engineered roof trusses, where cutting members is not an option and the route has to be found between them. Each leads to a different call, so the survey has to come before any choice.
Questions that settle the tube in a loft
- Can a straight line be drawn from dome to ceiling without a beam in it?
- Is there a water tank, duct or cable run along the route?
- Are the roof members trusses that must not be cut?
- How far apart are the roof and the ceiling?
- Is the loft floored, so that storage sits under the route?
Does the tube change warmth or condensation?
Yes. A tube passes through cold air in the roof void, so its insulation and seals matter. A well-sealed tube with a break at the ceiling keeps warmth in and damp out, whichever type you choose.
A tunnel is a short pipe joining two very different spaces: the outdoors above and your heated room below. Anything that bridges them can leak warmth or let moist air meet cold metal.
The cold surface problem
If warm, damp air from a bathroom reaches the cold wall of a tube, it can condense on it. A sealed ceiling diffuser stops room air getting up the pipe, and a sealed dome base stops outside air coming down. The tube does its part by being airtight at every joint.
Rigid sections
Rigid tubes have joints between each length, and each joint must be taped or sealed. The metal itself conducts cold, so in a cold loft we sleeve or wrap the tube to cut that effect. Fewer, longer sections mean fewer joints to get wrong.
Flexible ducts
A flexible tube has fewer joints, since it runs in one piece from top to bottom. It is often a thinner material, so it can run colder on its surface, and extra wrapping may be wanted. The bend radius also matters, because a kinked section can trap damp air.
The diffuser
At the ceiling, many diffusers have a double layer that gives a thermal break between the tube and the room. That layer matters more than the tube type. We choose it with the room in mind, with a bathroom getting the most care. Building Control looks at the opening as part of the notification we handle, and the thermal detail is part of what is built.
Do the rules differ for a rigid or flexible tunnel?
No. The rules look at the dome on the roof, not the tube inside it. A house usually relies on permitted development, a flat needs planning permission, and the new opening is notified to Building Control.
Planners see the part you can see from the street. The tube sits inside the roof and has no effect on the planning route, so the choice between rigid and flexible is made on practical grounds alone.
What counts is the dome
On a house, a rooflight is permitted development under Class C if it projects no more than 150mm beyond the plane of the roof slope and sits no higher than the highest part of the roof. Whether the tube below it is rigid or flexible, the dome is measured against that limit. A side-elevation slope brings an extra rule: the whole window must be obscure-glazed, and non-opening unless the parts that open are more than 1.7 metres above the floor of the room.
Flats and ownership
A Tyneside flat, upstairs or down, has no householder permitted development, so a new dome needs planning permission from the council. The tube may run through space that someone else owns, which is a matter for your title or lease. Read your papers and talk to the other owner first, before you commit to an order. Our planning answer for sun tunnels sets out each case.
Building Control
A new opening in the roof is building work, notified to Building Control. The Building Control notification, where one is due, comes from us, and it carries the thermal detail of the tube.
How do I decide which tube to have?
Choose rigid where the route is short and straight, and flexible where something is in the way. Where both would work, rigid keeps more light. The survey settles it.
The decision comes down to the loft, the length and how the light will be used. You do not need to be an engineer to follow the logic.
Start with the route
If you can draw a straight line from where the dome would sit to where the diffuser will hang, rigid is the natural choice. If a purlin, a duct or a trimmer is in the way, ask whether the dome can move. If it cannot, a flexible section or an angled rigid elbow is the way round.
Think about the room
A bathroom or a dark landing under a long run will gain from the brighter, cleaner path of a rigid tube. A walk-in wardrobe or a store cupboard under a short run will be fine with either. If the light is just there to stop a space feeling shut in, a flexible tube is entirely adequate.
What the quote will say
Once we have surveyed, the fixed written quote names the tube type and gives our reason for it. Nothing on the quote shifts unless the scope of the job does. Our guarantee page sets out the 10-year workmanship guarantee that covers the fitting.
Our view: go rigid where the route is straight and short, and use flexible only for the jog or offset the loft forces.
Tube choices answered briefly
Is a flexible sun tunnel a lesser product?
Not at all. It is built for a different job. Where a straight rigid drop is not possible, a well-fitted flexible tube gives a good result and costs you very little light over a short run. It only falls behind on long or kinked runs.
Can the two types be combined in one tunnel?
Yes. A run can have a rigid section for most of the drop and a short flexible length to deal with an offset. That mixes the clean light path of the first with the reach of the second,.
Does the tube type change what I see in the ceiling?
No. The diffuser at the ceiling looks the same whichever tube sits above it, and the room sees a white panel. The tube only changes how bright it is and how the loft is arranged above the ceiling.
Will I need to go into the loft often?
Not once the tunnel is in. There is nothing to service or adjust. Keep the tube clear of stored items so nothing presses on it, especially a flexible one. A tidy loft keeps the run straight and the light strong.
Is a sun tunnel allowed on my house without planning?
On a house, often yes under Class C, if the dome stays within the 150mm projection limit. A flat needs permission. Read our planning answer for the conditions that can change it.