A sun tunnel is only as good as the stretch of loft it has to cross. The dome on the roof and the diffuser on the ceiling are decisions about size and finish, but between them sits a tube, and the tube is made in two ways. One is a rigid sleeve of mirror-finish metal that arrives in straight lengths. The other is a ribbed, foil-lined duct that stretches out like the hose on a vacuum cleaner and bends round whatever is in its way. Both carry daylight down to a room with no outside wall, and which one suits your house depends on the timbers, tanks and pipework that share the space above that room.
If the room in question is a landing, a hall or a bathroom of your own, book a survey and we will trace the route from inside the loft. The wider picture on domes, diffusers and rooms is on our sun tunnels page.
What the lining does to the light
Daylight enters the dome as a wide wash from the whole sky and has to reach the diffuser by bouncing off the inside wall of the tube. Every bounce gives up a little of it to the wall. A tube that sends light down with few, shallow bounces delivers more at the bottom than one that sends it ricocheting.
A rigid tube has a smooth mirrored face from top to bottom, so a ray of light meets a continuous reflective surface and carries on down. The lengths push together at collars and are taped at each joint. The tube keeps its shape, which means the light path is the shape you designed.
A flexible tube is a spiral-ribbed duct with a reflective foil skin. The ribs are what let it bend, and they are also small ridges on the wall. Light hitting a ridge can be thrown back upward or sideways, so over the same distance a flexible tube normally delivers less than a rigid one. The loss is greater when the duct is slack and wrinkled, and smaller when it is stretched taut and held in a straight line.
None of that makes flexible the weaker choice for every house. A rigid tube that cannot be fitted delivers nothing, and a flexible one that reaches the ceiling delivers something. The sensible comparison is between two tubes that will both go in, which is why the loft is the first thing we look at.
Tube vocabulary
- Collar
- The short sleeve that joins two rigid lengths, taped so light and loft air stay out of the joint.
- Elbow
- An angled rigid piece that turns the route to one side.
- Corrugation
- The spiral ribbing on a flexible duct that lets it bend.
- Offset
- The sideways shift between where the dome sits and where the diffuser lands.
- Sag
- A drooping loop in a flexible duct that has not been pulled tight or supported.
When the loft has opinions
Look into a loft above a landing and the clear space is rarely where you want it. The ceiling joists run one way. Roof timbers cross at angles overhead, and in roofs built from factory trusses the diagonal webs often sit exactly where a straight tube would like to go. There may be a cold water tank, a run of cable, a bathroom extractor duct on its way to the roof, or the brick bulk of a chimney breast. On top of it all lies the insulation, which has to be kept around the tube.
A rigid tube needs a clear path as wide as itself, and a straight one if you want the best from it. When the dome position on the roof and the diffuser position on the ceiling line up, with nothing between them, a rigid run is hard to improve on. When they do not line up, we can shift the diffuser a few hundred millimetres, or add an elbow, or switch to flexible for the whole route or just part of it.
Flexible duct suits the places where the obstruction cannot move. A tank beside the stair head, a pair of trusses with a narrow gap, or a loft hatch that only admits a small hand: a rigid length would not go through any of those, and a ribbed duct can be fed through and then pulled into line. In an older terrace loft with a party wall on one side and a stack of stored boxes on the other, that flexibility can be the difference between a tube that goes in and a survey that ends with a roof window being suggested instead.
Rooms with a bedroom or bathroom directly overhead are another matter, since the tube would have to pass through a lived-in space. That is a limit of the house itself, and neither build solves it.
A tank beside the stair head can push the whole route sideways
Matching the build to the room
The room does not choose the tube, but it shapes the route, and the route chooses the tube.
A landing at the head of the stairs in a two-storey semi often has the roof directly above, a clear bay between joists and the dome facing open sky. That is a rigid case. A hall in a bungalow beneath a trussed roof is a closer call, because the roof timbers may cross the line of a straight drop, and a short flexible section can slip past them. A windowless bathroom brings the extractor duct into play, and the tube has to keep clear of it; our answer on a sun tunnel in a bathroom covers the damp-room details.
An inner utility room or boot room below the shallow ceiling void of a flat-roofed extension is a different shape of problem. The void between ceiling and roof deck is thin, and a rigid tube may have little room to be anything but short. A flexible duct can run sideways along the void to a better ceiling position. Where the extension is wide, though, one round diffuser lights only a patch of it, and a flat roof rooflight may serve the room better.
| Room and roof | What the route looks like | Build we lean towards |
|---|---|---|
| Landing under a pitched roof | Clear bay, straight drop from dome to ceiling | Rigid |
| Bungalow hall under trusses | Straight drop blocked by diagonal timbers | Rigid with a short flexible offset |
| Windowless bathroom | Must clear the extractor duct and stay sealed to the ceiling | Rigid where it fits, flexible to bend round the fan route |
| Utility under a flat-roofed extension | Thin ceiling void, sideways travel needed | Flexible, or a rooflight instead |
| Terrace loft with a tank or boxed storage | Tight, cluttered access, awkward alignment | Flexible |
Hanging, sealing and wrapping the run
Fitting is where the two builds feel most different in the hand. Rigid sections are lifted into the loft in lengths, pushed together at the collars and taped at every joint. They hold their own shape, and the main job is fixing the tube so that it stays plumb and clear of the roof timbers. Flexible duct arrives compressed and is drawn out to the length needed. It is pulled straight, and then it is held: tied or hung at intervals so that it cannot droop into a loop between supports.
That support matters more than it looks. A flexible tube that sags creates pockets where the light bounces and stalls, and a loop in a damp loft is a place where condensation can gather. A taut run, held up and kept as straight as the loft permits, is the one that delivers.
Keeping the loft's cold away from the tube
The ceiling is part of the thermal envelope of the house, and a tube crosses from the cold side of it to the warm. For energy efficiency, we seal the joint at the ceiling ring against draughts and wrap the length that lies in the cold loft with insulation, rigid or flexible alike. Without that, the tube's own surface can chill, the warm air in the room below meets it, and moisture forms. The insulation sleeve and the sealing tape are both lines on the quote, because they are part of the job.
Joints, ends and the cut at the roof
Each joint is a place where light can leak out or loft air can creep in. The tube is cut to the measured length rather than left long and bunched, and the ends are fixed to the dome base and to the ceiling ring so there is no gap to whistle in a wind. Both builds finish in the same way, with the diffuser clipping into a ring in the ceiling, and from below nobody can tell which sort is above it.
Worth knowing
Rigid or flexible, the tube is taped at every joint, supported along its length and wrapped in insulation where it crosses the cold loft.
What the quote should name about the tube
A sun tunnel quote that says only "sun tunnel, fitted" leaves out the part that changes the job. When we write one, the tube build is stated, so you can see whether the route is rigid, flexible or a mix of the two. The length is given as measured along the tube, from the dome to the ceiling, not as the distance between roof and ceiling in a straight line. The number of elbows is given. So is the insulation wrap, and any loft boarding that has to be lifted and put back.
Rigid and flexible tubes do not cost the same to supply, and they do not cost the same to fit. A straight rigid run through a clear loft is usually the quicker job. A long flexible route through a boarded, crowded loft takes more time and more care, because every support point is something we have to build. Each tube is cut to measure for the route, so it is made to suit the house. The full list of what moves the figure is in our guide to what a sun tunnel costs to fit.
The survey is free and the quote that follows is fixed and in writing. The 10-year workmanship guarantee is written down alongside it, and applies to a flexible run exactly as it does to a rigid one.
Lines that depend on the tube
- Build of tubeRigid lengths, flexible duct, or a rigid drop with a flexible offset.
- Elbows and jointsEach change of direction and each collar is counted.
- Loft insulation sleeveThe wrap that keeps cold loft air off the tube.
- Access and boardingBoards to lift and stored belongings to move before work starts.
Cutting the roof: rules and sign-off
Both builds need the same hole in the roof, because the dome and its flashing are the same whichever tube hangs below them. On a house, a dome that stands no more than 150 mm beyond the plane of the original roof slope usually falls within the Class C permitted development right for rooflights, and our planning guide sets out the conditions and the cases where that right does not apply.
Cutting through a roof is building work, so we handle the Building Control notification where the work is notifiable, and the Building Regulations guide explains what is looked at. For general advice, the Planning Portal is the national source.
How far a tube can run before the room goes dim is on the page about how long a sun tunnel can be, and what to expect from the light on an ordinary day is in do sun tunnels work. Where the room sits under a slope that can take a window you can see out of, a VELUX roof window may be the better answer, and the survey settles which.
We fit sun tunnels in Maidstone and across about 30 road miles of mid-Kent, in terraces, semis, bungalows and newer estate homes. Send a photo of the loft hatch and the dark room to WhatsApp on 07880 284133, or email info@orchardskylightsmaidstone.co.uk, and we will arrive knowing what to bring.
