Rigid or flexible sun tunnels

A sun tunnel gathers daylight in a dome on the roof, passes it down a tube and spreads it into the room through a diffuser. The tube is lined with a reflective surface, so the light bounces from one side to the other on its way down instead of being absorbed. Each bounce loses a little. A short, straight tube asks the light to bounce only a few times. A long or crooked one asks for many more, and the room gets less.

That is the whole reason the choice between rigid and flexible matters. Both kinds do the same job, and the difference is how many bounces the light has to survive and how easily the tube fits the space it has to cross. In a house with a simple loft and a ceiling not far below the roof, either will light the room. In a house with a deep roof space or a crowded one, the choice shows up in the room below.

A rigid tube is made of straight sections with a smooth, mirror-finish inner surface, usually polished aluminium. The sections push together to reach the height you need, and where the run has to change direction, an elbow joins two lengths at an angle. Many elbows can be set to different angles, so the tube can step sideways to miss a rafter or a trimmer before dropping to the ceiling.

The smooth surface is the rigid tube's strength. Light travels down it with fewer, cleaner reflections, so a straight run delivers more daylight than any corrugated alternative of the same width. The tube also holds its shape without help. It does not sag between supports, it can be wrapped in a single insulating sleeve and it meets the ceiling collar squarely, which makes the finish at the bottom end much easier to get right.

The cost of that stiffness is that a rigid tube has to follow a plan. Each elbow is a fixed shape and each length has a fixed size, so the route is worked out before anything is cut. Where the loft is cluttered, a rigid tube may need several elbows, and a tube with several elbows begins to lose its advantage.

A flexible tube is a corrugated, reflective tube, rather like a wide, shiny hose, that can be stretched to length and bent to a curve. It can be fixed at an angle, offset around a roof timber and pulled into the gap between ceiling joists without any joints. In a roof space where a straight route simply does not exist, it can be the difference between fitting a sun tunnel and not fitting one.

The corrugations are what let it bend, and they are also what costs it light. Every ridge scatters a share of the light instead of passing it on cleanly, so a flexible tube is a poorer carrier over the same length. That is acceptable on a short run, such as a bungalow corridor where the roof sits close above the ceiling, and less so on a long drop through a deep roof space.

Flexible tube also needs support. Left to hang, it sags into folds that choke the light and snag on whatever is near it. A properly fixed flexible tube is held at intervals and stretched taut, with no kinks and no tight curves. Sleeved in insulation it holds its shape better, but it takes care to wrap a corrugated surface neatly, and the sleeve has to follow the bends.

The table gives a pattern, not a rule. A long flexible run in a well-planned loft can beat a short rigid one with three awkward elbows. The decision comes from the route, which is why it is made at survey with the roof space open in front of you, never from the product list alone.

Every elbow, every metre and every kink costs light, and the costs add up. A single gentle bend barely registers. A pair of sharp ones can leave the room noticeably dimmer than the same tube run straight, and there comes a point where adding more tube is not worth the hole in the roof. Makers publish a maximum length for each tube diameter for exactly that reason, and the limit is lower once bends are included. Our answer on how long a sun tunnel can be explains where that limit sits and what to do when a room is further from the roof than a tunnel can sensibly reach.

Diameter works the other way. A wider tube carries more light and loses a smaller share of it per metre, but it needs a clear route between the rafters, and it needs a larger round opening in the ceiling. On a roof with closely spaced timbers a narrower tube may be the only one that fits without cutting structure, and then the quality of the tube material matters even more. If you want the full picture of what a tunnel gives and where it stops, our guide to a sun tunnel or a rooflight compares it with a conventional opening.

A sensible rule for the survey is to count what the light has to do. A straight rigid tube from roof to ceiling is the best case. A rigid tube with one stepped offset is a very good one. A route that needs two or more changes of direction, or a deep drop, is where we talk through the alternatives, which sometimes means a different position for the dome or the diffuser, or a different product altogether.

Both kinds of tube cross the cold part of the house. The stretch between the roof and the ceiling sits in loft air, and in winter it is only a thin wall of metal or foil away from the outside temperature. If warm, damp air from the room below reaches it, that air cools and drops its moisture on the tube wall. That is how a sun tunnel ends up with condensation on the inside of the dome or dripping at the diffuser, and it is a fitting question more than a product one. We deal with it directly in do sun tunnels cause condensation.

The two kinds of tube answer it a little differently. On a rigid tube, a single preformed sleeve of insulation wraps the straight lengths and is taped at the joints, and the job is quick and tidy. On a flexible tube, the sleeve has to be gathered around the corrugations and eased round each bend without crushing, because compressed insulation does less. In both, the seal at the ceiling matters more than anything. If the diffuser and collar are airtight, little warm air gets into the tube in the first place.

Loft insulation around the tube needs the same attention. Mineral wool should sit snugly against the tube and the sleeve, with no gap that lets cold air down beside it and no heap of squashed fibre that has stopped doing its job. A bathroom below makes all of this more important, and we treat that case in our guide to sun tunnels for bathrooms, landings and hallways.

From the room, a sun tunnel is a round disc in the ceiling and the line where it meets the plaster. The tube type changes how easy that line is to get right.

A rigid tube ends in a stiff collar that can be fixed square to the ceiling opening, and the ceiling is cut to a clean circle that the collar sits behind. The diffuser trim then covers the edge, and the plaster around it can be made good to a crisp line. A flexible tube is less obliging. It needs a stiff collar or short rigid adaptor at the bottom so that the opening stays round and flat, otherwise the tube pulls the collar out of true and the trim never quite sits flush.

Whichever tube goes in, the ceiling around the opening is finished as part of the job. On plasterboard, the cut edge is filled, taped where needed and sanded smooth, then the trim ring is fixed over it and the paint is cut in against the ring. On an older lath and plaster ceiling, the cut is made carefully to avoid breaking the keys that hold the plaster up, and the join is made good in the same material as the ceiling. We put that detail in writing with the quote, along with what sits above the ceiling, so you can see before work starts how the opening will be formed and finished.

A few questions settle most cases, and they are the ones we ask at survey.

How far is the ceiling from the roof?

A short drop, as in a bungalow or the top floor of a house with a low roof space, suits either tube, and a flexible one costs little in light. A long drop through a tall landing or a high ceiling is where a rigid tube pays off.

Is the route clear?

If you can look from the roof position straight to the ceiling position between two rafters and over one set of joists, a straight rigid tube will go in. If a tank, a chimney breast or a run of cable crosses the line, a stepped rigid tube may get round it, and a flexible section may be easier.

Is the loft used?

A floored or converted loft changes the route. The tube may have to be boxed in where it passes through a usable space, and a rigid tube is easier to box neatly. Our answer on whether a sun tunnel can go through a loft covers floored and stored-in roof spaces.

What is the roof?

The dome and flashing are chosen for the covering, whether that is slate, plain tile, interlocking tile or a flat roof upstand, and they are the same with either tube. The roof side of the job is set out in sun tunnels on flat and pitched roofs.

In older Devon houses, the answers can be less tidy. Cob and stone cottages in the Culm and Exe valleys have thick, uneven roof builds, and the old rafters seldom run straight or sit at the spacing a standard tube expects. A mix of rigid and flexible, planned section by section, is sometimes the honest fit. In a Georgian or Victorian terrace in Exeter, the roof space is often deeper and the ceiling lath and plaster, which makes the straight rigid route and the careful ceiling cut worth the extra thought.

A good quote for a sun tunnel names the tube type and says why it was chosen. It should state the diameter, the dome and flashing for your roof covering, the route between roof and ceiling including any bends, whether the tube is insulated and how, and how the ceiling end will be finished. If the quote only says "sun tunnel" with a price, the choices that decide how the room looks are still open.

Our quote carries a written finish schedule covering the build-up above the ceiling, how the opening in the ceiling is cut and made good, the trim and the paint line, and how dust is managed while we work. Read the whole picture on our sun tunnel installation page, and see what the schedule covers on the finish schedule page. When you are ready, ask for a survey, and we will look at the roof space, choose the straightest route and tell you plainly which tube suits your house.