What sets the limit
A sun tunnel is a roof dome, a reflective tube and a ceiling diffuser. Light enters the dome, bounces down the inside of the tube and spreads into the room through the diffuser. Every bounce loses a little light, so a longer tube with more bounces delivers less.
There is no single legal or industry maximum. Each maker publishes its own figures for each tube diameter and tube type, and a larger diameter generally copes with a longer run because light makes fewer bounces per metre in a wider tube. VELUX, for example, gives a recommended length of 0.9 to 1.7 m for its rigid tunnel and allows extension to 6 m with extension sections. The data sheet for the model gives the figure, and the installer works within it.
Two things shorten the practical limit: bends and tube quality. A straight vertical drop is the best case. Every angle you add in the run reduces output, and a tube with a poor reflective coating reduces it further.
Brighter days make the limit less visible. A tube that looks dim on an overcast morning can look generous in strong light, so the length you can accept depends on how the room is used. A landing or a hallway that is only passed through can tolerate a long run. A bathroom where you want daylight to dress by wants a shorter one.
Rigid tube and flexible tube
Rigid tube has a mirror-polished inner surface and runs in straight lengths, joined with elbow sections where it needs to turn. It carries light further than flexible tube.
Flexible tube is a ribbed, foil-lined duct that can be bent round obstacles. It is easier to fit through a crowded loft, but the ridges scatter light, so the output falls faster along its length. It suits short runs, or places where a rigid tube cannot reach.
- Rigid tube suits a straight or gently angled run through an open roof space.
- Flexible tube suits a short run that has to dodge a purlin, a water tank or a chimney breast.
- A mix of both is common: rigid for most of the drop and a short flexible section to clear an obstacle.
Keep any flexible section as short as possible. Its mirror finish is rougher than that of rigid tube, and light drops fastest where the walls are ribbed.
Joints matter in both types. Each join is a place where light can leak out or tape can fail, so the sections are joined and sealed as the maker's instructions set out.
Bends cost more than length
In a lot of lofts the tube cannot drop straight down. The dome has to sit on a clear stretch of roof, while the ceiling opening sits over the room that needs the light. The tube must travel sideways to join them, and that means angled sections.
A bend sends light into a wall of the tube instead of straight through, so each one removes more than a stretch of the same length that runs straight. A shallow angle generally costs less than a sharp one, and a shorter run with a bend can beat a longer straight run.
For that reason, the position of the ceiling opening matters as much as the total length. The best place for the tube is almost always directly beneath the dome, between two ceiling joists, with the shortest route across the roof space.
Where a tube must dodge a rafter or a purlin, a pair of elbows can offset it sideways and bring it back to vertical. That adds two bends, and the survey shows whether an offset is needed or whether the dome can move to a better spot on the roof.
Roof depth and a tall loft
The length of a run is the sum of three things: the thickness of the roof build-up, the height of the roof space above the ceiling, and any sideways travel. A shallow pitched roof over a low loft gives a short run. A steep roof, a deep loft or a chalet with a tall void gives a longer one.
On a roof with a tall loft, an extension section joins the standard tube to reach the ceiling. Makers sell these in set lengths; the VELUX extensions come in 62 cm and 124 cm pieces. Where the run is long, a larger diameter helps keep the output up, because a wider tube collects more light at the dome. Our page on what diameter of sun tunnel you need covers that choice.
The tube also has to avoid ceiling joists, wiring, pipes and insulation that would be compressed. We measure on the roof, in the loft and at the ceiling, and write the run onto the survey sheet you keep.
A short example: a roof build-up of about 300 mm, a loft void of about 1.2 m and no sideways travel gives a run of about 1.5 m. The same roof with a corner to turn adds length and bends at once. These are example readings only; the measured figures on your sheet are the ones that count.
Insulating a long run
The tube passes through a cold roof space, so it is a place where warm room air can meet a cold surface. A long tube with no insulation around it can become damp on the outside. Makers supply insulation sleeves, and wrapping the tube through the cold section helps limit condensation on it.
The ceiling collar at the bottom should seal tightly so warm air does not travel up the tube into the loft.
Loft insulation disturbed around the tube and the ceiling opening is put back as the maker's instructions set out. Where a long run is wanted, tell us at the start; it changes the materials on the quote. Compare the tube with a roof window on our sun tunnel or roof window guide, and see the range on the sun tunnel installation page.
faq
tube length questions
Does the tube have to be straight?
No. Elbow sections let it turn, but a straight drop gives the most light, so the run is planned to need as few turns as the loft allows.
Does a longer tube mean a dimmer room?
Yes, the same dome gives less light at the end of a longer tube. A wider tube compensates in part.
Can I extend a sun tunnel later?
Extension tubes are sold by the maker, so a run can be lengthened within the maker's limit, but the room will get less light from it.