Short answer
There is no single figure. Each manufacturer sets a maximum length for each tube type and diameter, and rigid and flexible tubes have different limits, so the datasheet for the chosen model decides. A longer tube and each added bend both lose light, so a shorter, straighter tube always performs better. The survey measures the route from roof to ceiling to see which tube fits.
Who sets the maximum length
A sun tunnel is sold as a tested kit, and the maker states how far it may run. Those limits come from the manufacturer's installation instructions for the model. They differ between brands, between tube diameters and between rigid and flexible tubes, so quoting one length for every sun tunnel would mislead you.
What we do is work within the limit for the exact product on the job plan. If a route is longer than a single tube allows, the options are a different model, a wider tube, or a different product altogether. The page on sun tunnel installation describes the standard fitting.
Rigid and flexible tubes
A rigid tube is a straight reflective pipe, usually in sections. It carries light well over a straight drop and is the first choice when the route allows. A flexible tube is a bellows-style liner that bends around a rafter, a duct or a chimney. It is easier to route and gives less light for the same length, because its folds scatter some of the light that a smooth surface would pass on.
Many installations combine both: rigid for the straight part, flexible for the short stretch that needs to turn. Each has its own length allowance in the maker's instructions. The comparison in rigid against flexible tubes goes through the trade-offs.
Using more flexible tube than the route needs is a common mistake. It is tempting because it is easy to push through a crowded loft, but each fold costs brightness, and a tube that is stretched taut works better than one left slack. The survey aims to use the least flexible length that clears the obstacle.
| Rigid tube | Flexible tube | |
|---|---|---|
| Best use | Straight drop | Short turn around an obstacle |
| Light carried | Better per metre | Less per metre |
| Routing | Needs a clear line | Bends round joists and ducts |
| Length limit | Set by the maker | Set by the maker |
How light loss grows with length and bends
Light bounces many times on its way down a tube, and each bounce costs a little. A longer tube means more bounces. A bend adds loss as well, because light strikes the inside surface at sharper angles. The maker's yardstick, from VELUX, sets a 14-inch tube against about a 60W bulb on a bright day. It is the manufacturer's comparison for a short, straight run, not a promise for a long one.
In practice the length is only part of the story. A long tube from a bright south-facing slope can still beat a short one from a shaded roof. The diameter matters too, since a wider tube passes more light. We note the orientation and any shade on the roof at survey.
The aim is a route that is as short and as straight as the house allows, and we say plainly when a proposed run would give a disappointing result.
Reflective tube is not all the same. A rigid tube with a mirror finish carries light further than a flexible one with ridges, so where a long route is unavoidable the rigid sections are used for the longest straight stretches. Joints are taped as the instructions require, because a gap in the tube lets light and warm air escape into the loft.
- Shorter tubes carry more light than longer ones
- Each bend adds loss on top of the length
- Wider tubes pass more light than narrow ones
- Roof orientation and shade change what arrives
Extension pieces and fitting a longer run
Where the loft is deeper than the basic kit, makers supply extension tubes in set lengths that join to the main tube, within the model's stated maximum. The pieces are sealed at the joints so that the tube stays dust-free and does not let loft air into the room.
A long run also needs support. A tube hanging between roof and ceiling is fixed at the points the instructions give, and held clear of insulation and joists. Insulation in the loft is kept in place around it, so the ceiling stays warm. Where a tube would pass near a chimney, a flue or a cable, the route is moved.
How the survey measures the route
At the free survey we go into the loft with a tape. The measure starts at the proposed spot on the roof slope and follows the likely tube line down to the ceiling position, noting the roof pitch, joist and rafter positions, and anything in the way. We record the total length and every point where the tube would have to bend.
That measurement is then checked against the limits for the models that could do the job. For a downstairs room beneath two storeys the numbers often rule out a tube, and sun tunnels for downstairs rooms looks at that case. Whether the result justifies the cost is the subject of are sun tunnels worth it.
What goes in the job plan
The written quote comes with a job plan that names the tube type, the diameter, the measured length, the number of bends and any extension pieces. Alongside sit the planning position of the street, the Building Control route, the wait for the unit to arrive and the date set for fitting.
Because the length is on paper before anything is ordered, there is no surprise on the day about a tube that falls short or a box that is bigger than expected. If the job needs notifying, we send it to Building Control, and the workmanship carries a guarantee of ten years.
If the measured route is beyond what any tube can sensibly deliver, the plan says so and suggests another way to bring daylight into the room, such as a rooflight on the nearest flat roof.