Rigid or Flexible Sun Tunnels

A rigid tube is the premium route to brightness. Its mirror lining is made once at the factory, so it has no folds, no ridges and nothing for light to snag on. In a straight drop, light that enters the dome travels down in a few clean bounces and arrives almost as it came in.

That makes rigid the first choice where the loft allows it. A landing under an open roof space, or a hall below a plain pitched slope, can take a straight pipe with no turns. The result is the brightest ceiling disc you can get from a given dome size.

Rigid tubes can turn, using angled elbows. A pair of elbows lets a pipe step sideways round a joist or a tank. Each elbow costs some brightness, and each one is another joint to seal. A rigid run with a single gentle offset works well; a rigid run with four angles is usually better replaced by a flexible one.

The limits are length and fit. Rigid sections come in set sizes, so the run is built up from lengths and cut to suit. They need space to be handled, and they cannot be threaded through a tight gap. They also feel stiff in a roof where rafters or purlins leave little clear height.

Joints deserve a word. Every join in a rigid run is taped and fixed, so that no daylight leaks out and no loft dust gets in. A tidy run with few joints is quicker to fit, easier to inspect and less likely to rattle in wind. When we look at a loft we count the joints a route would need before we count the metres, because a short route with many turns can be a worse choice than a longer, plainer one.

A flexible tube earns its place where the loft is crowded or the route is awkward. It stretches out to the length needed, bends round a beam, and tucks past a chimney breast. In a house of the 1930s or an older terrace, where lofts have been added to and altered, that freedom is often what makes a sun tunnel possible at all.

The price is brightness. The ribs in a flexible liner break the smooth surface, so light scatters more as it travels. A short flexible run gives a pleasant result; a long one with several bends gives a dimmer one. The tube must be pulled taut, because a sagging liner folds into the light path and darkens the disc.

Some flexible liners use a high-reflectance film for the inner surface, which narrows the gap with rigid. We say what is specified on every quote, so two quotes can be compared by lining as well as by length.

Handling matters here as well. The liner arrives compressed, and it needs stretching to its full length and fixing at intervals so it stays straight between bends. A loose liner droops, and a drooping liner narrows the passage. We secure it to the structure with ties and check from below that the disc glows evenly before we close up. On a long flexible run we also keep the bends wide and gentle, since a tight kink does more harm than two soft curves.

A flexible tube also fits more easily through a restricted ceiling void, which suits a retrofit. Where the ceiling below is plastered and finished, the diffuser has to go into a neat round hole, and a flexible tube can approach that hole from an angle that a rigid pipe cannot.

Here is how the two compare on the points that matter. It is a guide to the pattern, not a ranking.

The look from inside the room is the same in both cases, because only the diffuser shows. What you see is a round disc, and the tube behind it is invisible. So the decision is not about appearance. It is a plain trade between the light you get and the route you can take.

It is common to combine them. A rigid length drops from the dome through the main roof space, and a short flexible tail finishes the run into a ceiling that sits off to one side. This hybrid keeps most of the rigid brightness and still reaches the spot where the diffuser looks best.

Two numbers shape the result: how long the tube is and how many turns it makes. You do not need a formula, only the habit of asking.

1. How far is it from dome to ceiling, measured along the route?
2. How many bends does that route need?
3. Is the tube a standard width, or a wider one?
4. What is the lining?

A short, straight run from a loft directly over a landing is the best case. A long run down through a two-storey house, past an upper floor, loses more. Each bend sends some light back toward the dome, so the difference between a plain drop and a winding route is the difference between bright and soft.

A wider tube catches more and passes more, in the same way a wider window lets more light in. Some roofs and ceilings cannot take the wider sizes, and the survey sorts out what fits. If the answer is a narrow tube on a long route, we say so plainly and talk about whether a roof window or another position would serve better. The sun tunnel vs rooflight guide goes through that alternative.

The dome's direction matters too. A slope facing south collects more direct sun; a north slope collects soft, steady light. Either works, since daylight reaches the dome from every part of the sky. A tall wall or tree close to the roof is a bigger factor than compass direction.

A tube passes through the loft, which is cold in winter, and into a room that is warm. That raises two reasonable worries: heat loss and condensation on the tube.

The detail sits on do sun tunnels lose heat. In short, the tube is an air-filled passage with a diffuser at the bottom that closes it off from the room, and the insulation laid around the pipe keeps the loft separate from the room below. Some tubes are made with a double wall or an insulating sleeve for this reason.

Rigid and flexible tubes behave a little differently. A rigid aluminium pipe conducts heat, so a sleeve helps on a long run through a cold loft. A flexible liner, being thinner, may be wrapped in insulation as it is laid. In both cases the aim is the same: warm air in the room should not meet a cold surface it can condense on.

Where the tube is also a ventilation route, with a fan or vent fitted at the diffuser, the airflow adds a further factor. The sun tunnel ventilation kit page explains how that fits. Whichever tube you choose, the vent is a separate item and you can add it to a rigid or a flexible run.

The tube does not fix the roof; the roof fits the tube. A pitched slate or tile roof takes a dome with a flashing kit matched to the covering. A flat roof takes a dome on a kerb, as described in sun tunnel flat roof. The tube choice sits below either, and does not change what is on the roof. If you are weighing the whole system, the sun tunnel installation page lists every part.

On cost, a rigid run is often simpler to price, since it is a set of parts and a plain fit. A flexible run is cheaper in parts but can take longer in awkward lofts. Broad UK ranges, shown as typical and subject to the job, are on sun tunnel cost and the main costs page. What changes the final figure is the length, the number of bends and the roof access.

The hole through the slates is a structural change, so rafters are trimmed with care and the finished opening has to satisfy the regulations. Building Control gets its notification from us wherever one is due. For a bathroom or hall use, read sun tunnels for bathrooms, landings and hallways for how the diffuser and room fit together.

At the survey we go into the loft, find the clear path, measure the run and count the bends. The itemised quote names the tube type, its lining and its width, along with any opening or vent option. It is free, with no obligation. The roof is sealed against the weather by the end of each day, floors are covered, and the guarantee applies to the fitting for ten years.