Flexible vs rigid sun tunnels

Buyers beware when buying your sun tunnels

When searching the internet you’ll find that you encounter a variety of manufactures offing rigid or flexible systems, with a few offering both. Rigid sun tunnels are formed using sections of straight or adjustable aluminium tubing with a reflective surface. Flexible sun tunnels use a foil and wire ducting, similar to that used for air-conditioning, and are sold as being easier to fit and tend to be a lot cheaper.

So what are the differences and does it really matter which you go for?

Flexible sun tunnel solartube

The answer is a categorical and unreserved YES.

I read on another website, http://www.fixmyroof.co.uk/videos-and-guides/sun-tunnel/, that flexible systems ‘if used on a  south’ish facing and don’t require more than 1.8 metres in length, it can produce very good results.’ Whilst I’m sure that this is well-intentioned advice, in my experience, this couldn’t be further from the truth.

Here’s why:

  • the worst rigid tube will give you at least 5 times more daylight than the best flexible system
  • the best rigid system will give you around 17 times more light

In what possible respect could it be regarded as very good?

Yes, on a south facing roof in full sun at mid-day in late June, it might be giving a reasonable result. But a good system should be delivering good light levels, in all weathers, throughout the year.

The big irony is that flexible systems are often selected because the run of the tube is not straightforward. Yes, you can easily run these flexible duct systems around obstacles in the roof space but the moment you introduce a bend into one of these systems then you can forget getting any light through it.

Flexible tubes will only give you a tiny fraction of the amount of daylight that you will get from rigid sun tunnels like Solarspot and Solatube
Flexible tubes will only give you a tiny fraction of the amount of daylight that you will get from rigid sun tunnels like Solarspot and Solatube

Light moves in a totally predicable way, you’ll probably remember ‘angle of incident’ from your school days. The light moves through the system by reflecting of the surface tube. The smoother and more reflective the tube, the further and more efficiently the light travels. On a flexible unit half of the reflective surface of the tube faces back up towards the sky and so at least 50% of all the light goes back on the first bounce. And when you consider that the light has to make multiple bounces to travel the length of the tube you don’t have to be a rocket scientist to work out what’s going to happen to that light.

Poor refelctivity of solart ube flexible duct systemsA few years ago Solatube commissioned the Building Research Establishment (BRE) to test two of the UK’s leading 350mm flexible systems against their Solatube 290 DS. The tests were conducted using 2m long systems that were all perfectly straight and the results the BRE came back with should be printed as a warning on all flexible sun tunnels.


  • Solatube 290 DS – 40% efficient
  • Velux flexible Sun Tunnel  – 3.8% efficient
  • SolaSkylights Sky Tunnel – 1.5% efficient

Read the full report

To put this into context, a single Solatube 290 DS will deliver around 10 times more light than the Velux and around 26 times more than the SolaSkylights unit. Yes they are cheap, but cheap rarely equates to good. And when you consider that you will probably spend just the same to have it installed as you would a rigid unit, any saving will be minimal. We will happily fit a flexible unit for you but please don’t complain if the results aren’t what you imagined.

For all installation enquiries please call us at NorfloDaylight on 01922 433396 today.