BRE tests prove flexible light pipes are no match for Solatube® rigid tube systems

With recent entries to the daylighting market from a number of providers of products using flexible rather than rigid tubes, the BRE undertook tests to compare the relative light transmittance of the Solatube rigid pipe system to the two most common flexible duct systems. All the products were tested under both simulated sunlight and natural overcast conditions.

While the Solatube system achieved light transmittance values in the region of 43% in overcast condition, the flexible pipes managed values averaging only 1.5 - 3.8%. In full sun, the Solatube outperformed the flexible systems, being up to 74 times brighter at certain times of the day.

All three systems were assembled as per their manufacturer's instructions with top domes/panels and bottom ceiling diffusers in place. The Solatube system also included an integral reflector in the dome, designed to pick up more light at low sun angles. This is a standard enhancement provided with all Solatube systems and is designed to increase daylight harvesting efficiency.

To ensure that the tests were carried out equitably, the flexible tubes were also stretched for some readings, something virtually impossible to achieve in an actual installation. This showed some increase to light transmittance values of between 2% and 6.8% under overcast conditions, less than one sixth of the results of the rigid pipe, and between 1.3% and 3.6% under simulated sunlight conditions, compared with an average of 42.2% for the Solatube rigid pipe system.

Table 3.1 - test results - overcast sky

The table above shows that the Solatube rigid pipe system transmitted 10 - 28 times more light than the flexible pipe systems under overcast sky conditions.

Table 3.2 - test results - simulated sunlight

Under simulated sunlight tests the flexible pipe systems transmitted up to 28 times less light than the Solatube rigid pipe system. Note: stretched figures for the flexible systems are not shown in the tables above - see full report (below).

What the tests didn't show

To ensure that all three systems were shown in the best possible light, please excuse the pun, the simulated sunlight tests were carried out with the roof panel or dome of each system effectively facing due south. In effect, this meant that the light source was directed virtually straight down into the systems - a situation that is likely to occur only on a very small percentage of roofs. If the system was to be installed on a different aspect - north, west or east - the results would have been even less favourable for the systems that use panels with no light harvesting technology rather than domes with light-harvesting technology.

Owing to the Solatube Raybender® 3000 Fresnel lens technology and LightTracker™ reflector, the positional aspect of the Solatube system is less critical as it delivers a similar performance wherever it is located on the roof. In essence, regardless of the angle at which the light hits the dome, it will be refracted into the aperture of the system and down into the space below. Table 2 illustrates how low level sunlight, early and late in the day, is actively harvested by the Solatube, but lost by the other two systems. On overcast days, all three systems will deliver their respective performances regardless of aspect. This is because the light is diffused.

See the full report from the BRE.

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Last updated: 14th June 2010