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Design Considerations

What percentage or area of rooflights should I use?

That would depend on the operational requirements for that building and the specification of the rooflight because there are several options to suit different requirements. The provision of natural daylight rather than artificial light has been demonstrated to deliver more energy savings than can now be achieved through increasing insulation levels.

Studies have demonstrated that the optimum rooflight area is usually in the region of 12 to 18%. At higher rooflight levels, consideration should be given to the risk of solar over-heating at the hottest times of the year, and the ventilation strategy for the building.

What is the Daylight Factor for your rooflights?

The Daylight Factor cannot be stated for any stand-alone product. It is a comparison of the light levels experienced throughout a building as a percentage of the daylight that is available outside the building.

A room or building with a daylight factor of less than 2% will generally appear gloomy and artificial lighting will be required for most tasks, compared to a daylight factor of more than 5% where it will appear strongly lit and minimise the requirement for artificial lighting providing that the light is uniformly distributed.

For more information on this subject, please see our Estimating for Daylight Factors’ guidance document.

What is ‘diffused’ light and how can I be sure I’m getting what I need?

Diffused light is a soft light with neither the intensity nor the glare of direct light. The light is scattered when passing through diffusers and naturally diffusing materials such as GRP and avoids glare and harsh shadows giving a more even spread of light throughout the building. Diffused daylight is always recommended as the best option for rooflighting unless there is a requirement for a clear uninterrupted view through the rooflight. Diffused light can be anything from quite lightly diffused delivering an identifiable amount of light spread, up to heavily diffused where the light distribution is so widely scattered that its limits are difficult define.

How diffused is the daylight that your rooflights deliver?

The diffusion in GRP materials is created by the refraction of light passing through the resin and glass. The more changes that the light passes through, the more diffused the light becomes. Where diffusion may be stated as 100%, this has no useful meaning as it ignores any reference to how widely and evenly the light is diffused.

What is the uniformity factor for your rooflights?

Uniform lighting, whether by natural daylight or by artificial lighting, is important where there is a requirement to operate in an environment with significant differences in lighting levels.

Changes between well and poorly lit areas, especially where glare occurs, can cause significant eye discomfort, leading to stress and tiredness and therefore potentially jeopardizing safety as the human eye takes its time to adapt to changing lighting conditions.

The uniformity factor can be expressed as a ratio of the highest to lowest illuminated area in a given room or space. The closer it is to one, the more uniformly lit the space is. It is therefore the design of the daylighting rather than a function of the rooflight itself. Increased rooflight areas with high diffusion and lower light transmission levels provide the most uniform lighting, or highest uniformity factors.

What does U-value mean?

It’s a measure of the rate of heat flow through the building fabric or a building component; the lower the U-value or rate of flow, the better the insulating ability.  Thermal performance is expressed in units of energy (Watts) per m2 per temperature degree difference. It is an essential consideration of the building design under Building Regulations.

What is best, high levels of light transmission or better U-values?

Generally, more energy is saved by reducing the energy consumed by artificial lighting than is used for heating purposes. Low U-value rooflights are not always the best solution for a building if the light transmission is reduced. The more layers added into a rooflight, the more the light transmission is affected. The Zenon Insulator® core can be used to overcome this issue.

What is best, small areas of rooflight with high levels of light transmission or larger areas of rooflight with better U-values but lower levels of light transmission?

This would depend on the use of the building and being able to strike the best balance of rooflight specification choice for that building. Insufficient rooflight areas can lead to areas of gloom and shadow that requires the use of artificial lighting to overcome poor or inadequate daylight distribution. Using rooflights with high levels of light transmission in only small areas can increase the risk of glare and visual discomfort making the space in the building unusable.

A rooflight area at a minimum of 10 to 12% floor area would often be a little too low with the optimum being around 16 to 18% floor area. If better light distribution is required further, then the rooflight area can be increased, but solar gains would need to be controlled by using solutions with lower levels of light transmission.

Do your light transmission figures make an allowance for dirt and debris?

No, as this would be impossible due to different environments, cleaning regimes etc. When designing for the building’s energy performance, it is prudent to apply amaintenance factor depending on the location and environment of the building.

What is the ƒ-factor for your rooflights?

The ƒ-factor of a product or assembly is an internal surface temperature comparison against outside air temperatures to determine the risk of surface condensation. For a well-insulated wall or roof, the ƒ-factor would be close to 1 and condensation unlikely. A poorly insulated element or cold-bridge may have an ƒ-factor of 0.5 or less.

Rooflights, by their very nature, are unable to achieve the very low U-values of the opaque parts of the roof and still allow light transmission. Typically, a rooflight with a U-value of 1.8W/m²K would have an ƒ-factor of 0.82 and a rooflight with a U-value of 0.8W/m²K would have an ƒ-factor of 0.92.

For more information on this subject, please see the Thermal Transmission section in our Zenon Technical Manual.

How can your rooflights reduce energy bills?

Not many construction products can deliver something valuable that is entirely free.  Maximising the benefit of this natural resource, in conjunction with automated lighting controls, can save more energy in a building now that increased levels of insulation are delivering significantly reduced returns.

During the colder months, rooflights can still contribute to the heating of a building through solar gains. The use of a rooflight with good levels of light transmission and improved thermal performance to reduce heat loss can deliver significant savings.

The specification asks for ‘walkable’ rooflights. Will your rooflights meet this requirement?

No rooflight supplied for any roofing application should be walked on unless specifically designed to do so. Only glass rooflights designed to floor-loading type applications should be considered for this use. No rooflight suitable for use in lightweight metal clad buildings can be considered suitable.

It is important to understand the difference between ‘walkable’ and ‘non-fragile’. For further guidance see the Non-Fragility section of our Technical Manual.


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Long March, Daventry, Northamptonshire, NN11 4NR, UK
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