Barrel vaulted rooflight incorporating Nanogel technology

Various design guides – such as BB 90 Lighting Design for Schools - already recognise that an extensive use of natural lighting can provide considerable energy savings as well as improve the working environment. According to a leading services engineer, horizontal rooflights provide two and a half times more light than vertical windows, so it is not surprising that they are growing in popularity. It is essential to consider rooflights as effective light sources in their own right, able to substantially reduce energy used by artificial lighting.

To help with this, comprehensive research carried out by De Montfort University is now available quantifying the energy savings that can be made, linking thermal performance and heating energy with illumination effects and energy used for artificial lighting. From this work it is clear that rooflights provide overall energy benefits in a range of different building types. It shows that by increasing the rooflight area, the need for artificial light is reduced, cutting the energy requirement of the building and reducing CO₂ emissions.

OVERALL ENERGY BENEFITS
The research proves that, for a building occupied primarily during the day, passive solar gain through the rooflights actually balances the reduced insulation value (compared with the roof itself). So, heating requirements are barely affected and the most dominant effect by far is the decreasing requirement for artificial lighting as the rooflight area is increased. Even for a building occupied 24 hours a day - the worst case scenario for rooflights – they still provide a very significant energy benefit. In almost all cases, a rooflight area of 15%-20% will achieve almost all the available savings in overall energy use and CO₂ emissions.

Using rooflights is therefore a straightforward means of meeting a building’s target emission levels under Part L of the Building Regulations.

But the Part L ‘Approved Documents’ (AD L) and other, similar national regulatory guidelines are reliant upon accurate thermal performance information for their effectiveness. We often take for granted insulation values put forward by manufacturers for complex components such as rooflights - so there are calls for more rigorous investigation by building control officers and specifiers of claimed U-values and how they are established. AD L clearly states that: ‘U-values shall be determined in accordance with the methods and conventions as set out in BR 443: Conventions for U-value calculations.’ The current 2006 edition of this BRE document takes the general view that: ‘While calculated U-values are acceptable for Building Regulations and most other purposes…test results should be preferred when available provided that the values have been obtained in accordance with the appropriate measurement standards.’

Specifically for windows and rooflights, BR 443: 2006 requires the U-value to be that of the complete unit, including the frame and upstand or kerb - which are particularly complex on modern rooflights. Just one look at the complex cross-section of a modern rooflight kerb shows how difficult it is for anyone to develop convincing heat-loss values using calculation alone. For testing, BR 443: 2006 specifically refers to measurement in a ‘hot box’ apparatus and this gives accurate measurements for a complete rooflight, no matter how complex the construction.

MATERIAL INNOVATIONS
New material technologies can also play a major role in improving the energy efficiency of rooflights, while maintaining good lighting characteristics as well. For example, Xtralite has introduced Nanogel technology which combines impressive thermal, light transmission and acoustic characteristics. Nanogel consists of translucent aerogel granules - filling clear polycarbonate multi-wall rooflight constructions – that allow light to pass through whilst serving as a highly effective thermal insulation. The extremely small pore size means that air molecules collide with the silica lattice, rather than each other, transferring energy to it, substantially limiting heat conduction. So, insulation performance is vastly improved, with U-values for a triple walled rooflight reduced from 2.4W/m2K without Nanogel to 1.3W/m2K with the new technology. Nanogel also offers excellent light transmission with diffusion giving a ‘shadow-less’ light quality, as well as a 25% reduction in sound transmittance (at 1, 000Hz frequency).

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