The provision of natural daylight within the built environment can deliver genuine, positive benefits to the finished construction; benefits that can enhance the financial and environmental performance of the building in service, benefits that can improve the internal environment and make it a better, more pleasant place to be. Benefits that can make a real, measurable contribution to sustainability and the drive towards carbon neutrality.

While considering the daylighting plan of any building, designers need to remain aware of the potential for overheating caused by excessive solar gain especially where there is no adequate ventilation strategy. The design of the building and provision of rooflights must balance several factors, in particular the perceived conflict between providing daylight and natural heat energy into the building together with the risk of overheating and the demands on the roof space to provide photovoltaic energy generation.

For any building, there is an optimum target percentage of rooflight area that will deliver the optimum level of natural daylight into the building, making the optimum saving in energy consumption and costs. Beyond that point, solar gain can add to the energy consumption if powered cooling systems become necessary.

The total solar gain, defined as the ‘g-value’, is a measure of the total amount of solar heat energy that passes through a window or rooflight expressed as either a percentage or a factor of 1. Most of this energy is transmitted directly through a transparent or translucent material in the visible light spectrum.

The total solar heat energy combines the full spectrum of directly transmitted solar heat and solar radiation absorbed into the rooflight materials which is then re-radiated and conducted into the internal space. The amount of absorbed solar radiation depends on the mass of the window or rooflight assembly and the materials making up that mass. For this reason, the solar heat transmission generally correlates quite closely with the light transmittance.

The transmission characteristics of plastics such as polycarbonate, and polyesters used in glass reinforced polyester (GRP) products, tend to allow more heat energy through the material in the near infrared regions than glass. However, when clear polyesters are combined with glass in GRP for rooflight sheets, the glass reinforcement tends to reduce this infrared element in proportion to the glass content of the material.

It is important that this relationship between the two properties is understood as specifications are now routinely being produced where it is very apparent that the values for both light transmittance and g-value are being entered independently of each other into building energy modelling software, such as SBEM, until the optimum output results are achieved.

While there are additives, coatings, films etc available to reduce the solar heat gains through windows and rooflights, these also reduce the light transmittance of the materials in all systems other than some high specification glazing systems.

The consequences of using data not applicable to any rooflight system can result in a compliant energy modelled design that cannot be delivered on site because the performance of the required product is simply unachievable. To avoid this situation arising, the manufacturers of rooflights can advise on options that most closely meet the required performance criteria.

For all your commercial, industrial and warehouse type building needs, please see the range of Zenon Premier and Optimum specifications presented concisely to aid selection. These specifications are on NBS Source, with a full breakdown of performance data. For any queries, please contact Hambleside Danelaw on marketing@hambleside-danelaw.co.uk, or call them on 01327 701 920.