Too Much of a Good Thing?

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 the Government’s original target of project carbon neutrality for non-domestic buildings by 2019.

While considering the daylighting plan of any building, particularly relatively lightweight buildings such as those with metal clad envelopes, designers need to remain aware of the potential for overheating caused by excessive solar gain where there is no adequate ventilation strategy, or where there are significant heat gains due to internal processes. The design of the building and provision of rooflights has to balance several factors, in particular the perceived conflict between providing daylight and the associated energy savings together with the risk of overheating.

For any building, there is an optimum target percentage of rooflights which will deliver the optimum level of natural daylight into the building, making the optimum saving in energy usage and costs. Beyond that point, solar gain can add to the energy consumption if powered cooling systems become necessary. In high internal volume industrial buildings, the most appropriate, but uncontrolled action, to counter overheating in the building will often be to increase the ventilation by opening doors.

The ‘g-value’ is a measure of the total solar heat energy that passes through a window or rooflight, most of which is directly transmitted through the material or construction in the visible light spectrum. Total solar heat gain includes directly transmitted solar heat and absorbed solar radiation, which is then re-radiated and conducted into the space. For this reason, the solar heat transmission correlates closely with light transmission. If properly considered at the design stage and well managed during the building’s service life, in a relatively temperate climate such as the UK, passive solar gain usually provides a benefit for most of the year, with the overheating effects only being a consideration for the hottest few weeks of the summer months.

sunA study by De Montfort University concluded that the optimum rooflight area for minimum energy consumption is generally in the region of 15% to 18% of the buildings floor area, for a large single storey industrial type building. If wall-lights are being considered, then the figure can be increased to 20%. A Rooflight Association (RA) study commissioned with Oxford Brookes University concluded that evenly distributed rooflights up to 20% of the roof area can be used without significant solar overheating.

Section 12: Lighting of HM Government’s Non-Domestic Building Services Guide 2013 edition states that for a space below rooflights to be classed as ‘daylit’, the rooflight area should be at least 10% of the floor area with light transmittance at least 70%. If the transmittance is below 70% then the rooflight area should be increased proportionately.

Notwithstanding this, Criterion 3 in Building Regulations Approved Document L2A requires the effects of heat gains in summer to be limited to reduce the need for air-conditioning, or the energy consumption of any air-conditioning system that is installed. For buildings defined in the National Calculation Methodology database as ‘top lit’, those whose zone height is less than 6m, a total of 10% rooflight area with a framing factor of 25% and a g-value of 68% is recommended. For buildings whose zone height is greater than 6m, the rooflight area increases to 20% with a framing factor of 15% and a g-value of 46% to go some way into taking account the effects of stratification, or the effect of internal temperatures being cooler at lower level occupied spaces.

These recommended rooflight areas are calculated making certain assumptions on a limited range of light, thermal and solar transmission values for rooflights, whereas the designer has the freedom to specify a wider variation of product to meet the specific needs of the building. It is therefore recommended that the designer uses the data provided from the manufacturer, obtained through responsible independent physical testing. In the absence of any specific BS or EN standard governing the measurement of solar energy transmission through ‘plastic’ type rooflights, there is a clear need to ensure that the information provided on this issue by rooflight manufacturers, is relevant and specific. Hambleside Danelaw use full solar spectrum transmission data from physical testing by The National Physical Laboratory calculated in accordance with BS EN 410 for all rooflight assembly data produced.

For more specific technical requirements, please email or call 01327 701920