060 RCI 0717

RCI July 2017

ROOFLIGHTS & ROOF VENTS Establishing the optimum rooflight area NARM, The National Association of Rooflight Manufacturers, has played an important role representing the UK’s rooflight industry in consultation with the Government and its subcontractors, CIBSE, AECOM and the BRE, on issues relating to Part L of The Building Regulations. The following article by NARM’s Technical Committee explains how small differences in rooflight It is now well understood that correctly area can have a significant impact on light levels and energy usage specified rooflights save energy and reduce CO² emissions, making them a critical contributor to meeting Part L of The Building Regulations covering the Conservation of Fuel & Power and the equivalent regional regulations. Independent research by the De Montfort University’s Institute of Energy and Sustainable Development has shown that minimal losses in thermal insulation are greatly offset by energy savings resulting from reduced demand for electric lighting and that generally speaking, the greater the rooflight area the greater the potential savings. However, there is a limit before overheating may become an issue, so an optimum area needs to be identified. There is never a specific solution in respect of rooflight area – judgement is required on a project by project basis. However, the example shown below (see graph) demonstrates how data can be interpreted to inform a decision. NARM can supply data for different building locations, for rooflights with vary-ing degrees of light transmission and for different daily time windows. Building use The first consideration in establishing rooflight area, is the use of the building. As a typical example, in retail and manufacturing areas, the recommended light level is 500 Lux. Establishing the appropriate rooflight area to achieve the desired light level is the next point to consider. The larger the rooflight area, the more hours each year the required light level will be provided by natural light. This is the crucial factor affecting energy and emissions reductions – as during these hours, the need for electric lighting is removed. This graph shows how rooflight areas will affect illumination levels 060 JULY 2017 RCIMAG.COM “The first consideration in establishing rooflight area, is the use of the building. As a typical example, in retail and manufacturing areas, the recommended light level is 500 Lux” In this example for a single storey building located in London, between the hours of 6am and 6pm daily, with rooflights providing 50% light transmission, you can see that as the rooflight area along the bottom of the graph increases, the length of time a given illumination level is achieved, is extended. So for a building requiring 500 Lux, looking at the yellow line you can see that with 10% rooflights, 500 lux would be achieved for approximately 2000 hours or 46% of the working year. If rooflight area is increased to 15%, 500 lux is then available for just over 2,500 hours, or 58% of the working year. So a building with 10% rooflights will require electric lights to be turned on for approximately 30% more hours per year than if the building was fitted with 15% rooflights. Note that the savings will only be made if the lights are switched off during periods when they are not required. For this reason automatic lighting controls should always be specified as part of the project. Elmhurst Energy research Recent research carried out by Elmhurst Energy supports these figures and highlights savings resulting from improving daylight levels and installing lighting controls in existing buildings. The results show savings of up to £5.92/m²/yr, and savings in CO² emissions up to 28.7kg CO²/m²/yr can be achieved, with results from industrial and retail buildings very consistent. Slightly lower savings were achieved in the rooflit areas of a school (savings in running costs of £2.92/m²/yr, and savings of CO² emissions of 14.7kg CO²/m²/yr) primarily because significant sections of the rooflit areas were also well lit by windows, reducing the effect of improved illumination through rooflights. These figures demonstrate that correctly specified rooflights will provide outstanding long-term returns on investment with short payback periods – as well as contributing to improved sustainability credentials for the building occupier. For further information, the following documents can be downloaded free of charge from: www.narm.org.uk NARM Technical Document NTD 06.2 ‘Designing with Rooflights Supporting Part L Building Regulation’ NARM Technical Document NTD 10 “An independent report by Elmhurst Energy, on Improving Daylighting and Lighting Controls on existing non-domestic buildings’. “There is never a specific solution in respect of rooflight area – judgement is required on a project by project basis. However, the example shown here demonstrates how data can be interpreted to inform a decision”


RCI July 2017
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