Roofing

Rooflights
How Rooflight Area Affects Light Levels, Energy Costs and CO 2
Emissions
NARM’s Technical Committee, explains how small differences in rooflight area can have a significant impact on light levels and energy usage.
It is now well understood that correctly specified rooflights
save energy and reduce CO 2 emissions, making them a critical
contributor to meeting Part L of The Building Regulations
covering the Con-servation of Fuel & Power and the equivalent
regional regulations.
Independent research by the De Montfort University’s Institute
of Energy and Sustainable Devel-opment 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, 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.
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
(as listed in CIBSE Guide A).
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. In this example - for a single storey building located in London, between the
hours of 6am and 6pm daily, with rooflights provid-ing 50% light transmission - you
can see that as the rooflight area along the bottom of the graph in-creases, 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 the rooflight area is increased to 15%, 500 lux is then available for just over 2500
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.
Automatic Controls
It’s important to note that the savings will only be made if the lights are switched off
during periods when they are not required and for this reason automatic lighting
controls should always be speci-fied as part of the project.
Recent research carried out by Elmhurst Energy supports these figures and gives
detailed savings in building running costs resulting from improving daylight levels
and installing lighting controls in existing buildings. The results show savings in
running costs of up to £5.92/m 2 /yr, and savings of CO 2 emissions up to 28.7kg
CO 2 /m 2 /yr can be achieved, with results from industrial and retail buildings being
very consistent. Slightly lower savings were achieved in the rooflit areas of a school
(savings in running costs of £2.92/m 2 /yr, and savings of CO 2 emissions of
14.7kg CO 2 /m 2 /yr) pri-marily 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 an
outstanding long-term return on investment with short payback periods – as
well as contributing to improved sustainability credentials for the building
occupier.
For a deeper insight into energy saving with rooflights, 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’.
Call 01483 271371 or
email
sales@whitesales.co.uk for
more information.
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