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Trademarks of CorusColorcoat, Colorcoat Connection, Confidex,Confidex Sustain, HPS200, Prisma and are trademarks of Corus.

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Copyright 2007CorusLanguage English 0807

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Corus Colors

Integrated lighting solutions for low energy buildings

Colorcoat® Technical Paper

August 2007

There’s only one true

Ensure that it’s by Corus

2 Colorcoat Connection® helpline +44 (0)1244 892434

Corus and lowenergy designSince the 1970s, Corus have been at theforefront of technology in pre-finished steelbuilding envelopes. In more recent times, theneed to conserve energy and reduce the buildingcontribution to climate change has informed thedirection for envelope design. 90% of the CO2

emitted from a building comes from the usephase and Corus are now actively researchingmethods for the building envelope to contributeto the minimisation of this. The first Colorcoat®

Technical Paper “Creating an air-tight buildingenvelope” gave building designers and installerspractical guidance on minimising heat-lossthrough air-leakage. This paper continues thistheme of low-energy buildings, examining thebalance between natural and artificial lighting.

In providing an ongoing commitment to the future of the building envelopemarket, Corus have established theColorcoat® Centre for the buildingenvelope at Oxford Brookes University.Located within the Oxford Institute forSustainable Development and one ofthe largest schools of architecture inthe UK, the Centre is commited toproviding cutting-edge research to develop the future of pre-finishedsteel building envelopes.

The work reported here has utilised theleading expertise of Oxford BrookesUniversity, together with advancedcomputer modelling from the Corus R&Dlaboratories to provide a definitive viewof lighting strategies for low-energycommercial and industrial buildings.

Working together to drive design

28Colorcoat Connection® helpline +44 (0)1244 892434

The Colorcoat® brand

The Colorcoat® brand is the recognisedmark of quality and metal envelopeexpertise from Corus. With over 40 years experience, we activelydevelop Colorcoat® products andprocesses to reduce their environmentalimpact to a level beyond merecompliance. All Colorcoat® productsare manufactured in factory controlled conditions, providing clear advantages onsite in terms of speed of construction andminimising social disruption.

Colorcoat® products manufactured inany UK Corus site are certified to theindependently verified internationalmanagement system, ISO14001 and100% recyclable, unlike most otherconstruction products.

Colorcoat® products offer the ultimate in durability and guaranteed performancereducing building life cycle costs andenvironmental impact.

Corus has detailed Life Cycle Costing andLife Cycle Assessment information thatdemonstrates the positive performanceof Colorcoat® products when comparedwith other alternatives. This is availablefrom www.colorcoat-online.com

Colorcoat HPS200®

Corus has demonstrated that ColorcoatHPS200® can be recycled without additionalburden to the environment. It has beeneco-designed to exceed future legislativerequirements and reduce its environmentalimpact, providing a long-term sustainablebuilding envelope solution.

More information about the environmentalperformance of Colorcoat HPS200® isprovided in an Environmental ProductDeclaration. This is available fromwww.colorcoat-online.com

Colorcoat Prisma®

The ideal choice to deliver eye-catchingbuildings that will stand the test of time.Technically and aesthetically superior toPVDF (PVF2), Colorcoat Prisma® is readilyavailable in the most popular solid andmetallic colours. All backed up by thecomprehensive Confidex® Guaranteeproviding cover for up to 25 years on walls.

Confidex® GuaranteeOffers the most comprehensive guaranteefor pre-finished steel products in Europe

and provides peace of mind for up to 30 years. Unlike other guarantees,Confidex® covers cut edges for the entirety of the guarantee period and does not require mandatory annual inspections.

Confidex Sustain™

Provides a combined guarantee whichcovers the durability of the Colorcoat®

pre-finished steel product and makesthe pre-finished steel building envelopeCarbonNeutral – the first in the world.Unavoidable CO2 emissions from thepre-finished steel cladding systemincluding fixings and insulation, aremeasured from cradle to grave and the impact offset. More than justoffsetting, the aim is to encouragespecification of the most sustainablepre-finished steel products and cladding systems.

Colorcoat® Building manualDeveloped in consultation witharchitects and other constructionprofessionals, the Colorcoat® Buildingmanual incorporates over 40 years of Colorcoat® expertise. It containsinformation about sustainabledevelopment and the creation of a sustainable specification.

If you require any further informationplease contact the Colorcoat Connection®

helpline on +44 (0)1244 892434.Alternatively further information can befound in the Colorcoat® Building manualor at www.colorcoat-online.com

Colorcoat® products and services1 Approved document L: Conservation of fuel and power (2006 edition).

2 Lighting levels: CIBSE Concise Handbook, Chartered Institute of Building ServicesEngineers, 2001.

3 ACR(CP) 001: 2003 Recommended Practice for Work on Profiled Sheeted Roofs (orange book).

4 TM37: CIBSE Technical Manual TM37 ‘Design for Improved Solar ShadingControl’, CIBSE 2006.

5 CIBSE Guide A, Environmental Design CIBSE, 2006.

Further information

27www.colorcoat-online.com

ContentsOverview 3

Lighting levels 4

Daylighting 5

Artificial lighting 6

Use of roof lights 7

Building modelling 9

The effect of rooflight area on availability of natural light 11

Solar gain and overheating 14

The air-conditioned building 19

The naturally or mechanically ventilated building 20

The effect of internal racking 21

The effect of geographical location 23

The effect of rooflight transmittance 24

The effects of global warming 25

Conclusions 26

Further information 27

The Colorcoat® brand 28

Overview

Half of all energy consumed in the UK isused in buildings, mainly in heating andlighting but increasingly also in cooling.Minimising energy use is essential both in reducing the building operating costsand as part of an approach to tackle CO2

emissions and global climate change.This requirement to provide moreefficient buildings is the driver behind thelatest revisions of building regulations forthe conservation of fuel and power,namely Approval Document L in Englandand Wales, Part F in Northern Irelandand Section 6 in Scotland.

The latest building regulations provide a new format for the compliance methodfor new buildings other than dwellings. A minimum overall energy performancein terms of a limit on CO2 emissions mustbe met for each new building and iscalculated by using the Simplified BuildingEnergy Model (SBEM). This signals achange away from considering individual

elements of a building towards an all-encompassing view of the buildingwhere various factors each have aneffect on overall energy use and can be considered on their merits.

Lighting accounts for 20 to 25% of allelectricity consumed in Northern Europe.Typically industrial and commercialestablishments consume 20 to 30% of their total energy just for lighting.Rooflights can be readily integrated with profiled pre-finished steel claddingsystems as part of a strategy to reduce overall energy consumption by providing natural daylight.

This Colorcoat® Technical Paper uses full-building energy modelling to assessthe benefits provided by rooflights in largecommercial and industrial buildings.Rooflights can readily be incorporated intoa pre-finished steel roofing system andwhen combined with an effective control

system can help to minimise energyusage by providing natural lighting anduseful solar gain to offset the increasedenergy losses through the increased areaof low insulation. Large areas of rooflightscan lead to excessive solar gain causingthe building to overheat, a goodapproach is to use a low level of naturalbackground light with point lightingwhere required. Covering 10% of the roofarea with rooflights gives a good startingpoint for designing a partially daylitinterior. Natural lighting from rooflights is most effective in wide, open spaces,where a slightly increased rooflight areamay be considered. High-bay rackingand similar bulky equipment can have a dramatic effect on light availability. A reduced level of rooflight area andsuitably positioned aisle lighting willprovide an effective strategy. Theguidance here explores how to do this tooptimum effect without creating excessivesolar heat gains or thermal losses.



Artificial lighting

At the start of the lighting design process,it is important to define the requiredlevel of lighting. The building designermust select the most appropriate lightinglevel for the proposed activity within thebuilding. Increased levels of concentrationand intricate tasks will require higherlevels of illuminance. Even in speculativebuildings, the general class of final use

is often known, but assumptions mayhave to be made. Consideration shouldalso be given to potential future changeof use of the building.

It is generally accepted that a degree of natural lighting will enhance theworking environment. The colour andreflectivity of the internal surface of

the walls and roof will affect the internalilluminance. Use of a “bright white” pre-finished steel liner sheet will providean excellent internal surface finish forgeneral lighting requirements.

The following table gives some generalCIBSE guidelines2 for recommendedilluminance levels for different activities:

Lighting levels

Light output (luminous flux) is measuredin lumens (lm). The lumen is defined asthe amount of light emitted per secondon unit area placed at unit distance froma one candela light source. Illumination ismeasured in lux (lx), defined as one lumenper square metre.

Daylight Factor (DF) is defined as theratio of the actual illuminance at a pointin a room to the illuminance from anidentical unobstructed sky and is the

standard method of expressing thedaylighting performance of an internalspace. A design value for sky illuminanceof 5,000 lux is often used, being theequivalent of a heavily overcast, diffusesky. In the UK, this value is exceeded for 85% of working hours. In this case, a space with a Daylight Factor of 6%would have a illuminance of 300 lux.Daylight factor within a building will vary and can best be represented by a contour diagram.

Units of light

Standard maintained Activity/ interiorilluminance (lux)

200 Foyers, entrances, automatic processes

300 Libraries, sports halls, food court packing, warehouses

500 General offices, assembly, retail shops

750 Drawing office, supermarkets, showrooms

1,000 DIY superstore

Table 1. Recommended illuminance

It should be noted that the lightingdesign level has a major impact on theenergy consumption and resultant CO2

emissions. For low energy design, thelowest sensible lighting level should bespecified. A practical approach is tospecify a low level of backgroundlighting which can come mainly fromwindows or rooflights, with localised“point” lighting in areas where higherilluminance is required.


Daylighting can provide psychological,physiological and energy-saving benefitsproviding it is sensibly applied. For mostpeople, who spend over 80% of theirwaking hours inside buildings, daylightis welcomed, providing a link with theexternal, natural environment and itschanging conditions. Although studieshave not shown that productivity isdemonstrably increased, a positivecorrelation has been proven betweenoccupant satisfaction with the indoorenvironment and natural light levels.Overall, it is recognized that a sense of well-being is engendered in daylitinteriors, leading to improved moraleand loyalty of staff. In practical terms,good quality, diffuse daylighting canreduce strong shadow effects on vertical surfaces such as boxes inracking, enabling easier identification.

All building lighting regimes shouldcombine the available natural daylightwith efficient artificial lighting and aneffective control system.

There are two options for providingdaylight to the interior of large singlestorey buildings:

High-level vertical glazing elementsKnown as clerestory glazing within the exterior walls. The glazing ratio is limited by the proportionally low area ofwall, in relation to roof area. Their maindisadvantage is the rapid decay in natural lighting (daylight factor) withdistance from the window, an importantconsideration given the deep plan natureof many buildings. For this reason, theyare not suitable for multi-bay buildings.Positioning of high bay racking or bulkyequipment will also obstruct natural lightpenetration from the sides. Daylight qualityand quantity is highly dependent uponorientation of the building. Large areas ofvertical glazing can also result in localisedsolar gains and can make some kinds of work difficult.

RooflightsConsisting of translucent elementswithin the roof cladding construction,through which natural daylight can bewell distributed, with little dependenceupon orientation at low roof pitches.

There is a wide range of rooflight typesand constructions, which are dependanton the choice of cladding system, butthese fall into two categories:

• In-plane, where the rooflight is profiled to match the roof cladding.This is the predominant design usedwith pre-finished steel cladding onportal framed buildings. These aregenerally manufactured from GlassReinforced Polyester (GRP), Poly VinylChloride (PVC) or Polycarbonates.

The thickness varies from 1mm to3 mm dependant on the material used and the structural requirements.

• Out-of-plane, which include barrelvault and domed designs. These aredesigned to sit above the roof level.They are fixed to a kerb or upstand,which is attached to the cladding. The upstand must be insulated butwill introduce some additional heatlosses. This type of rooflight isgenerally only used on membrane or standing seam type roofs where in plane rooflights are not possible.Out-of-plane lights are generally more expensive, provide less light and generate more potentiallyproblematic interfaces.

Rooflights are generally constructed as double or triple skin arrangementsand can either be fabricated on site orpreassembled in a factory. In practicemost rooflights will need to be triple skin to achieve the limiting U valuestandard of 2.2 W/m2K as specified in the latest building regulations. Lower U values are available with some systems. Surface coatings can be applied to the rooflights to improvedurability and reduce build up of dirt, which would reduce the level of light transmittance.

Benefits of daylighting

Daylighting

Provision of daylight inlarge single storey buildings

A daylit space is defined withinApproved Document L1 as a space:

• within 6 metres of a window wallprovided that the glazing area is at least 20% of the internal area of the window wall; or

• below rooflights and similar providedthat the glazing area is at least 10% of the floor area.

In practical terms for large single storey buildings, 10% rooflight area can be considered as a goodstarting point when considering a daylit requirement.

Definition of daylit spaces

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Artificial lighting in large volume portalframe buildings has traditionally beenprovided by high or low bay reflectorluminaries with high outputs of up to1000 W. These luminaries typically useMetal Halide lamps which produce anintense bluish bright light that is easy onthe eyes. Although Metal Halide lampshave high energy efficiencies they havedisadvantages when designing toreduce CO2 emission levels. Somelamps have a ‘warm up’ period that cantake up to 7 minutes for the lamps toreach the maximum output making themunsuitable for on / off controls and moreexpensive specialist lamps are requiredif dimming is required. To take fulladvantage of the benefits of daylighting,an efficient lighting control system isrequired to maintain constant light levelswhich is difficult to implement with highoutput Metal Halide lamps.

A modern and energy efficient approachis to use T5 Fluorescent High Bay fixtures.Although the initial cost is higher, T5fluorescent fixtures require less energythan metal halide, and offer improvedcontrol capability and lower maintenancecosts. A typical 54 Watt T5 lamp producesup to 5,000 lumens and will lose only 5-6%of lumen output over it’s life whilst metalhalide lamps can deteriorate by 35% overthe same period. Office lighting fixturescan also utilise the benefits of T5 lamptechnology. In this case, T5 lamps providean even ambient light on the ceiling surface,eliminating high contrast light and reducingthe risk of eyestrain. T5 fluorescent lampsare ideal for use with occupancy sensorsand photo cells, and are cost effectiveand versatile when dimmed.

It is important to note that any strategy fora low energy building should start withintrinsically efficient components; the useof energy efficient lighting with a goodcontrol system should be considered aspart of a strategy in conjunction withappropriate areas of rooflight.

Artificial lighting types

Artificial lighting

An effective operational control system is essential to minimise the use of artificial lighting and gain themaximum benefit of natural daylightfrom installed rooflights. Without anefficient control system, natural daylight from windows or rooflights will have limited beneficial effects on energy usage or carbon footprint.

Control systems can work on the basis of:

• Hi-Lo dimming – switching betweentwo power output settings. This canbe used with most lamps, but whenoperating at the low level setting, lampsoperate at a much lower efficiency.This is a relatively basic control systemwhich is inexpensive, but does notdeliver the energy-saving reductionsof more sophisticated systems.

• Automatic switching – switchingindividual lamps on and off. This isonly really applicable with fluorescentfixtures where there are several lower-output lamps. Again, this is a relatively simple option, but doesnot achieve the best benefits.

• Continuous dimming. This is onlyapplicable to fluorescent lamps such as T5 and is the most complex system,but does deliver the best energy-savingresults as the lighting is most closelymatched to the availability of natural light.

All control systems usually incorporate a time delay or difference between the light intensities at which lamps areswitched on and off to prevent overfrequent switching, for example whenclouds pass over the sun.

Good commissioning of control systemsis essential to ensure that performanceis “as designed” and meets operationalrequirements. A poorly designed systemmay result in manual over-ride and failureto deliver the designed energy savings.

The computer modelling used to generatethe recommendations given in thisColorcoat® technical paper assumes theuse of an efficient continuous dimmingcontrol system. It is important to note that the energy savings reported here will not be achieved unless this approachis adopted in practice.

Lighting controls

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When considering the percentagerooflight area for a building, the designerwill need to look at the interaction of anumber of conflicting requirements and

aim to achieve a balance, which satisfiesthe building operations and regulatoryrequirements as well as minimisingoverall energy usage.

Factors, which must be consideredwhen selecting rooflight area

Hours of operation

Fig. 1. Effects of rooflights on building operations and environment

• Provision of natural daylight.• Reduced CO2 from lighting.• Solar heat gains in winter.

Positive effects

• Increased fabric heat losses.• Excessive solar gains and

overheating during summer.• Cost of installation.• Increased maintenance requirement.

Negative effects

Lighting type /efficiency

Control system

Heat generation

Air conditioningpassive cooling

Internal colour and reflection

Solar overheating

Natural daylighting

Solar gain

Increasedfabric loss

Rooflight• Size /% area• Transmittance• U values• Layout

Required lighting level for operations (lux)

Summary of effects of increased level of rooflights

Use of rooflights


Artificial lighting

The arrangement of rooflights shouldaim to give an even distribution of light.In some circumstances additional orreduced areas of rooflight could beconsidered for areas of different activitywithin the building. However, thisapproach could cause issues if there isa future change of use of the building,so in general, rooflights are distributedwith relative uniformity over the roof area.

There are four general approaches for installation of rooflights:

1 Chequerboard2 Continuous3 Ridge to eaves4 Mid-slope

Fig. 2. Options for rooflight layout

Chequerboard Continuous Ridge to eaves Mid slope

Considerations when specifying rooflight layout

Rooflights are significantly more fragile than pre-finished steel cladding.Even when specified as “non fragile” it is wise to design such that they willnever be walked over. This considerationmust be taken into account whenspecifying the layout and wouldgenerally discount the 'continuous' and 'ridge to eaves' arrangements. In all cases the designer should ensurethat the position of rooflights is obviousthroughout the life of the building, taking account of any colour fade/discolouration, which may occur. It iscommon practice to use fasteners with a Poppy Red coloured head aroundrooflights to ensure their visibility. For further information regarding safetypractices for work on roofs androoflights, refer to ACR(CP)001:20033,“recommended practices for work on profiled sheeted roofs”.

The supplied and installed cost ofrooflights vary greatly dependant on type and ease of installation. As ageneral guideline, in-plane rooflights

will cost in excess of double the price of a similar area of insulated pre-finishedsteel cladding. For this reason, it iseconomically sensible to specify theminimum area of rooflights, which will give the majority of the daylighting benefits.

Out-of-plane rooflights are significantlymore expensive and are far morecomplex to install. This must be takeninto account when comparing the overall costs of a profiled metal roof with in-plane rooflights against otherconstructions, such as flat roofs, which require out-of-plane rooflights.

As rooflight area increases, there arediminishing returns on natural lighting,combined with increasing risk of solaroverheating. The optimum area willdepend on a number of factors. The results of full building energymodelling reported in this Colorcoat®

Technical Paper give guidelines as to this optimum level for a variety of building operations.

In general a mid-slope approach offersthe most practical solution with a goodbalance between an even distribution of natural light, without the increasednumber of potentially problematic

interfaces created by a chequerboardlayout and maintains easy access over the entire roof structure formaintenance, unlike ridge to eaves and continuous arrangements.


The Corus Colorcoat® centre for theBuilding Envelope based at OxfordBrookes University and Corus Researchand Development have examined howvarying rooflight percentage area affects:

1 The available natural lighting at agiven illuminance.

2 The risk of solar overheating.3 The risk of overheating from the

combined effect of solar gain andinternal processes heat generation.

4 The differences between air conditionedand naturally ventilated buildings.

5 The light distribution in a building with internal fitments.

6 The effect of different geographicallocation.

7 The effect of reduced lighttransmittance typical throughdeterioration in service.

The results have been derived from acombination of dynamic thermal simulationand daylighting analysis. Two differentanalysis packages have been used toensure consistency of results. In eachanalysis the effect on overall buildingenergy use and CO2 emissions wasdetermined. Thermal simulation wasundertaken with both Tas and IES, usinghourly recorded weather data for theUK. These tools require the geometry of the building and all constructions and thermo physical details of thematerials used. In addition, information

on internal heat gains (people lightingand equipment) has been used tocalculate heat loads and thermal comfortindicators throughout the year.

In all cases, lighting gains were controlledin accordance with modelled availabledaylight in the space to give the requiredilluminance. Daylighting analysis wascarried out using both Lumen Designerand Radiance. In the former case, anaverage daylight factor (percentage ofoutside illuminance) for the entire spacewas calculated. In the latter, daylightfactor was calculated per square metrethroughout the space, enabling moreprecise control of lighting. In practice,lighting would be controlled by zoning,each zone being provided with a lightsensor to either switch off or dim thelamps within that zone in response tolocal daylight availability. In practice,modelling will always overestimate theenergy efficiency of the building as controlsystems are not 100% efficient andlamp efficiency will degrade over time.

Principle calculations have not used the Simplified Building Energy Method(SBEM) as used for regulatorycompliance since this is not advancedenough for these kind of calculations.The recommendations arrived at fromthe modelling will provide a goodstarting points for compliance throughSBEM or other available software.

Modelling tools

Building modelling


Building modelling

A typical 66x48 m (3,168 m2) out of town retail building was taken as thebase building. It was assumed to haveinsulated pre-finished steel claddingwith U-values sufficient to comply with 2006 regulations.

Rooflights were assumed to be evenlydistributed across the roof and varied in length to give different percentages of total roof area. They were of tripleskin polycarbonate construction, with U-value of 2.0 W/m2K and lighttransmission of 0.64, which is typical for commercially available triple skin rooflights.

A number of base parameter for the building were set, some of which were then varied to simulatedifferent situations:

• Lighting: 5.6 W/m2

• Equipment: 2 W/m2

• Ventilation: 0.12 ac/h• Infiltration: 0.5 ac/h• Cooling: 25°C (when used)• Heating: 19°C• Occupation: 6 days 0800 h -1800 h

or 7 days 24 hour operation

• Location: S.E. England

Carbon dioxide emissions resulting from gas heating and electricity usewere taken to be as quoted in the 2006revision of Approved Document L1:

Natural gas: 0.194 kgCO2/kWhGrid electricity: 0.422 kgCO2/kWh

The results obtained are specific to themodelled building, however the principlescan be applied to most buildings of thisform. It should be noted that changes to building geometry, inclusion of largeareas of glazed façade and buildingorientation will all have an effect on the actual results.

The modelled building

Fig. 3. Tas representation of modelled building

From the daylighting model, usingdifferent settings for required lightinglevel, it was possible to determine thepercentage of occupied hours thatartificial lighting could be switched off.The model has assumed that the lightingcontrol system is 100% efficient andthat the artificial lighting requirement is continuously varied to maintain therequired light intensity. In practice thecontrol system would be set such that it did not respond until the light level fell outside of preset upper and lower

levels. The result of this is that thecalculations over-estimate the energysaving from lights being switched off or dimmed due to available naturallighting. For day-time operations a more practical assumption would be an additional 10% CO2 emissions fromartificial lighting, to prevent excessiveswitching, dependant on the controlsystem and its settings. 24-houroperations will be more efficient as there will be long periods at night whenthe control is effectively constant.

Daylight availability and lighting control

The graphs below illustrate the modelledlight intensity at different percentagerooflight areas for:

1 8am to 5pm, day time operation. 2 24 hour operations.

It should be considered that operatingpatterns for the building may changeduring the design life.

The building has been modelled withoutany internal fittings (for example rackingor process equipment) and the resultsare most applicable to wide, openspaces such as sports halls. The effectsof internal racking are considered later.

8am to 5pm operationFor operations requiring 300 lux, theadditional natural daylight availabilitythrough increased percentage rooflightarea is very significant up to 10%. (whichprovides 75% of the maximum availablenatural lighting). Increasing rooflight areabeyond 10% does not yield a substantialincrease in daylight availability.

For operations requiring 1,000 lux, theadditional natural daylight availabilitythrough increased percentage rooflightarea is very significant up to 14%. (whichprovides ~60% of the available naturallighting). Increasing rooflight area beyond14% does not yield a substantial increasein daylight availability for the additionalinvestment. It should be noted that themaximum available natural lighting will be less than 100% occupied hours.

24 hour operationThe total maximum light availability for 24-hour operation is approximatelyhalf that for a day-time operation. During night-time operations rooflightsdo not provide any natural lighting orsolar gains and should be regarded as a source only of additional fabric heat loss. Although the shape of thecurves suggest that a higher level ofrooflights may be beneficial, this mustbe balanced against the additional fabric heat losses. In practice, a strategywhich adopts a slightly lower level ofrooflights for buildings where 24 houroperation is likely will give the lowestoverall energy usage.


10

20

30

40

50

5% 10% 15% 20% 25% 30%

0

Percentage rooflight area

Per

cent

age

hour

s (%

)

% occ hours 300 lx daylight % occ hours 500 lx daylight

% occ hours 1,000 lx daylight

The effect of rooflight area on availability of natural light

20

40

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5% 10% 15% 20% 25% 30%

0

Percentage rooflight area

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cent

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hour

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Fig. 4. Annual daylight availability (London, 8h-17h weekdays) Fig. 5. Annual daylight availability (London, 24/7)

The available natural light intensity for a range of percentage rooflight areas,has been modelled throughout the yearat different times of the day. The chartsbelow illustrate the light intensities for10% rooflight area. For considerableperiods of time during the day, the light intensity (lux) inside the building isconsiderably higher than the designed

requirement, particularly during thesummer months. This can be as high as1,200 lux. The main requirement foradditional artificial lighting will be duringthe winter months when there is limiteddaylight. Increasing percentage rooflight area, does not alter the lightavailability pattern but does increase theilluminance, particularly at higher levels.

200

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0 2 4 6 8 10 12 14 16 18 20 22 24

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The effect of rooflight area on availability of natural light

Mar

May

Jul

Sep

Nov

0 2 4 6 8 10 12 14 16 18 20 22 24Jan

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Hour of the day

- Illuminance (lux)

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nth

0-100 100-200

300-400 400-500

200-300

500-600

600-700 700-800 800-900

900-1,000 1,000-1,100 1,000-1,200

Fig. 6. Contour map of daylighting levels within the modelledbuilding using 10% rooflights throughout the day and year

Fig. 7. Slice through the contour map showing lighting levelsthrough the day on four specific dates

The graphs below show the naturaldaylight available at different dates in the year for 10% and 15% rooflightareas. The graphs demonstrate thatincreasing the percentage area ofrooflights has a small effect on the time below a design illuminance

value (eg. 300 lux) when artificial lighting will be required, but has a very dramatic effect on the maximum light available, particularly during the summer. This in turn can lead to increased solar over-heating.


300

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Fig. 8. Internal illuminance at different periods in the year

March

September

June

December

The solar gain through transparent or translucent elements of a buildingare dependant on the orientation of that element. The solar loads persquare metre averaged over day timehours for a July day are summarised in table 2.

The actual solar heating load inside the building is dependant on thepercentage area and the solar energytransmission of the rooflights.

Peak solar loads


In addition to letting beneficial daylightinto the building, rooflights also transmitinfrared radiation, which generates heat within the building. This process is referred to as “solar gain”. In winter,solar gains can provide additional heatwhich can reduce the peak heating load, however solar gains need to bemanaged to prevent overheating duringsummer periods. Overheating can resultin increased ventilation requirements orhigh air conditioning loads which willsignificantly increase energy demandand resultant CO2 emissions to maintaina comfortable working environment.

Studies of global warming carried out by UK climate impacts program (UKCIP)predict that in all scenarios an increase insummer temperatures will occur. This willincrease the risk of solar overheatingand highlights the importance of takingthis into account during the buildingdesign. The key concern is that there is a potential for significant increases in building energy consumption and

consequently CO2 emissions due to the increased use of mechanicalcomfort cooling systems.

Approved Document L1 requires that buildings should be constructed so that:

1 Naturally ventilated spaces do notoverheat when subject to a moderatelevel of heat gain.

2 Mechanically ventilated or cooledspaces do not require excessivecooling plant.

For Approved Document L1 compliance,a calculation is required to check theeffects of solar gain in summer to limithigh internal temperatures usingprocedures detailed in CIBSE TM37:Design for improved solar shadingcontrol, CIBSE 20064. The regulationsrequire that to achieve compliance, a number of passive measures are used to limit the negative impact of solar gain.

Approved Document L states thatreasonable provision would be to showfor every occupied space which is notair-conditioned that either:

a When the building is subject to the solar irradiances for July as given in the table of designirradiances in CIBSE Design Guide A5, the combined solar and internal casual gains (people,lighting and equipment) per unit floor area averaged over the period 06:30 to 16:30 solar time(GMT) is not greater than 35 W/m2. or

b The operating temperature within the building does not exceed 28°C for 1% of the occupied hours. This is the benchmark for thermalcomfort as used in CIBSE guide A5,dependant on the building use.

These criteria give a good basis forinvestigating the effects of solar gainthrough rooflights.

Criteria for solar overheating

Solar gain and overheating

Orientation Solar load on external surface W/m2

London Manchester Edinburgh

North 124 127 125

NE/NW 203 206 198

East/West 319 332 326

SE/SW 367 395 404

South 355 391 413

Flat / low pitch roof 655 672 647

Table 2. Peak solar loads for July


30

60

90

120

150

180

210

0

Solar gain

Day

s p

er y

ear

14 W

m^-2

16 W

m^-2

18 W

m^-2

20 W

m^-2

22 W

m^-2

24 W

m^-2

26 W

m^-2

28 W

m^-2

30 W

m^-2

32 W

m^-2

34 W

m^-2

36 W

m^-2

15% rooflights 12.5% rooflights10% rooflights 7.5% rooflights

The solar gains for increasing percentagerooflight areas have been calculated forthe model building and are illustrated in fig 9, which shows the number ofdays in a year when the solar heat gainswill reach a given value. The datapresented is for 7.5%, 10%, 12.5% and 15% rooflight area.

For a building with over 10% rooflightarea, there is an increasing risk of solar overheating. A building with 15% rooflight area will always have days with more than 35 W/m2 solar gains. The additional effects of internalprocess heat generation must also be taken into account and must beadded to the solar heat loads. This iscovered in the following section.

Effect of rooflight area on solar gainFig. 9. Days per year (y axis) for which solar gainsthrough the rooflights will exceed a given value (x axis)

Note. Approved Document L1 stipulatesthat 35 W/m2 should not be exceeded tolimit the risk of overheating.



This is further illustrated by the graphsbelow which are based on actualweather data. These show the internalsolar gains on specific days during theyear and demonstrate that with rooflight

area greater than 10% there is a muchgreater risk of the building overheatingbased on 35W/m2 average heating load. The overall solar gain is directlyproportional to the rooflight area.

15

30

45

60

75

0 2 4 6 8 10 12 14 16 18 20 22 24

0

Hour of the day

So

lar

gai

n (W

/m2 )

15

30

45

60

75

0 2 4 6 8 10 12 14 16 18 20 22 24

0

Hour of the day

So

lar

gai

n (W

/m2 )

15

30

45

60

75

0 2 4 6 8 10 12 14 16 18 20 22 24

0

Hour of the day

So

lar

gai

n (W

/m2 )

15

30

45

60

75

0 2 4 6 8 10 12 14 16 18 20 22 24

0

Hour of the day

So

lar

gai

n (W

/m2 )

15.0% 12.5% 10.0% 7.5%

Fig. 10. Solar gains at different periods during the year for varying percentage rooflight area

March

September

June

December

In most buildings, there is an additionaleffect of internal gains from the process,artificial lighting and people in the building.To assess whether overheating is likely,these internal gains need to be added tosolar gains to determine the total heat gain.

To demonstrate the effect of total heatgain in practical conditions, three internalprocesses have been modelled and the results presented here.

2 W/m2 Typical warehouse 24 hour operations.

25 W/m2 Retail outlet/DIY store. Daytime operations.

50 W/m2 Food processing plant 24 hour operations.


2

4

6

8

10

>25 >27 >29 >31 >33 >35 >37 >39

0

Internal temperature

Per

cent

age

of

occ

upie

d h

our

s

15.0% 12.5% 10.0% 7.5%

Effect of internal process

5

10

15

20

25

>25 >27 >29 >31 >33 >35 >37 >39 >41

0

Temperature

Per

cent

age

of

occ

upie

d h

our

s

15.0% 12.5% 10.0% 7.5%

Fig. 11. Time when temperature will exceed a given value (day time operation)

The low internal heat generation and lowlighting levels (300 lux), have a very limitedeffect on the overall internal heat gain,which is dominated by the solar effects.

Warehouse operations (2 W/m2 process heat gain)

Fig. 12. Time when temperature will exceed a given value (day time operation)

The internal heat gains from highergeneral lighting levels (500 lux) andlocalised point /display lighting aresubstantial. When combined with the additional solar heat gains, there is a significant period when 28ºC is exceeded.

When specifying rooflights, some formof ventilation or air conditioning shouldbe considered which will haveassociated additional CO2 emissions.

Retail outlet/DIY store (25 W/m2 process heat gain)



The matrix to the right summarises the risk of a building overheating forincreasing process heat generation and increasing rooflight area. Rooflight areas greater than 15% will almostcertainly lead to a certain amount ofoverheating. For buildings, which havehigh internal process heat gains, thedesigner will need to establish how he can minimise additional gains, combined with an effective ventilation or air-conditioning system.

10

20

30

40

50

60

70

>25 >27 >29 >31 >33 >35 >37 >39 >41 >43 >45 >47 >49

0

Temperature

- % rooflight area

Per

cent

age

of

occ

upie

d h

our

s

15.0% 7.5%

Fig. 13. Time when temperature will exceed a given value (24 hour operation)

The effects of the process heat generationare dominant and the building willoverheat and will require mechanicalventilation or air conditioning. In thissituation the percentage area of rooflightsmakes very little overall difference. The lower fabric U value of the rooflightsallows additional heat to escape, excepton very sunny days, when additionalsolar gain, through rooflights will occur.This can be seen where the modelledcases (7.5% and 15%) actually crossover on the graph. For this type ofapplication, careful consideration must be given to cooling or ventilation requirements.

Food processing plant (50 W/m2 process heat gain)

Effect of process heat generation and rooflight area on risk of overheating

Process heat gain Percentage area rooflights

5.0% 7.5% 10.0% 12.5% 15.0% 17.5% 20.0%

2 W/m2

25 W/m2

50 W/m2

Fig. 14. Risk of overheating for varying processes

Total heat gains are manageable

Careful consideration required to manage total heat gains

Total heat gains likely to cause over heating


Many buildings such as retail outletsand supermarkets have air-conditioninginstalled to maintain the internal operatingtemperature within a set range. This maybe a requirement of the processes within the building, storage requirementfor perishable goods, or to maintainoccupier comfort.

Air-conditioning control systems willusually be operated to maintain thetemperature between set upper andlower limits. Decreasing the acceptabletemperature range will have a verysignificant impact on the amount of time the heating/cooling system is

operating and will increase the total CO2 emissions. This is demonstrated by the graphs below for two differentcontrols.

The effect of percentage area of rooflightson overall CO2 generation is very marginal,as can be seen from these two graphs,is more dependant on other factors inthe building design. It should be notedin both cases that as the area ofrooflights is increased, the heating andcooling requirements also increase. Theeffect of heating and cooling on total CO2 emissions is balanced by the beneficialeffect of natural lighting, although this

will only be achieved if an effectivelighting control system is installed.

It must also be considered that manyretail premises have high levels of displaywindow glazing, which dependant on thegeographic orientation will also contributeto the effective solar load and subsequentair-conditioning requirement within thebuilding, as well as providing additionalfabric heat losses and additional heatingrequirement. However, display glazing oftenhas little effect on lighting levels and anysaving through reduced artificial lighting,so the effect of display glazing is generallyto increase the overall energy requirement.

10

20

30

40

50

60

70

10% 11% 12% 13% 14% 15%

0

Percentage rooflights

kgC

O2/

m2 /y

ear

The air-conditioned building

Fig. 15. Retail building (heated to 19ºC, cooled to 24ºC, 24/7): CO2 emissions vs % rooflight

10

20

30

40

50

60

70

10% 11% 12% 13% 14% 15%

0


kgC

O2/m

2 /yea

r

Heating (1)Cooling (2)

Heat + cool + lightHeating + cooling

Fig. 16. Retail building (heated to 20ºC, cooled to 22ºC, 24/7): CO2 emissions vs % rooflight


The majority of industrial buildings arenot air-conditioned and rely on eithernatural or mechanically generatedventilation. The use of mechanical ornatural ventilation generates far lowerCO2 emissions than air-conditioningequipment. In many cases, ventilation is created by simply opening accessdoors. This can create a securityproblem and allow dust and debris to be blown into the building.

The effect of solar gain and internaltemperature variations should always beconsidered to ensure that a satisfactoryworking environment is maintained.Increasing rooflight area has been shownto have a significant effect on increasing

the solar gain and temperatures within thebuilding. The graph below demonstratesthe large increase in internal temperaturevariation, with increased percentagerooflight area, for the modelled building.The temperature ranges have beencalculated, based on real weather datafor the summer period. It can clearly beseen from this graph that on the sunniestdays, percentage rooflight areas in excessof 10% can cause particularly large anduncomfortable variations in internalworking temperatures.

The temperature variations within thebuilding will be most extreme during the summer months when there are highlevels of solar irradiance, leading to very

high day-time operating temperatures,combined with ambient night timetemperatures. This effect can also be significant through out the year. Large temperature variations on individualdays will only result in small averagevariations when calculated throughout theyear. The graph below demonstrates the average daily temperature variation forincreasing rooflight area. Individual dailyvariations on the sunniest of days will be in the region of four times the averagelevel. As can be seen, increasing rooflightarea increases both the average andactual operating temperature range withinthe building. This increases the risk ofsolar overheating and unacceptabletemperature variation within the building.

3.5

4.0

4.5

5.0

5.5

6.0

5.0% 7.5% 10.0% 12.5% 15.0% 17.5% 20.0%

3.0


Average temperature variation

Ave

rag

e te

mp

erat

ure

vari

atio

n

5

10

15

20

25

100 120 140 160 180 200

0

Day of year

Tem

per

atur

e va

riat

ion

Maximum variation (WIP, 20.0, 24hrs)




The naturally or mechanically ventilated building

Fig. 17. Daily temperature variation throughout the summer period modelled with 5%, 10%, 15% and 20% rooflight area

Fig. 18. Average daily temperature variation for increasing rooflight area


The distribution of both natural daylightand artificial light within a building will behighly dependant on the presence andnature of internal equipment or racking.A building such as a sports hall with a wide open space and evenly spacedmid-slope rooflights, will have a fairlyconstant light intensity. However, theinstallation of internal equipment, and in particular high bay racking in a warehouse or distribution centre, will create areas of full and partialshadow, with much lower lightintensities. In this case, the available natural daylight will not be fully realised and high levels of additionalartificial lighting will be necessary.

A typical DIY retail building has beenmodelled in three cases:

1 With no racking in the building.2 With high bay racking installed with:

a The rooflights positioned directlyover each aisle.

b The rooflights positioned directlyover the racking.

In practice there will be a range ofpositions between these 2 extremes.

The following parameters for the building and racking have been used:

Height to the rooflight of 8mRacking height of 3 and 6mRacking width 3mAisle width 3mRooflight area up to 16.6%Standard CIE overcast sky

The building was modelled with 16.6%rooflight area, as this would correspondto 1m wide rooflights at a pitch of 6m,which corresponds to the pitch of theracking. In practice the pitch of therooflights may not correspond to thepitch of the racking. Lighting levels werecalculated at three different heights onthe front of the racking. The results areshown in table 3.

The effect of internal racking

Fig. 19. Modelled cases 19(a) and 19(b) showing examples of aligning rooflights either directly above aisles or directly above racking

Fig. 19(a). Rooflight over aisle

Fig. 19(b). Rooflight over racking


The effect of internal racking

Case Illuminance (lux)Top of rack @ 1m height Floor level

No racking, mid-slope rooflights 1,700

Rooflight over aisles. 3m racks 750 560 520

Rooflight over aisles. 6m racks 1,200 350 350

Rooflight over racks. 3m racks 580 520 400

Rooflight over racks. 6m racks 300 50 20

Table 3. Light intensities for the modelled cases

Effect of increasing rooflight areaAs the rooflight area increases, theoverall light intensity within the buildingwill increase, however this will alsoincrease the shadow effects in areaswhich are not directly lit. There may also be some areas, which are in directsunlight and may be subject to glare.Even with 16.6% rooflight area, there is inadequate lighting at the base of the taller racking. In general it is notalways practical to design the rooflightpositions around the racking layout. It must also be considered that theinternal material use or layout of the building may change during theservice life of the building.

It is generally a better option to use therooflights to provide a good backgroundlight level with artificial lighting alignedwith the aisles.

Effect of altering racking /aisle separation androoflight /racking heightIncreasing the aisle width in relation to the racking will increase the lightavailability in the lower positions on the racking and will decrease thecontrast between the top and bottom of the racking. However, to maximisewarehouse capacities there is a tendencyto minimise aisle width and maximiserack width and height. In both cases this will reduce the available daylightreaching the lower portions of the racking.To maximise the warehouse capacityracking is usually constructed as high asis practical within the bay. Decreasing theseparation between the rooflights andthe top of the racking, will again reducethe available daylight within the aisles.

In summary, where high bay racking (or large equipment) is to be installedwithin the building, the lighting fromrooflights is limited. A good approachwould be to use a modest level ofrooflights (approximately 7.5%) to give a good background light level with point lighting where required.

Practical experience of SBEM (used for proving compliance with ApprovedDocument L1) indicates that buildingswith less than 10% rooflight area havemore difficulty in achieving the targetCO2 emission rate. However SBEM does not take into account the reducedlighting effect created by high bayracking. The building designer will needto balance the SBEM requirementsagainst effective lighting gains.


As the latitude increases from SouthernEngland (London 51.5ºN) to Scotland(Edinburgh 55.9ºN), the solar altitudedecreases and weather patterns aredifferent. Sky luminance is decreased,reducing the mean interior illuminancefor the same daylight factor by 10-11%,as shown in Figure 20.

The total CO2 generated from heatingand lighting were calculated forSouthern England (Slough 51.5ºN) and Scotland (Edinburgh 55.9ºN) and are shown in Figure 21.For the cases studied, carbon dioxide emissions from heating were17% greater in Edinburgh, whilstemissions for lighting increased by 9%. It should be noted that theadditional lighting will also havecontributed to building heating.

In general with increasing latitude:

• The external ambient temperature will decrease.

• The average sky illuminance will decrease.

• Increased rooflight areas will be required to achieve the sameinternal level of natural daylight.

• Increased use of electrical lighting willgenerate internal heat gains which will partly offset the additional heatingrequirements generated by the lower external ambient temperatures.This creates a large increase in CO2 generation.

• The gains from increasing rooflightarea to maintain internal illuminancewill need to be carefully balancedagainst the increased fabric heatlosses through the rooflights

incurred due to the lower externalambient temperatures.

• The risk of solar overheating will be reduced, however this can behighly dependant on the local weather patterns.

In summary, comparing buildings further north to those in the south,overall building energy consumption and CO2 generated will increase onmoving north, although there are somany factors in play that this effect is difficult to predict. On moving north,the positive benefits of rooflights areless pronounced due to the lower lightintensities from the sky. However, usingthe approach of a modest level ofrooflights coupled to specific lightingwhere necessary is still the mostpractical solution.

200

400

600

800

1000

1200

1400

7.5% 10.0% 12.5% 15.0%

0

Rooflight percentage

London: average light level (lux)

Illum

inan

ce (l

ux)

Edinburgh: average light level (lux)

The effect of geographical location

5

10

15

20

25

13% rooflights (Slough) 13% rooflights (Edinburgh)

0

Location

Heating

CO

2 em

issi

on

s (k

gC

O2/

m2 /y

ear)

Lighting

Fig. 20. Effect of latitude on average light levels at midday for different percentage rooflights (London 51.5 ºN, Edinburgh 55.9 ºN)

Fig. 21. Effect of latitude on CO2 emission


Depending upon the materials used(polycarbonate or GRP) and the numberof layers (double or triple-skin), in-planerooflights can have a light transmissionfactor of 0.6-0.8 when newly installed,meaning that between 60% and80% of available light is transmitted.However, over the 20-25 year servicelife, a number of factors combine toreduce light transmission:

• Discolouration (yellowing) due toageing of the material by exposure to solar ultraviolet light. Modernrooflights suffer less from this than has historically been the case.

• Moss, lichen and mould growth.• Atmospheric particulate

deposition (soot, dust, etc).• Bird fouling.

The extent of the light transmissiondecrease depends upon the materialused and the cleaning schedule.The effects can be minimized byusing low discolouration polycarbonate,as specified by some manufacturersand by regular inspection and cleaningof the rooflights. Some deteriorationin daylight performance is unavoidableand should be allowed for in theinitial calculations.

Reduction of light transmission reducesdaylight factor and solar gain and henceaffects energy requirements for bothartificial lighting and heating. An extremecase for a 40% reduction in rooflighttransmission shows that lightingrequirements rise as expected, whilstthe decrease in solar gain, is largelybalanced by the extra heat produced bylighting, leaving the heating requirementslittle changed. See the table below.

In summary, as rooflight transmissiondecreases with age, total energy use will rise through a combination offactors, in this case by 6%.

850

900

950

1000

1050

1100

0% 5% 10% 15% 20%

800

Percentage reduction in light transmission

Effect of reduction in light transmission

Illum

inan

ce (l

ux)

The effect of rooflight transmittanceFig. 22. Effect of reduction in light transmission upon average light level at noon (12.5% rooflight area)

New rooflight Old rooflight withas installed. 40% reduction in lightTransmission 0.64 transmission to 0.384

Heating CO2 kg/m2/yr 16.00 15.55

Lighting CO2 kg/m2/yr 3.45 5.01

Total heat and light CO2 kg/m2/yr 19.45 20.56

Table 4. Effect of rooflight deterioration on CO2 generation for a building with 10% rooflight area


It is generally accepted that UK peakand average temperatures will increasesubstantially over the course of thiscentury, triggered mainly by rising man-made carbon dioxide emissions.Summers will become hotter and drier, and winters milder and wetter. In the shorter term, UK summers are already getting warmer, with 2003 and 2006 ranking fourth and first hottest respectively since records began in 1659 (source:Meteorological Office).

Using reference climate data from the1990’s, a future weather scenario hasbeen created in line with UKCIP (UKClimate Impact Programme) predictionsfor medium to high CO2 emissions in the 2050’s. The retail building modelwith 12.5% rooflight area has beenmodelled to compare overheating for current and future climates in the southeast of England. This used

typical available climate data for the 1990’s and the predicted data for the 2050’s.

It can be seen from the graph that hours overheating are increasedsubstantially for the future scenario,implying that cooling would be needed if thermal comfort for the occupants is to be assured. It can be concludedthat limiting the extent of solar gain by using a moderate percentage ofrooflights is one method of ‘futureproofing’ a building if the trend towards a warmer climate is assumed.

Solar radiation will increase by a similar factor to average temperature,making over-provision of rooflights a potential source of unmanageablesolar gain. It can be concluded that the problems associated with solar gainand overheating will become morepronounced with global climate change.

10

20

30

40

50

60

>20 >21 >22 >23 >24 >25 >26 >27 >28 >29 >30 >31 >32 >33 >34 >35

0

Degrees C

External – London 1990s Building internal 1990s

External – London 2050s Building internal 2050s

Per

cent

age

occ

upie

d h

our

s

The effects of global warming

Fig. 23. Retail building (9-5 Mon-Fri): 12.5% roof light area. No cooling. Percentage occupied hours over set temperature


The modelling work reported here hasconfirmed that rooflights provide anefficient means of incorporating naturaldaylight into large single-storeycommercial and industrial buildings.However, this also highlights that tooptimise the design of the building, anintegrated approach to building designmust be adopted. Rooflights cannot beseen to be a function of the envelopealone, but must be considered as part ofa lighting strategy which also includesefficient lamps and control systems.Likewise, rooflights and electric lightingalso have an effect on heating andcooling requirements, so reinforcing the need for a holistic approach tobuilding design.

In-plane rooflights can be readily andeconomically incorporated into profiledpre-finished steel roofs, indeed this is one of the benefits of the ubiquitouslow-pitch pre-finished steel roof. Care

must be taken in designing-in rooflightsto ensure that a good distribution oflight is achieved without impairingaccess to the roof, so the mid-slopeapproach is generally adopted.

When designing for a partially daylitinterior, it is sensible to consider usingrooflights for 10% of the total roof areaas a starting point. This provides aneconomical solution, which gives thevast majority of the benefits of higherlevels of rooflighting without raising thelikelihood of over-heating to a greatdegree. Where internal heat gains arelikely to be limited, high intensities oflight are required and the building is intended to be used for day-time only operation, it may be sensible toraise this value towards 14%. For 24-hour operation in lower light levels,with considerable internal heat gains, it could be beneficial to reduce rooflight area below 10%.

There are many factors to be balancedin designing the lighting, heating andcooling strategy for a building andwhere the final use is not known at thetime of erection, as is the case for manyspeculative constructions, it is difficult to adopt an all-encompassing approachto design. In this case, it would usuallybe wise to use 10% rooflights to providebackground lighting, with localised pointlighting being installed in areas, whichrequire higher levels. Most of thecalculations reported here considered an open building, but it has also beenshown that the presence of bulkyequipment or high-bay racking can have a dramatic effect on theeffectiveness of rooflighting, reinforcingthe optimum strategy of providing a low level of background light throughrooflighting, backed up by point lightingwhere required. In these cases a figure of 7.5% rooflights would provide a good starting point.

Conclusions

• Rooflights can help to minimiseenergy usage by providing natural light and can be readilyincorporated into a pre-finished steel roofing system.

• The incorporation of rooflightsincreases natural light availability but at the same time introduces areas of low insulation into the roof.Thermal losses here are virtuallymatched by solar gains through therooflights, although in the summer this can become problematical.

• The energy savings gained from rooflights will only be realised if they are combined with an

effective lighting operations control system.

• Covering 10% of the roof area withrooflights gives a good starting pointfor designing a partially daylit interior.In some cases, up to 14% could be beneficial, but at high levels,consideration needs to be given to the total heat gains in the building.

• High-bay racking and similar bulkyequipment can have a dramatic effect on light availability. A goodapproach is to use a lower area ofrooflights to provide a level of naturalbackground light with point lightingwhere required.

In summary


Corus and lowenergy designSince the 1970s, Corus have been at theforefront of technology in pre-finished steelbuilding envelopes. In more recent times, theneed to conserve energy and reduce the buildingcontribution to climate change has informed thedirection for envelope design. 90% of the CO2

emitted from a building comes from the usephase and Corus are now actively researchingmethods for the building envelope to contributeto the minimisation of this. The first Colorcoat®

Technical Paper “Creating an air-tight buildingenvelope” gave building designers and installerspractical guidance on minimising heat-lossthrough air-leakage. This paper continues thistheme of low-energy buildings, examining thebalance between natural and artificial lighting.

In providing an ongoing commitment to the future of the building envelopemarket, Corus have established theColorcoat® Centre for the buildingenvelope at Oxford Brookes University.Located within the Oxford Institute forSustainable Development and one ofthe largest schools of architecture inthe UK, the Centre is commited toproviding cutting-edge research to develop the future of pre-finishedsteel building envelopes.

The work reported here has utilised theleading expertise of Oxford BrookesUniversity, together with advancedcomputer modelling from the Corus R&Dlaboratories to provide a definitive viewof lighting strategies for low-energycommercial and industrial buildings.

Working together to drive design



The Colorcoat® brand is the recognisedmark of quality and metal envelopeexpertise from Corus. With over 40 years experience, we activelydevelop Colorcoat® products andprocesses to reduce their environmentalimpact to a level beyond merecompliance. All Colorcoat® productsare manufactured in factory controlled conditions, providing clear advantages onsite in terms of speed of construction andminimising social disruption.


Colorcoat® products offer the ultimate in durability and guaranteed performancereducing building life cycle costs andenvironmental impact.

Corus has detailed Life Cycle Costing andLife Cycle Assessment information thatdemonstrates the positive performanceof Colorcoat® products when comparedwith other alternatives. This is availablefrom www.colorcoat-online.com

Colorcoat HPS200®

Corus has demonstrated that ColorcoatHPS200® can be recycled without additionalburden to the environment. It has beeneco-designed to exceed future legislativerequirements and reduce its environmentalimpact, providing a long-term sustainablebuilding envelope solution.

More information about the environmentalperformance of Colorcoat HPS200® isprovided in an Environmental ProductDeclaration. This is available fromwww.colorcoat-online.com

Colorcoat Prisma®

The ideal choice to deliver eye-catchingbuildings that will stand the test of time.Technically and aesthetically superior toPVDF (PVF2), Colorcoat Prisma® is readilyavailable in the most popular solid andmetallic colours. All backed up by thecomprehensive Confidex® Guaranteeproviding cover for up to 25 years on walls.

Confidex® GuaranteeOffers the most comprehensive guaranteefor pre-finished steel products in Europe

and provides peace of mind for up to 30 years. Unlike other guarantees,Confidex® covers cut edges for the entirety of the guarantee period and does not require mandatory annual inspections.

Confidex Sustain™


pre-finished steel product and makesthe pre-finished steel building envelopeCarbonNeutral – the first in the world.Unavoidable CO2 emissions from thepre-finished steel cladding systemincluding fixings and insulation, aremeasured from cradle to grave and the impact offset. More than justoffsetting, the aim is to encouragespecification of the most sustainablepre-finished steel products and cladding systems.

Colorcoat® Building manualDeveloped in consultation witharchitects and other constructionprofessionals, the Colorcoat® Buildingmanual incorporates over 40 years of Colorcoat® expertise. It containsinformation about sustainabledevelopment and the creation of a sustainable specification.

If you require any further informationplease contact the Colorcoat Connection®

helpline on +44 (0)1244 892434.Alternatively further information can befound in the Colorcoat® Building manualor at www.colorcoat-online.com

Colorcoat® products and services1 Approved document L: Conservation of fuel and power (2006 edition).

2 Lighting levels: CIBSE Concise Handbook, Chartered Institute of Building ServicesEngineers, 2001.

3 ACR(CP) 001: 2003 Recommended Practice for Work on Profiled Sheeted Roofs (orange book).

4 TM37: CIBSE Technical Manual TM37 ‘Design for Improved Solar ShadingControl’, CIBSE 2006.

5 CIBSE Guide A, Environmental Design CIBSE, 2006.

Further information




The Colorcoat® brand is the recognised mark of quality and metal envelope expertise from Corus. With over40 years experience, we activelydevelop Colorcoat® products andprocesses to reduce their environmental impact to a level beyond mere compliance. All Colorcoat® products are manufactured in factory controlled conditions, providing clear advantages onsite in terms of speed of construction and minimising social disruption.


Colorcoat® products offer the ultimatein durability and guaranteed performancereducing building life cycle costs andenvironmental impact.

Corus has detailed Life Cycle Costing and Life Cycle Assessment information that demonstrates the positive performance of Colorcoat® products when compared with other alternatives. This is available from www.colorcoat-online.com

Colorcoat HPS200® UltraThe latest generation product for roof and wall cladding, Colorcoat HPS200® Ultra offers an exciting new colour rangeand dramatically improved colour andgloss performance. Maintenance free,Colorcoat HPS200® Ultra delivers twicethe colour and gloss retention of standard plastisols, and is now guaranteed for up to 40 years, combining outstanding performance with unrivalled reliability.

Colorcoat Prisma®

The ideal choice to deliver eye-catchingbuildings that will stand the test of time.Technically and aesthetically superior toPVDF (PVF2), Colorcoat Prisma® is readily available in the most popular solid and metallic colours. All backed up by the comprehensive Confidex® Guarantee providing cover for up to 25 years on walls.

Confidex® GuaranteeOffers the most comprehensive guarantee for pre-finished steel products in Europe and provides peace of mind for up to 40 years. Unlike other guarantees, Confidex® covers cut edges for the entirety of the guarantee period and does not require mandatory annual inspections.

Confidex Sustain®


pre-finished steel product and makesthe pre-finished steel building envelopeCarbonNeutral – the first in the world.Unavoidable CO2 emissions from thepre-finished steel cladding systemincluding fixings and insulation, aremeasured from cradle to cradle and theimpact offset. More than just offsetting,the aim is to encourage specification ofthe most sustainable pre-finished steelproducts and cladding systems.

Colorcoat® Building ManualDeveloped in consultation with architectsand other construction professionals, theColorcoat® Building Manual incorporatesover 40 years of Colorcoat® expertise.It contains information about sustainabledevelopment and the creation of asustainable specification.

If you require any further informationplease contact the Colorcoat Connection® helpline on +44 (0)1244 892434.

Alternatively further information can befound in the Colorcoat® Building Manualor at www.colorcoat-online.com

Colorcoat® products and services








Corus Colors



August 2007










Corus Colors



August 2007



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