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Corus Colors

An approach to the design of cost effective low carbon buildings

Colorcoat® Technical Paper

September 2009

Corus and low carbon buildingsCorus have worked closely with a number of strategic supply chain partners and technology providers to establish a hierarchy of measures, which can be taken to reduce the energy consumption and associated carbon dioxide emissions from buildings.

This guidance aims to help specifiers to integrate cost effective low carbon design into buildings and also exceed current Building Regulation requirements.

The results reported in this technical paper have utilised the expertise of Oxford Brookes University, the Steel Construction Institute and Corus R&D to model the effects of changes to the building envelope and services.Corus have worked together with CA Group, Corus Panels & Profiles, Eurobond, Euroclad and Tegral Metal Forming to ensure that the modelled results can be delivered in practice. These supply chain partners offer pre-finished

steel roof and wall cladding systems with thermally enhanced details, which will allow the design and construction of a highly efficient building envelope, a fundamental element of a building with low carbon dioxide emissions.

Working together to deliver low carbon buildings

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Contents

Overview 3

Building modelling versus actual building performance data 4

Operational and embodied CO2 5

Hierarchy of CO2 saving measures 6

Reduce service demand 6

Optimise the building envelope 10

Maximise service efficiency 15

Source low and zero carbon energy 18

Conclusions 21

References 22

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Overview

The UK Government has stated that long term targets for the reduction of carbon dioxide (CO2) emissions are implemented through the Energy Performance of Buildings Directive1 and through the relevant Building Regulation approved documents.

In the UK it is estimated that nearly 50% of all energy consumption is related to the operation of buildings and the Government has placed great emphasis on improving the energy efficiency of buildings as a means of reducing CO2 emissions. This fact is highlighted in the Energy White Paper2 and Approved Document L of the England and Wales Building Regulations3 (and the equivalent regulations in Scotland4 and Northern Ireland5). The amendments to this legislation came about as a result of the EU Directive 2002/91/EC1, which requires Member States to incorporate ways of reducing energy use in buildings, into their domestic legislation.

The introduction of revised Approved Document L3 in 2006, introduced CO2 emissions targets as one of the primary compliance criteria as well as minimum standards for insulation and air-tightness. Part L3 is to be strengthened again in 2010 and there is now a trajectory set by the UK government towards zero carbon buildings within the next decade.

As well as legislation to reduce CO2 emissions, increasing energy prices, an increased emphasis on business sustainability credentials and corporate image are also acting as drivers to increase building energy performance.

In 2008 Energy Performance Certificates for all buildings were introduced, which allows a potential tenant to assess the relative performance of a building.A building with lower energy consumption and subsequently lower CO2 emissions will command a higher rental value and will also hold this value for longer.

A recent McGraw-Hill6 study found that thermally efficient buildings command 3% higher rental rates and an average increase of 7.5% in building value. They also deliver 3.5% higher occupancy rates and ultimately improve return on investment by an average of 6.6%.

There are a large number of factors contributing to the performance of a building and focus of this technical document is to present an overview of the design criteria and technologies that have the greatest influence on the energy performance of buildings and to evaluate their potential benefits in terms of reduced CO2 emissions.

In order to minimise the CO2 emissions associated with a building, designers should consider the whole lifecycle, taking a cradle to cradle approach, from building component manufacture, construction through operation to demolition and recycling.


Building modelling versus actual building performance data When evaluating the performance of building design parameters, either modelled data or actual building performance data can be used.

When modelled data is used, the accuracy of the results is dependant upon the sophistication of the modelling package and the level of detail input into the model. When used for comparative purposes the model inaccuracies will tend to cancel each other out.

When actual building performance data is used, this gives an accurate picture

of the real performance, but does not take into account the large number of variables on how the building is actually being operated and for this reason accurate comparison between buildings is very difficult.

For the purpose of this technical document, to enable meaningful comparisons to be made, modelled data using the TAS energy simulation software7 has been used. In each case, the energy demand in kWh/m2/yr and associated CO2/m2/yr were compared with the base case described below in table 1.

The base case building was a medium-sized retail warehouse measuring 120 m long x 60 m wide x 10 m high with a 5° roof pitch and 10% rooflight area. The TAS7 model for this building is shown in figure 1. Building fabric and services were assumed in the base case to just comply with the current (2006) version of Part L3 of the Building Regulations and the occupancy pattern as defined in the National Calculation Method, SBEM database8.

Table 1. Values assumed for base case

Occupancy profile: Retail sales (“Retware_RetSales”.)People: 3.5 W/m2

Occupancy: 0800-1800Weather data: BRE 94 (Garston 1994)U-values Wall: 0.35 W/m2/K Roof: 0.25 W/m2/K Floor: 0.25 W/m2/K Rooflights (10% area): 2.20 W/m2/KAir-tightness: 10 m3/h/m2@50 PaLighting: 600 lux during operating hours; 6.72 W/m2

Display lighting: 5.00 W/m2

Lighting control: Photocell dimmer (when fitted)Equipment: 2.00 W/m2

Cooling: CoP = 2.5, 0800 to 1800, 7 days, 25°C set pointHeating: Efficiency = 0.9, 0600 to 1700, 7 days, 19°C set point, 16°C setbackNatural ventilation: 1% (floor area) open area when temperature exceeds 22°C

Figure 1. TAS7 representation of modelled medium-sized retail shed


Operational and embodied CO2

In order to minimise the CO2 emissions associated with a building, designers should consider the whole life cycle, taking a cradle to cradle approach, from building component manufacture, construction through operation to demolition and recycling.

However, in practice, the CO2 emissions generated to heat, light, ventilate and cool a building, often referred to as the operational CO2, far outweigh the embodied CO2 associated with the building fabric and construction.

The greatest savings in CO2 emissions can be made by optimising the operational energy performance of the building in use.

A simple analysis of three example buildings, which represent a small industrial unit, a medium-sized out of town retail unit and a large distribution warehouse, illustrates this point. In each case, realistic material quantities have been obtained from which the equivalent quantity of embodied CO2 was calculated.

These values are shown in table 2 alongside the corresponding quantities of

CO2 emissions associated with the buildings’ operation over a 30 year period. Figure 2 shows a graphical representation of these results, including a breakdown of the four main components of operational CO2 emissions.

The CO2 emissions associated with a building of this type are dominated by operational activities. It follows that significant reductions in a building’s carbon footprint can only be achieved if sufficient attention is paid to reducing the operational energy demand of the building, particularly that relating to heating, cooling and lighting.

Table 2. CO2 emissions in tonne/m2

Small Medium LargeDimensions 60 m x 30 m x 6 m 120 m x 60 m x 10 m 240 m x 120 m x 17 mEmbodied CO2 0.17 0.15 0.14Operational CO2 0.95 1.08 1.37Total CO2 1.12 1.23 1.51Operational CO2 as a % of the total 85% 88% 91%

Based on 30 year building life.

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Figure 2. Embodied and operational CO2 emissions

Small shed Medium Large shed

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Hierarchy of CO2 saving measures

In order to minimise operational CO2 emissions, building designers need to develop a reduction strategy and ensure that it forms an integral part of the design process.

The detail of this strategy will depend on the type, size and use of the building, but as a general rule, it should follow the hierarchy of priorities shown in figure 3.

The first priority is to minimise the demand for heating, ventilation and artificial lighting by a combination of efficient building design and energy conscious building use.

Secondly, the designer should look to optimise the building envelope, to balance the heat losses and, solar and lighting gains in the most cost effective manner.

Thirdly, the heating, ventilation and lighting should be delivered as efficiently as possible using the most appropriate technology for the building type and use, together with energy saving control systems.

Finally, and only after taking all practical steps to minimise the demand for energy, building designers should consider the use of renewable or low carbon energy sources to meet the building’s remaining energy needs.

Minimise demand for heating, ventilation and lighting.

Cost effective enhancement of the building envelope.

Meet demand for heating, ventilation and lighting as efficiently as possible.

Install renewable energy sources to reduce CO2 impact of residual energy requirements.

Reduce service demand

Optimise envelope efficiency

Maximise service efficiency

Source low carbon energy


LocationSize and geometryOrientationInternal temperatureVentilationHumidity


Step 1

Step 3

Step 4

Step 2

Figure 3. Hierarchy of priorities when designing low carbon buildings


Whole building design By considering the energy performance of the building at the concept design stage, it may be possible to optimise the shape and internal layout in order to minimise the demand for artificial lighting and mechanical ventilation. However, care must be taken to avoid compromising the functional performance of the building or the commercial requirements of the building owner or developer. The following factors have the potential to influence the building’s energy performance and each has been investigated further as part of this study: • Location• Buildingsizeandaspectratio• Orientation

LocationThe location of a building can influence CO2 emissions in two ways. There is the direct impact of the climate and available daylight between different regions of the country. It is very unlikely that this issue will ever influence the choice of location of a building, since this decision will be dominated by other factors.

The building location will almost certainly have an impact on the CO2 emissions associated with the use of the building by its owner, employees, suppliers and customers. Potentially the largest impact relates to the transport of people and goods to and from the site.

This is often the dominant factor in the siting of distribution warehouses and is also an important consideration for planning authorities when considering planning applications.

Building size and aspect ratioThe building dimensions are one of the most important factors to consider when designing to minimise the CO2 emissions of a building. Excessive floor area, height and volume, will increase energy during use, as well as embodied energy. Where future expansion is envisaged, rather than opting for an over-sized building, consideration should be given to a building that can be easily extended when required or one where the expansion could be accommodated by adjacent buildings.

A comparative modelling exercise of four buildings has been used to demonstrate the effects. The results are summarised in table 3.

Design for use of building floor areaThe study found that reducing the eaves height reduced the CO2 emissions. This is largely due to the reduction in energy required to heat the building since there is a smaller volume of air to heat per m2 of floor area.

This dependence on building height is confirmed by the two 10 m high buildings which, despite differences in floor area, have near identical CO2 emission values.

The main conclusion to be drawn is that buildings should not be designed to be any higher than is necessary for their predicted use.

Design for use of building volumeFor warehouses and similar buildings it may be more useful to consider the CO2 emissions per m3 of building volume, since modern racking systems allow efficient use of the whole building from floor to eaves level. The large 17 m high shed performed best at 2.6 Kg CO2/m3/yr.

The governing factor is the building surface area to volume ratio. Where efficient use can be made of the height and volume such as with modern racking systems, a taller building will reduce total CO2 emissions for a given volume.

Building shapeThe medium-sized building has been modelled with two different aspect ratios, giving a square (90 m x 80 m) or rectangular (120 m x 60 m) plan, but with equal internal volume. The total CO2 emissions are almost identical for these two cases. In the examples considered, the square building required less energy for heating and lighting, but this was almost exactly balanced by the increase in energy required for cooling. For a warehouse without cooling requirements, a square building will have the lowest emissions as it has the lowest ratio of cladding area to floor space and cladding area to volume.

Table 3. Effect of shape and size

Case Size (L x B x H) Kg CO2/m2/yr Change from base Kg CO2/m3/yr Change from baseSmall low 60 m x 30 m x 6 m 31.7 -12.0% 4.9 +41.3%Small high 60 m x 30 m x 10 m 36.2 +0.4% 3.4 -0.2%Medium 120 m x 60 m x 10 m 36.1 Base 3.5 BaseLarge 240 m x 120 m x 17 m 45.5 +26.1% 2.6 -24.7%

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OrientationThe orientation of a building can sometimes impact on it’s CO2 emissions however this effect is usually very limited.The inclusion of a glazed wall, as is common in retail applications, does introduce a dependence on orientation, due to the directional nature of the solar gain.

However, as the results in table 4 illustrate, the effect is only marginal. The small benefit observed when the glazed wall faces south is due to a reduced demand for heating in the winter, which is partly offset by an increased demand for cooling in the summer.

The orientation of loading bay doors away from the prevailing wind can help to reduce

heat loss when they are opened in the winter, but this effect is extremely complicated, not feasible for modelling and would be highly site dependant, so no conclusions can be drawn on this other than to keep openings away from the prevailing wind direction. This is generally good practice as it also reduces the wind loading on the structure, but is clearly dependent on the availability of suitable access.

The effect of orientation on overall energy performance is very small and so this is unlikely to be a significant consideration in building design, where the site and available access will to a large extent determine the plan and layout.

Table 4. Effect of orientation with one wall glazed

Orientation N E S WKg CO2/m2/yr 36.8 36.8 36.2 36.6

Reducing demand for heating, lighting and ventilation

Careful assessment of the requirements for the internal operating environment can have a very significant reduction in energy demand and CO2 emissions. The following factors can have a significant impact on the service energy demand required:• Internaltemperatureandcontrol parameters• Ventilation• Humidity• Internallightinglevels

Internal temperature and control parametersThermal comfort criteria should be established at the design stage and should take into account the heating, cooling and ventilation systems employed. The human perception of comfort depends on a number of factors including air temperature, radiant temperature and air movement. These are combined in a parameter known as the ‘operative temperature’.

CIBSE Guide A – Environmental design9, gives the nominal range of operative temperature for comfort for several common building applications. The wider the acceptable temperature range, the smaller the demand for heating and cooling will be. The impact of adjusting the thermostat settings on CO2 emissions has been investigated for the medium- sized retail shed described earlier. The temperature ranges considered, along with the associated CO2 emissions are given in table 5.

* Temperature to which the building is heated during unoccupied hours (18.00 to 08.00).

Table 5. Effect of adjusting the thermostat settings

Case Heating set point °C Heating set back °C* Cooling set point °C Kg CO2/m2/yr Change from baseBase case 19 16 25 36.1Heat to 21°C 21 16 25 37.7 +4.4%Heat to 17°C 17 16 25 35.0 +3.1%Set back 12°C 19 12 25 31.2 -13.5%Cool to 23°C 19 16 23 37.7 +4.4%Cool to 21°C 19 16 21 40.3 +11.7%



A change in the heating or cooling set point by only 2°C results in a change in the CO2 emissions by 3-4%. The CO2 savings relating to the cooling and heating are independent of each other, since they occur at different times of the year, so changing both set points could reduce emissions by as much as 8% over the whole year.

The most significant change of the cases modelled occurred with the reduction of the set back from 16°C to 12°C. This is the temperature of the building outside occupied hours and has minimal effect on the operating conditions. There is a practical lower limit to the set back temperature, due to the risk of condensation or damage to the building’s contents by low temperatures. A low set back temperature may require a higher capacity heating plant to raise the temperature quickly to the desired daytime level each morning.

VentilationIt is particularly important when designing buildings with very good air-tightness to consider the requirements for ventilation within the building. The standard measure of ventilation is the air change rate. This is expressed in ‘air changes per hour’ (ach). Guideline values for typical environments are given by CIBSE guide A9.

Mechanical ventilation is driven by electric power and so increasing the design ventilation rate does increase the CO2 emissions associated with the building. In many cases, particularly for large industrial and warehouse buildings, sufficient ventilation can be achieved through natural air movement and leakage.

Good practice is to create a very air-tight building envelope with controlled ventilation. Where ventilation is controlled, heat ‘lost’ through ventilation may be recovered and returned to the building, reducing the energy demand and associated CO2 emissions significantly. Heat recovery systems can be up to 70% efficient, permitting relatively high air change rates without excessive CO2 emissions.

HumidityThe humidity of the internal environment affects the thermal comfort of the building occupants and their perception of the internal temperature.

Optimising the humidity can actually save energy by increasing the thermal comfort of the occupants, thereby reducing the demand for heating and/or cooling. The overall impact of humidity control on CO2 will depend on the building use, the internal temperature range and ventilation and should be assessed by a building services engineer at the design stage.

Internal lighting levelsLighting can account for up to 50% of the CO2 emissions from a building. The CO2 emissions per kW/h of electrical energy are more than twice as much as for gas and so although lighting only accounts for approximately 25% of the operational energy, its contribution to CO2 emissions is much greater.

The specification of the internal lighting levels and selection of the most appropriate lighting strategy and control systems to ensure that the best use is made of available daylight are critical to reducing the overall level of CO2

emissions. This is covered in depth in the Colorcoat® Technical Paper ‘In-plane rooflights for low energy buildings’10.

Table 6 presents guidelines for recommended illuminance levels for different activities based on CIBSE guidance9.

Table 6. Recommended lighting levels

Standard maintained illuminance (lux) 200 300

500 750 1000

Activity interior Foyers, entrances, automatic processesLibraries, sports halls, food court packing, warehousesGeneral offices, assembly, retail shopsDrawing office, supermarkets, showroomsDIY superstore

There are many simple ways to minimise energy usage in a building, but one of the most significant is to optimise the acceptable temperature set points for the internal environment. Whilst this is a matter of building management rather than design, it is important for the designer of low carbon buildings to understand the effect of control regimes and, where possible, influence this through the specification.


Optimise the building envelope

InsulationAir-tightnessLinear thermal bridgingIncorporation of rooflights


Minimising heat loss through the building envelope should be regarded as a priority in any building design, as there are no adverse consequences beyond the direct cost implications of specifying a better quality of construction.

Of particular interest is how this may be reduced through the use of improved insulation, air-tightness, envelope detailing and careful incorporation of rooflights.

InsulationThe thermal transmittance of a construction is given by the U-value. This is the heat in Watts (W) passing through a m2 of construction per °C temperature difference from inside to outside. Maximum average U-values are given in the Building Regulations3, as are maximum local U-values for any part of a construction. Increasing insulation thickness, to lower U-values, will reduce fabric heat losses from the building, but the advantages become proportionately less as thickness is increased. U-values are already low, so the advantage of adding more insulation is limited.

Figure 4 shows the impact of insulation thickness on U-value for two typical insulation materials. Actual cladding systems will vary due to factors such as repeating thermal bridging. It is always important to consult the cladding manufacturer for actual U-values of a particular system.

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Figure 4. U-value for different insulation thickness

1 2 3 4 5

Insulation thickness mm

U (W

/m2 K

)

Key

Polyurethane foamMineral Wool

0.000000

0.208333

0.416667

0.625000

0.833333

1.041667

1.250000

1.458333

1.666667

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2.291667

2.500000

0.0000000.2083330.4166670.6250000.8333331.0416671.2500001.4583331.6666671.8750002.0833332.2916672.500000

20 40 60 80 100 120 140 160 180 200 220 240 260


To halve a given U-value, requires the insulation thickness to be approximately doubled. Insulation levels have a greater impact on the total heat loss and CO2 emissions, of smaller buildings than for larger ones. This is due to the ratio of volume to surface area. So, for typical industrial, warehouse or retail buildings, with relatively high volume, increasing insulation has much less effect than in small buildings.

The medium-sized retail building described earlier has been remodelled with enhanced insulation to investigate the impact on building energy performance and CO2 emissions. The original and enhanced U-values along with the resulting CO2 emissions are presented in figure 5. It should be noted that the improved insulation values are achieved at a significant cost, not only of the additional insulation, but also the other components of the wall or roof pre-finished steel cladding system such as fixings and spacer systems. The systems modelled here are, in some cases, more than 50% thicker for the better insulation values. The improvement in operation CO2 emissions achieved should be balanced against this additional cost and the slight increase in embodied CO2.

Panel edge effectsThe insulation performance at the edge of panel systems is usually lower than at the centre of a panel. The effect of joints and through fasteners, which are known to repeat at a given frequency, should be included in the U-value calculations.

The joints for factory insulated composite panels, have been designed to minimise any additional edge losses.

Some architectural wall panels and insulated rainscreen systems can have significant thermal losses around the panel edges. This can be clearly seen by thermographic imaging of the building in figure 6, which shows up the relatively warm outlines of the panels.

Figure 6. Thermographic imaging of building

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Figure 5. Effect of insulation only

0.35/0.25 0.3/0.2 0.2/0.10.25/0.15

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Figure 7. Effect of panel size on overall U-value for 1 m wide cladding panel

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Panel length m

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alue

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Figure 7. Effect of panel size on overall U-value for 1 m wide cladding panel

1 2 3 4 5

Panel length m

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Overall U-valueJoint heat flow W/m2K

Centre U-value

Air-tightnessCold air infiltration has a very significant effect on the heat loss through the external envelope and the overall CO2 emissions associated with the building.

The air-tightness of a building is quantified in terms of its air permeability, which is defined as the volume flow rate of air per m2 of building envelope including the floor slab at an applied pressure difference of 50 pascal (Pa).

Compliance with the regulations is enforced through pre-completion testing, which is now mandatory for all types of buildings. ATTMA TS1: 200611 gives the methodology for air pressure testing.

All buildings must be pressure tested, unless the floor area is less than 500 m2, when a default value of 15 m3/h/m2 can be used. For buildings over 500 m2 a maximum reasonable design limit for air-tightness is 10 m3/h/m2.

It will be difficult to meet the overall building CO2 emission rate without a reasonably air-tight building envelope.

Where and how air-tightness is generatedIn a typical pre-finished steel clad building, the air-tightness barrier is provided by the interior or liner side of the envelope. It is important to be aware of this and to construct appropriately. While the outer sheet of an insulated envelope will give weather protection, and will be sealed in order to prevent water leaks, it is important that this is not relied upon to give air-tightness.

Full guidance on sealing of pre-finished steel cladding joints are given in the Colorcoat® Technical Paper ‘Creating an air-tight building envelope’12.

When considering the detailing of a pre-finished steel building envelope, it is important to consider:• Jointsandjunctionsbetweencladding sheets and also with different elements such as floor slabs.• Plannedpenetrations,forwhichwell designed specifications should be given.• Unplannedpenetrations,suchasfor building services.

As the overall panel size decreases, the ratio of edge to panel area increases and this decreases the effective overall U-value of the pre-finished steel cladding system for a given panel thickness, see figure 7. The building designer needs to balance the aesthetic requirements of panel size against the cladding performance.

Pre-finished steel building envelopes are inherently air-tight and with correct design and construction of the detailing, it is possible to achieve levels of air-tightness below 3 m3/m2 hr at 50 Pa.


The results shown in figure 8 clearly demonstrate the effectiveness of air-tightness in reducing CO2 emissions compared with the relatively small improvements that are achievable by increasing insulation. The relative importance of air-tightness compared to insulation thickness has increased over recent years as buildings have

become better insulated. Further increases in insulation thickness above the current regulatory requirements will have an ever-diminishing impact on energy consumption and CO2, whilst reducing unwanted air infiltration can reduce heating demand dramatically. It is important to note that improved air-tightness is achieved without

significant changes to the specification, merely through attention to detail in the construction of the building, so the additional cost of this benefit is minimal when compared to most other means of significantly reducing the CO2 emissions associated with a building.

Minimisation of thermal bridging A thermal bridge is a localised area of lower thermal resistance. The importance of envelope detailing in reducing CO2 emissions is often overlooked. However, it is only by careful attention to the detailing and workmanship, especially at joints and junctions between building elements, that a well performing pre-finished steel clad building envelope will be achieved. In particular, there are two aspects of envelope detailing which are important to minimising heat losses:• Minimisationofthermalbridging• Continuityofinsulation

Thermal imaging checks can be made after construction to ensure that insulation and sealants have been correctly installed, and that thermal bridges have been avoided. However, the rectification of poor workmanship at this stage could have severe cost and time implications.

Good performance is routinely achievable using standard pre-finished steel cladding systems, but design guidance must be followed with attention to detail on site.

Repeating thermal bridges, such as fasteners, must be included in the U-value calculation, whereas non-repeating thermal bridges such as junctions between different building elements must be accounted for separately. The additional heat flow through a junction is known as the psi (ψ) value and is expressed as W/m/K for each junction type.

The relatively high thermal conductivity of steel, approximately 52 W/m/K, compared with mineral wool, 0.04 W/m/K, means that careful detailing is required to ensure that thermal bridging does not occur in certain applications. For example, pre-finished steel flashings should not be allowed to penetrate

from inside to outside, instead two separate flashings should be used, which do not contact each other. It is also important to cut back the ends of pre-finished steel sheets at corners and verges to prevent them from penetrating the insulation. Thermal bridging at joints, junctions and penetrations can typically account for an additional 10% of heat transfer through the building envelope. Well designed and installed details can half this figure.

As well as increasing heat loss from the building envelope, thermal bridging can cause localised condensation as surface temperatures may be reduced below the dew point of the air in the space. Cladding manufacturers are able to supply standard details, which follow these rules, sometimes referred to as ‘enhanced’ details, and it is important to ensure that these are adopted in practice.

Figure 8. Effects of insulation and air-tightness

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Kg CO2/m2/yr

0.000000

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0.0000000.2083330.4166670.6250000.8333331.0416671.2500001.4583331.6666671.8750002.0833332.2916672.500000

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Insulation Part L BackstopAir-tightness Combined

10 m3/hr/m2

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Kg CO2/m 2 /yr

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-5%

-17%


Continuity of insulationEnsuring continuity of insulation is critical to maintain the design U-value of the wall or roof construction and meet the design ψ values for the junctions and interfaces.

For built-up systems it is important that insulation is fed well into all gaps. Where pre-finished steel foam filled composite panels are used it is equally important that gaps between rigid insulation, for example at corner or ridge details and penetrations, are filled with either loose fill fibre or self-expanding foam insulation.

The images in figure 9 demonstrate poor continuity of insulation around two penetrations in the gable wall of a small building, resulting in hot spots on the cladding exterior, cold spots on the building interior and additional heat flow out of the building.

Figure 9. Poor continuity of insulation

Incorporation of rooflights In-plane rooflights can be easily integrated into built-up pre-finished steel roof cladding. A detailed study into the optimum proportion and layout of rooflights has been undertaken and is reported in the Colorcoat® Technical Paper ‘In-plane rooflights for low energy buildings’10. This study concludes that, in the correct proportion, the use of rooflights can significantly reduce a building’s energy consumption. However, in order to realise the energy savings, rooflights must be used in conjunction with effective lighting controls.

From an energy perspective, a potential disadvantage of in-plane rooflights is heat loss through or around the rooflights due to relatively poor U-values and air-leakage at joints. However, energy modelling indicates that heat losses through the rooflights are largely offset by useful solar gains and reduced reliance on electrical lighting.

A potentially more serious problem is the risk of overheating during the summer months. This can result in an increased demand for energy for cooling where it is used, or reduced occupier comfort in uncooled buildings. The precise balance between solar gain, heat loss and energy saving from the use of natural daylight is complex and also depends on occupancy patterns. The optimum area of rooflights will differ from one building to another. As a general guide, for daytime use, 10% of the roof area is a good starting point for most applications, although in some cases, areas up to 15% may be beneficial. For 24 hour operations, 8-12% rooflights are recommended. In-plane rooflights require strict cleaning schedules as dirt deposition can severely reduce light transmission. The light transmittance of the rooflight material may also deteriorate with age.

Installation of high bay racking can severely reduce the amount of daylight reaching floor level. While racking is unavoidable in many applications, its impact on lighting levels could be minimised by careful positioning in relation to the rooflights.

North light structures are occasionally used as an alternative to in-plane rooflights and have the advantage of reducing solar gain, thereby minimising the risk of overheating in the summer, but at the expense of increased winter heating bills. The energy balance between a low-pitched structure with in-plane rooflights and a north light structure is very complex and will depend very much on the building application.

42°C

-38°C

4

2

0

-2



Maximise service efficiency HeatingVentilationLighting


Having taken the necessary steps to ensure that the demand for heating, ventilation and artificial lighting have been minimised, and waste avoided wherever possible, the next stage is to specify the most energy efficient means of delivery. The best choice of services will vary from one building to the next and will depend

on a number of factors such as roof height, required ventilation rate, allowable temperature range, fabric losses and gains, mounting restrictions and availability and cost of fuel. Selecting the appropriate system for the application is key to maximising the energy efficiency of the building services.

Heating plant efficiency and control systems There are several methods for supplying heat into a building in common use in the UK. Each has its own advantages and disadvantages, which should be considered

carefully before selecting one method over another. All of the heating options described in table 7 are suitable for new build and retrofit.

Table 7. Typical heating systems for large spaces

Appliance Description Advantages DisadvantagesIndirect fired warm air heaters

Direct fired warm air heaters

Radiant tube heater

Indirect radiant tube heater

Oil or gas fired heaters providing a high velocity supply of hot air into the space. Used with de-stratification fans to improve overall efficiency.Gas fired heaters providing a high volume supply of heated air. The products of combustion are included in the air supply.Gas fired tubular heaters suspended from the roof. May be flued or unflued depending on ventilation rates.Oil or gas fired. Heat is emitted from the surface of a large diameter suspended tube through which very hot air is circulated.

Relatively low cost. Can be floor mounted or suspended.

Efficient means of heating space. Low temperature of supply air reduces stratification. No need for separate flue.Efficient means of heating spaces with intermittently high ventilation rates. Comfortable environment at low air temperature.Can be used in areas where conventional high surface temperature tubes might not be permitted.

Requires fuel supply and flue. High velocity air can create local discomfort. High temperature output promotes stratification. Can be noisy. Requires suitably high ventilation rates.

Shadow effects. Not suitable for some environments with corrosive atmospheres.

Lower surface temperature also means lower radiant efficiency.


It is vital, for achieving low CO2 emissions from a building, to specify and use the most efficient heating system matched to the particular requirements of that building. Many modern heating systems work at efficiencies in excess of 90% and where boilers are used, the condensing mode can increase this further.

In any heating system it is important to fit the appropriate sensors control system to manage the operation of the plant and distribution of heat for maximum efficiency. It is also important to implement zoning of the building to account for varying operating patterns and heating loads. Table 8 summarises the control strategy for different heating systems.

Table 8. Control strategies for different heating systems

Heating system Primary control sensor Control strategyRadiant tubes

Warm air heatersRadiators

Under floor

Black bulb radiant temperature sensorAir temperature sensorIndoor air temperature sensor

Return temperature sensor

On/off

Optimum startOptimum start, weather compensation Weather compensation

De-stratification fans Basic heat loss calculations assume that the air is well mixed in the heated space. However, in most large spaces with moderately high temperature heat sources there will be a tendency to thermal stratification, as illustrated in figure 10. This means that the temperature immediately under the roof is much higher than at floor level. The difference can be 10°C for air heaters and even higher for radiant tubes. This is important because the heat losses through the building fabric depend on the temperature difference between inside and outside.

The heat losses through the roof of a building with stratification can be double that which would occur if the air were fully mixed. This could represent 5% to 10% of the total heating energy consumption. There are practical means of reducing stratification from conventional high temperature heating systems, such as de-stratification fans.

The overall effect on CO2 emissions will be specific to each building and are dependant on the height of the building, the overall heating within the building and the electrical power used by the fans.

Figure 10. Stratification in a building schematic

High tempheater

High tempheater

High tempheater

High tempheater

Stratification - Hot air is trapped at roof level Reduced stratification with roof fans - Hot air

returned to ground level

Maximum service efficiency


VentilationThere are two primary approaches to achieve necessary levels of ventilation:• Naturalventilation• Mechanicalventilation

Modern buildings are constructed to minimise uncontrolled ventilation and natural ventilation is carefully managed through purpose designed ventilation systems.

Natural ventilation does not necessarily imply natural cooling, as the air supplied will be at the external ambient temperature. Air-leakage should not be regarded as satisfactory natural ventilation since draughty buildings are rarely comfortable for their occupants and are difficult to control.

The effectiveness of natural ventilation depends on the size and orientation of the building. Roof vents are a common option for natural ventilation in buildings without suitably large openings. These need to be carefully positioned so as to maximize their performance.

Mechanical ventilation uses fans to generate air movement. It is more predictable and therefore easier to design than natural ventilation.

The trend is towards hybrid systems that use predominantly natural ventilation, but with mechanically driven fans to improve predictability of performance over a wider range of weather conditions.

MechanicalHeatandVentilationRecovery(MHVR)systemsusetheheatfrom the exiting warm stale air to heat up the fresh colder air as it enters the building. The warm air is vented out of the building alongside the incoming fresh air, allowing heat transfer from the exiting airtotheincomingair.TheuseofMHVRsystems can significantly reduce the amount of energy required to warm the fresh air to a comfortable level and so reduce CO2 emissions.

Lighting typesTraditionally, large industrial type buildings have been illuminated by metal halide luminaires. Although these lamps have high energy efficiencies, they have a ‘warm up’ period of about four minutes before they reach maximum output and are unsuitable for on/off sensor control. Modern T5 fluorescent fixtures require less energy than metal halide, and offer improved control and lower maintenance costs. T5 technology is highly energy efficient, continuously dimmable, and is ideal for use with occupancy sensors and photocells.

Lighting controlsIn large single storey buildings, as detailed earlier, careful use of in-plane rooflights can give very good levels of daylight. This will only result in a reduction in CO2 emissions, if the artificial lighting system is well controlled.Detailed below is a range of automatic control systems which are available:• Photocellsrespondtonaturallight levels. For example, they can be used to switch outdoor lights on at dusk and off at dawn. Some advanced designs gradually raise and lower artificial light levels with changing daylight levels. • Mechanicalorelectronictimers automatically turn on and off indoor or outdoor lights. • Occupancysensorsactivatethe lights when a building or portion of a building is occupied. • Dimmersreducethewattage and output of incandescent and fluorescent lamps but will save energy only when used consistently. Dimming fluorescent lamps requires a special dimming ballast and lamp holder but does not reduce their efficiency.

The effect of installing an efficient control system and utilising the available natural daylight, can reduce CO2 emissions from the medium-sized building modelled in figure 1 by nearly 20%.

Artificial lighting


Source low and zero carbon energy

Install renewable energy sources to reduce carbon impact of residual energy requirements.Electrical power.Heat.


Having designed a building to minimise the requirement for energy, and then specified the most efficient services for the intended building use, in almost all buildings, there will be a residual demand for energy. This residual energy demand will usually be for both electrical power and heat. If the designer is to truly minimise the operating CO2 emissions of the building and

potentially create a ‘zero carbon’ building, then sources of energy which generate either no or low levels of CO2 emissions will be required. There are many low and zero carbon (LZC) technologies now available for incorporation into the building including photovoltaics, wind turbines, hot water from solar thermal panels, heating and cooling via heat pumps and various fuels

derived from biomass. Although renewable energy is ‘low carbon’ it is not necessarily low cost. Typically, most of these technologies have a relatively high capital cost and so the designer should put every effort into minimising their requirement by adopting the strategies described earlier in this technical paper.

HeatSolar thermalSolar thermal systems capture the sun’s energy and either store it for a limited period of time or distribute it for use withinthebuilding.Variousmethods and products are available that use the sun’s energy in this way.

Flat plate type collectors use the suns energy to heat liquid in duct pipes which is then used in the building. The temperature rise of the system can typically be up to 50ºC. Evacuated tube collectors are more expensive than plate and tube collectors, but are also more efficient. The increase in temperature in these systems can be up to 150ºC. The higher temperatures are due to the liquid being allowed to evaporate and subsequently condense within the system’s evacuated double tubes. This type of solar thermal system is now commonly used for water heating as well as hot water based heating systems, but is always treated as a bolt-on to the building envelope rather than being fully integrated.

Ground source heat pumps Ground source heat pumps (GSHP) work on the same principle as a refrigerator or a conventional air conditioning system in that they extract heat from one location and transfer it to another. Unlike geothermal heating, GSHP technology does not require a high temperature heat source, since the temperature of the air or water supplied to the building is not related to the temperature of the ground or other ambient heat source.

Heat pumps are able to extract useful heat from the ground at normal winter temperatures and use it to heat water to 40ºC. This makes GSHP a good complement to under-floor heating.

GSHP, while using a renewable source of heat, does demand an electrical input and so could not be considered to be zero carbon unless linked to a source of zero carbon electricity. However, the efficiency achieved makes this a very low carbon technology.

Combined heat and power and use of biomassCombined heat and power (CHP) is another technology, which is not zero carbon, but the improved efficiency over conventional energy allows it to be classed as low carbon. CHP works on the principle that waste heat produced during the electricity generation process may be used for heating or cooling purposes.

CHP only makes economic sense if there is a real and continuous demand for heat, the value of which can be discounted against the operating cost, along with the electricity generated.

Further reductions in CO2 emissions may be obtained by using biomass as the fuel for the CHP plant, but consideration should be given to the need to transport and store the biomass fuel.

Recent developments have allowed an extension of CHP to those areas that require significant cooling through the year, particularly in connection with the intensive use of computers. In such applications the heat from the CHP can be used to drive absorption chilling. This is termed ‘tri-generation’ as the plant provides electricity, heating and cooling.


SolarWall®A system highly applicable to pre-finished steel clad buildings, is the perforated Transpired Solar Collector (pTSC). The system consists of a perforated sheet collector (manufactured from pre-finished steel cladding), installed as an additional skin to the southerly elevation of a building in order to create an air cavity. The solar heated air (generated on the external surface of the panel) is drawn through the perforations into the cavity space where it is then collected at the top of the elevation before being distributed into the building.

The solar heated air can then be used directly as building ventilation air, or if required utilised as a pre-heater for the building’s main heating system, thereby reducing the amount of energy required to heat the building and the resulting CO2 emissions.

On a bright, clear day (irrelevant of outside temperature) the system can be used to heat air by as much as 25-30°C above its ambient temperature, with each m2 of collector providing the equivalent output of a 0.5kW heater.

The system is capable of delivering useful energy throughout the year. During the summer, when there is no demand for

heating, the heated air is naturally vented back to the atmosphere via the perforations at the top of the elevation, with the additional skin acting as a sun-screen; reducing heat flow through the façade.

Alternatively, with the addition of a heat exchanger, the heated air could be used for pre-heating water.

Practical testing of this type of system has shown that for a medium-sized industrial unit, heating bills can be cut by up to 50% across the annual cycle, thereby cutting the total CO2 emissions for the modelled building by up to 20%.

SolarWall® Panels

Distribution DuctingFan Unit

Air Space

Heat lost through the wall is brought back by incoming air

The heated boundary layer of air is drawn through the perforations into the air cavity

The air space inside the SolarWall® is under negative pressure, drawing the warm air upwards

Air Gap

Figure 12. SolarWall® perforated Transpired Solar Collector

Solarwall® installation in

Colorcoat Prisma®,

Grey Aluminium, at the

Community Healthcare Campus,

BRE Innovations Park, Watford.


Source low and zero carbon energy

Electrical powerPhotovoltaic panelsPhotovoltaicpanels(PVs)convertenergyfrom the sun directly into electricity. They come in various forms, such as panels, rain screen cladding, thin film sheeting, semitransparent modules, roof tiles and solar shading panels. They can be used as a bolt-on system, or can be fully integrated into the building envelope. There is generally a good publicperceptionofthebenefitsofPVs,so their installation can boost the environmental image of the building owner or occupier. Silicon is the most common active component in the panel, although dye based and cadmium telluride systems are also available. The silicon can be either crystalline or amorphous.

Photovoltaic panels can be placed on a building’s wall or roof, although their angle and position relative to the sun will affecttheirperformance.CrystallinePVunits are generally quite heavy, especially those supplied on metal frameworks, so care must be taken to ensure that the supporting structure can carry the additional loads.

PVsmountedoninclinedframescanalsogenerate high wind uplift loads, which might impact on the design of the purlins. If add-on units are to be used, a structural engineer should be consulted.

AmorphoussiliconPVlaminatesystemscan be applied directly onto the pre-finished steel cladding system. Laminate systems are much lighter and have minimal effect on the building structure as shown in figure 11. They generally have lower absolute efficiencies than crystalline silicon systems, but are less affected by

orientation, light levels and the lower efficiency is reflected in their cost.

ActualoutputsfromPVarraysdependon many factors, including the actual system specification, local solar irradiance levels, orientation towards the sun and absence of any shading. TosizeupaPVarrayforagivenpoweroutput it is important to employ a specialist engineer, but for the medium-sized building modelled here, the roof area would be more than sufficient to generate all of the electricity demands.

Figure 11. PV laminate on factory insulated bonded mineral wool composite panel

Wind turbines Wind turbines combine the latest engineering technology wind power to generate mechanical energy, which is then converted into electricity by means of a generator. Achieving a reliable and economic supply of electricity means that the design, siting and control of wind turbines can be complex. The technical difficulties are often compounded by objections raised during the planning process arising from the visual impact of wind turbines, noise and their potential impact on wildlife.

Variousformsofbuilding-mountedturbine have been developed recently, but since the overall efficiency of wind turbines depends on size and clear wind flow, these tend to be much less efficient than large, free-standing turbines.

The stated output of wind turbines in the UK is usually based on a wind speed of 5 m/s. Any reduction in the speed of the wind below the assumed design value will have a significant adverse impact on the turbine’s output and the economic viability of the installation.

In many areas of the UK, the wind speed regularly exceeds 7.5 m/s, but these tend to be in exposed locations, on the coast or on hills. In towns and cities, wind speeds are usually significantly lower. Specifiers should note that all wind turbines have a specified minimum speed below which they become inactive, known as the cut-in speed.

The elevation of the turbine is also important, in terms of altitude above sea level and height above the ground, as increasing either will result in higher wind speeds and, therefore, greater turbine output. In the latter case, the essential requirement is to raise the turbine above any potential obstructions to air flow, for example trees or other buildings. Clearly, the ability to achieve this with a roof-mounted turbine will depend on the height of the building relative to its neighbours.

Experience has shown that those located in well exposed areas of high wind speeds work very well, but for the building designer, in most cases, these criteria are not achieved.


Conclusions

Building Regulations3 stipulate a minimum level of energy efficiency, by the use of minimum backstop values and whole building CO2 emissions. However, in many cases, designers and clients wish to go beyond this level and future iterations of the Building Regulations3 will require lower carbon buildings.

Even for basic buildings, with low servicing requirements and relatively short design lives, the operational phase of the building still accounts for more than 80% of the total life cycle CO2 emissions.

Clearly, it is vital to focus on improvements in energy performance in use to make significant improvements to a building’s associated CO2 emissions.

In designing low carbon buildings, there is a clear hierarchy for the designer to follow, first minimising the requirement for servicing, optimizing the building envelope, then using efficient servicing strategies and finally sourcing the residual energy requirement, as far as possible, from low carbon or renewable means.

1. Optimise the building geometry to the proposed operations within the building, while allowing for future expansion and or change of use.2. Select the most suitable internal environmental conditions which meet the requirements for the building operations.





1. The most significant gains can be made from increasing the air-tightness of the building envelope.2. Gains from increased insulation are quite limited.3. Attention to detailing and removal of cold bridges through good design and attention to detail.4. Careful inclusion of rooflights to provide natural daylight without excessive additional fabric losses or high summer solar gains.

1. Use of lighting and heating controls is essential to realise the benefits from the building envelope design.2. Select intrinsically efficient heating, lighting and ventilation systems and operate as designed.

1. The CO2 reduction/cost will usually be much less than that spent on the envelope or service enhancements.2. Various systems exist for production of electricity and heat. The suitability of each system will be dependant on the building type, location and application.

Finally, it is useful to note that the calculations reported in this technical paper show that low CO2 targets can be achieved relatively simply for pre-finished steel clad buildings. The combined effect of improving lighting control, air-tightness and insulation can reduce the total operational CO2 emissions by up to 45% which represents a major step towards more sustainable buildings.


References

1. EU issued Directive 2002/91/EC (Energy Performance of Buildings Directive 2003)

2. The Energy White Paper “Our energy future – creating a low carbon economy” 2003

3. Building Regulations Approved Document L: Conservation of fuel and power (2006 edition)

4. Scottish Building Standards. Section 6 – Energy 2009

5. Department of the Environment for Northern Ireland. Technical booklet E: 2005

6. 2006 McGraw-Hill Green Building SmartMarket Report; McGraw Hill Construction 2005

7. TAS energy simulation software version 9.0.9

8. SBEM. Simplified Building Energy Model, iSBEM version 3.3b 2009

9. CIBSE Guide A – Environmental design; 7th Edition 2006

10. Colorcoat® Technical Paper; In-plane rooflights for low energy buildings.

11. ATTMA TS1: 2006

12. Colorcoat® Technical Paper; Creating an air-tight building envelope.


The Colorcoat® brand

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 Ultra®

The 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),ColorcoatPrisma® is readily available in the most popular solid and metallic colours. All backed up by the comprehensive Confidex® Guarantee.

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. Available only with Colorcoat HPS200 Ultra® and Colorcoat Prisma®.

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.Corus and their Confidex Sustain® assessed supply chain partners endeavour to reduce the CO2 emissions generated in the manufacture of pre-finished steel cladding systems but there will always be some unavoidable CO2 emissions. These unavoidable CO2 emissions are measured from cradle to cradle and the impact offset. Our aim is to encourage specification of the most sustainable pre-finished steel products and cladding systems. Available only with Colorcoat HPS200 Ultra® and Colorcoat Prisma®.

Colorcoat Verso®

A superior alternative to leathergrain plastisol. Providing improved durability at a competitive price. Available in 25 popular colours, with a unique Corus emboss and guaranteed for up to 30 years via the Colorcoat® supply chain partners.

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 servicesThe Colorcoat® brand is the recognised mark of quality and metal envelope expertise from Corus. With over 40 years experience, we actively develop Colorcoat® products and processes 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 manufactured inany UK Corus site are certified to theindependently verified internationalmanagement system, ISO14001 and100% recyclable, unlike most otherconstruction products.

www.colorcoat-online.com

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

Trademarks of Conserval EngineeringSolarWall is a trademark of Conserval Engineering Canada.

CarbonNeutral is a registered trademark of the CarbonNeutral Company.

Corus is part of the Tata Steel group.

Care has been taken to ensure that the contents of this publication are accurate, but Tata Steel Europe Limited and its subsidiaries do not accept responsibility or liability for errors or information that is found to be misleading. Suggestions for, or descriptions of, the end use or application of products or methods of working are for information only and Tata Steel Europe Limited and its subsidiaries accept no liability in respect thereof.

Before using products or services supplied or manufactured by Tata Steel Europe Limited and its subsidiaries, customers should satisfy themselves as to their suitability.

Corus cares about the environment – thisbrochure is printed with biodegradable vegetable inks and using material with at least 80% recycled content.

Copyright 2009 Corus

Language English 0909

Sales contact detailsCorus Colors Shotton Works Deeside Flintshire CH5 2NH United KingdomT: +44 (0)1244 812345F: +44 (0)1244 831132www.colorcoat-online.com

Colorcoat Connection® helplineT: +44 (0)1244 892434F: +44 (0)1244 836134 T: EIRE +353 (0) 1 2973365E: [email protected]

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