BUILDINGS &#38; ENERGY

Guides to thermal performance of curtain walls Revisions to Part L of the Building Regulations have prompted the Centre for Window and Cladding

Technology to re-publish its guidance on thermal performance of curtain walls. H

eat flow through curtain walling, and other forms of cladding with metal and glass components, is complex due to lateral heat flow within the components, and to cold bridging. Calculation of U-values (heat transfer values) should take full account of all heat flow through the wall by conduction. Furthermore it is necessary to know the temperatures accurately when determining the risk of condensation. The revised CWCT guides draw together previous work on heat flow through glazing systems and more recent work on lateral heat flow in insulated metal panels. The U-value of a wall may be

determined by applying the area-weighted method to sum the U-values of separate parts of the wall. Individual parts of the wall may be assessed for U-value using either a hot-box or calculation by finite element or finite difference method. However, it is normally incorrect to divide a wall into its individual components and correct assessment of an overall U-value for a wall is dependent on the analyst’s skills when dividing the wall into elements. When applying the area-weighted method, it is essential that heat flows only through the assessed element of the wall and not laterally between elements. The wall must be divided

into a series of elements separated by adiabatic boundaries. Thus an element for the purposes of U-value assessment may comprise half of a glazing unit, a glazing frame and half of an insulated panel. The elements to be assessed are thus complex and may not be realisable, as they comprise several incomplete components. It is very difficult to test elements in a hot-box if they include partial components with little structural integrity such as half of a gas-filled glazing unit. It is easier and more flexible to assess U-value by calculation than measurement. Analysis allows the rapid assessment of part components and substitution of one component for another and may be used to show whether correct boundaries have been assumed. The complex geometry of many components used in building envelopes leads to a risk of inclusion of cold bridges within the envelope. This effect becomes more significant for walls that are otherwise better insulated with lower overall U-value. The problem is particularly acute where materials with very low thermal conductivity are used. Whilst hot- box testing may show surface temperatures, calculation by the finite difference or finite element methods will clearly show temperatures throughout the wall and the presence of cold bridges within the wall. Cold areas of the inner surface may cause surface condensation that is normally a nuisance rather than a cause of serious damage. However, cold bridges can reduce the temperature within the wall below the local dew-point and cause interstitial condensation. This may lead to corrosion of metals or degradation of insulation. Presence of

HIGHWAYS &#38; ENVIRONMENT

Winter challenges on the highway network

For many years, the practice of salting highways has been derived through ‘custom and practice’, rather than being based upon scientific research. Prompted by the challenge of procurement on the basis of Best Value, organisations across the UK are now asking questions such as – With budget and resource constraints, is the service currently delivered the best that can be achieved?

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(Top)Thermal distribution at panel edges (Above) Temperature distribution at a window head showing cold bridging

Research Focus NO. 48 FEBRUARY 2002

RL was asked by the National Salt Spreading Research Group to under- take independent trials to advance the understanding of salt spreader performance with different salt systems, the effective de-icing of highways under the effects of traffic, and measurement of residual salt. The research will lead to a far greater

understanding of the salting of carriageways which will, in turn, lead to greater efficiency, cost savings, improved service delivery and safety, and environmental benefits.

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moisture within any insulation will increase its thermal conductivity and the overall U- value of the wall. Accurate condensation risk analysis is necessary to ensure that cold bridging is minimised and that adequate vapour barriers are included.

The CWCT thermal guides comprise:

• Standard for specifying and assessing for heat transfer (the U-value)

• Guide to good practice for assessing glazing frame U-values

• Standard for specifying and assessing for condensation risk

• Guide to good practice for assessing heat transfer and condensation risk for a curtain wall.

The guides show worked examples of U- value calculation and give guidance on:

• specification of U-value assessment; • the selection of appropriate elements for calculation;

• calculation of overall U-values; • identification of cold bridges; • condensation risk analysis; • appropriate software for analysis; • current UK codes and regulations.

Part L of the Building Regulations (England and Wales) may be found at http://www.safety.dtlr.gov.uk/bregs/brads .htm. The CWCT maintains a website, http://www.cwct.co.uk/thermal that gives guidance on standards, software and certification schemes.