The rules

On 21 July 2004 the government published its proposals for changes to Part L of the Building Regulations as part of its aim to cut carbon dioxide emissions 60% by 2050. The construction industry has less than two months left in which to respond to the document.

For specifiers of cladding there is plenty to mull over. Here we summarise 10 key proposed changes that would affect the specification of cladding for non-dwellings.

1 Broadening the definition of “material alteration” in the regulations. This will bring more maintenance and alteration work within the scope of Part L and so improve the energy efficiency of the existing building stock. The document suggests that when replacing complete external walls or replacing their internal renderings and plaster, a reasonable thickness of insulation should be applied and the building should be made airtight. Where doors and windows are replaced, the new units should meet the requirements for new buildings.

2 Allowing a trade off between construction elements and boiler efficiency to permit greater design flexibility. Within limits, U-values could be relaxed if the heating performs better. However, fabric insulation is a longer term measure than boiler efficiency, because an efficient boiler could be replaced with an inefficient model later in the building’s life. Therefore, greater latitude is allowed in terms of trading off reduced heating system efficiency for better fabric performance than vice-versa.

3 Significantly increasing a building fabric’s insulating performance. This is proposed to be implemented in two phases (see below) – one six months after publication of the amendment and the second 24 months after publication.

4 Increasing the standards of detail design and site workmanship to reduce the incidence of gaps in insulation and the effects of cold bridging. Breaches of the insulation can be caused by joists and rafters, flanking around the edges of windows and doors, or by a reduction in insulation thickness. The document calls for the adoption of robust design details as a means of showing compliance. Designers may be able to use alternative calculations as a way of meeting the requirements, although the guidance on how this can be done is yet to be published.

5 Raising the standard of the fabric’s airtightness to reduce the amount of heat lost through uncontrolled ventilation. Air leakage should be no more than 10 m³ per hour per m² of external surface area at a pressure difference of 50 Pa. The changes propose that fan pressurisation tests should always be undertaken where buildings have a floor area greater than 1000 m². For buildings with a smaller floor area, proof of compliance will be through the use of robust design details.

6 Introducing performance standards to avoid solar overheating. These would include controlling the size and orientation of windows and shading and using the building’s thermal mass to absorb excess heat during the day. This requirement has been included to prevent any subsequent user of a building from being forced to use comfort cooling just because the envelope performance is so poor. For offices and shops, windows and doors will be allowed to make up no more than 40% of the exposed walls; in the case of industrial buildings, the figure drops to just 15%. Rooflights will be allowed to make up no more than 20% of the roof area for any type of building.

7 Introducing performance standards for buildings that are air-conditioned or mechanically ventilated and have a floor area greater than 200 m². For offices a method of demonstrating compliance is proposed that uses a carbon performance index alongside pass levels based on current typical performance. This is intended “to eliminate a significant proportion of current ‘typical’ performance that currently falls short of reasonable practice”.

8 Rating the building’s carbon emission potential in terms of kg of CO₂ per m² of floor area as a means of demonstrating compliance. One way of calculating this value is to adapt the carbon performance index used for air-conditioning (see point 7) to include the energy used in heating and lighting the building. This approach allows greater flexibility in trading off between the building envelope and the building services while maintaining a satisfactory level of performance. Alternatively, the annual carbon emission of the proposed building can be compared with that of a similar building that complies with the elemental method of calculation. To meet this standard, the building envelope has to provide certain minimum levels of insulation and the various building services have to each meet defined standards of energy efficiency.

9 Introducing requirements for checking that completed buildings with a floor area greater than 1000 m² match their approved design. Ways of showing compliance include certification by the installer, surveys with infrared cameras and airtightness pressure testing. The proposals include standards for remedial action in the event of test failures.

10 Enabling occupying organisations to monitor the energy consumption of the building services – such as the heating, lighting and air-conditioning – and introducing energy consumption meters. Occupiers will be able to compare energy consumption with national benchmarks.

• To see the proposals, visit www.odpm.gov.uk, then select “Building Regulations” and click on “consultation papers”.