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Geoff Wilkinson’s Regs: Part L arrives

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The controversial new Part L is not only confusing, parts aren’t even ready

So October is here and Part L 2010 is with us, but the controversy surrounding its implementation won’t die down. Every new piece of legislation is subject to an implementation period, during which exemptions or former rules continue to apply. The transitional arrangements for Part L 2010 meant that any application submitted prior to 1 October would continue to be assessed under the 2006 requirements, provided work was started by 1 October 2011.

Communities and Local Government (CLG) has been criticised previously for allowing house builders to submit multiple applications for projects that were not ready to be built and introduced a backstop clause that works must commence within one year (i.e. by October 2011).

CLG has caused consternation amongst green groups by releasing a circular letter on 28 September, clarifying the transitional arrangements that widen its definition of commencing works. The guidance says that, in the department’s opinion, commencement would usually be marked by work such as:

  • Excavation for strip or trench foundations or for pad footings,
  • Digging out and preparation of ground for raft foundations; vibroflotation piling (with stone columns), boring for piles or pile driving,
  • Drainage work specific to the buildings concerned.

This is in-keeping with previous guidance, but the letter goes on to say that ‘where there are a number of buildings on a site covered by a single application, it is generally the commencement of work on the first of the buildings within the application that determines whether all the building work can take advantage of the transitional provisions, not each individual building’. There was a rush of applications to beat the deadline; Local Authority Building Control (LABC) reported record numbers of applications last September.

Andy Hardy, president of LABC, says: ‘The previous commencement of work rule allowing individual units to continue under existing standards was fair. But now developers are being encouraged to preregister whole sites that will be carried out in different phases for years to come. This means that new homes will still be being built to 2006 standards in 2013 or 2014. Purchasers won’t understand why, commentators won’t understand why, and we don’t understand why.’

The controversy doesn’t stop there, as CLG has admitted that a key part of the 2010 requirements, relating to thermal bridges, isn’t ready. As mentioned in AJ 01.07.10, the plan was to reduce heat loss through thermal bridging by introducing confidence factors in design and construction.

Under SAP 2005, the heat loss through thermal bridging was dealt with by a simple formula. This multiplied the total exposed area by a single factor ‘Y’, applicable to all dwellings, which was added to other heat losses through the building fabric. For junctions conforming to the Accredited Construction Details (including masonry and timber frame structures) a global Y-value was taken, typically 0.08W/mK.

The 2010 changes mean that this simple formula will no longer be acceptable. Instead, the simple Y-value used in SAP 2009 will be substituted by a dwelling-specific Y-value, which must be calculated individually for each dwelling, based on the length of each thermal bridge multiplied by the heat loss factor,  (psi), associated with the junction detail. The temperature factor ‘ƒ’ should also be calculated to determine surface temperature and the risk of mould growth, which can have significant health implications.

As the calculations can be complex, another way to comply involves adopting a quality-assured accredited construction detail that includes a confidence factor of 25 per cent or more than the value for an unaccredited detail. At the time of writing, no accredited construction details schemes had been approved, and any detail can be used in SAP without a performance penalty. CLG says that when it approves a scheme, it will reinstate confidence factors.

Dyfrig Hughes, National Energy Services’ technical manager, says: ‘House builders will be delighted at this news as it will make it easier to meet Target Emission Rates in the short term. However, the danger is that it will slow down the process of setting up schemes as the incentive of removing confidence factors will no longer be there.’

Hughes may not have much longer to wait, as the Timber Research and Development Association’s Technology’s Building Performance Service has calculated the -value for 20 common details in timber frame construction, based on ‘standard’ details it previously published. The new guides, Timber frame: standard details for thermal performance Volumes 1 (Mineral wool insulation) and 2 (Rigid foam insulation), are available now.

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