BUILDING designers face a minefield of calculations when working out energy-efficiency values of new homes.
Proposed amendments to Building Regulations Part L include moves to improve energy efficiency and to reduce carbon dioxide levels in new homes.
But two vital calculation methods do not stack up.Designers must use the Target Carbon Emissions Rating (TCER) equation to set the efficiency goal, then another procedure to work out if the final design will reach that target.
This procedure, the Standard Assessment Procedure for Energy Rating of Dwellings (SAP2001), is also under review.
The new SAP2005 is out for consultation until the end of October.
But the two SAP equations do not give a consistent performance level, so the TCER equation must also be rethought, according to Construction Products Association industry affairs director John Tebbit.
He said: 'The problem comes when the industry changes to SAP2005.That means the TCER will have to be changed too. Currently there is no correlation between the two SAPs.'
This is down to the changes outlined in the SAP2005, which includes provision for thermal bridging at junctions and around openings, solar gain and photovoltaic energy installations.
These changes would need to be echoed by alterations to the TCER equation to ensure a target-compliant house designed under SAP2001 would still comply with SAP2005.
Mr Tebbit said: 'There is likely to be a tweaking of all the factors in the TCER equation, but the most obvious change would be to adjust the 'shape' factor as the new SAP takes thermal bridging into consideration and the two are closely linked.'
The new Part L also includes standardised house designs that comply with the latest laws and are aimed at small-volume builders that do not have in-house design capabilities.
And such wholesale changes from the existing SAP will affect the development of standard or 'Model' designed houses, claimed Mr Tebbit.
He said it was essential the methodology behind the energy efficiency calculations were finalised before the industry began working on standard designs.
'We do not want to start designing standard models before confirming the methodology. It would be a waste of time.'
Mr Tebbit revealed the CPA and House Builders Federation were due to meet over the next month to thrash out a solution.