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The road to zero carbon

The proposed changes to SAP: why they are required and their impact. By Sean Lockie

The Government’s target for “zero carbon” (full definition pending) homes by 2016 will be met, in part, by revisions to Part L (Conservation of Fuel and Power) of the Building Regulations (which is currently out for consultation) in 2010 and 2013. 

To meet these targets, the Department for Energy and Climate Change (DECC) and Building Research Establishment (BRE) have improved the existing Standard Assessment Procedure (SAP) model, the tool used for calculating carbon emissions on domestic buildings. They are currently seeking opinion under a consultation process, with a deadline for all responses of 4 September 2009.

Herein lies one of the challenges. The Building Regulations (Part L) are owned by the Department of Communities and Local Government (DCLG), but the model used to calculate emissions in residential projects (SAP), is owned and managed by DECC, a different Government department. 

So what are the proposed revisions to SAP?

The ten biggest changes are:

  • Seasonal profiles - Calculations for space and water heating were previously done on a per annum basis. These will now be monthly profiles which will allow better predictions of summer and winter emissions.
  • Weather data improvements - The weather data used for calculating the outside temperature has been updated.
  • Space cooling - The old SAP model didn’t take into account space cooling (air conditioning) which affected its overall accuracy. Aircon is becoming increasingly popular in residential developments, particularly in inner city flats where ‘heat island’ and noise pollution effects are greatest. Now if there is ‘fixed’ air-conditioning in a proposed development it will need to be modelled.  Mobile aircon units are exempt.
  • Thermal mass parameter - Thermal mass gains are now included.
  • Party walls now included - Research by the Energy Saving Trust has shown that a dwelling can lose between 12 per cent and 24 per cent of its heat from the party wall, depending on the size and type of wall. To rectify this, party walls will be treated (insulation and air tightness) to the same levels as the building envelope.
  • Internal heat gains now included - Internal heat gains, like appliances, lighting and cooking, are now recognised and contribute towards a buildings internal heat gains.
  • Boilers - The old SAP model used a single efficiency level for boilers. This has been changed to allow a user to apply their own efficiency level depending on the make and type installed and monthly profiles which differentiate between summer and winter loads.
  • Hot water - The generation of hot water through gas boilers generates a significant amount of CO2 (e) over the life of the building. In the past it was one profile fits all. Now if the model shows less than 125 litres per person per day then a five per cent saving on the emissions associated with the hot water will be recognised.
  • CO2 emissions factors - The CO2 conversion factors (like gas, electricity, oil) have been extensively increased.
  • Thermal Bridging - Users can now overwrite the standard defaults, provided that they can prove the construction details are from an appropriate accreditation scheme.

There will, however, still be critics who will call for more to be done to reward renewables and to model user behaviour.

What will be the effects of these changes?

These changes to SAP will have a major impact on how residential developments are designed in the future. The improved SAP model means that developers will now get more credit for passive design measures and controls, which previously would not have been picked up because the older model was too blunt. With the changes, a developer and the supply chain will have new options to consider and be credited for, as listed above. These long overdue changes could mean a rewriting of the rule book as new systems get tested on residential led projects.

There is still a gap between what was modelled by a design team and what is constructed on site and this will only be overcome by tougher commissioning regimes and post construction tests, such as air tightness and possibly thermograph tests to check insulation levels. And in the past, people in building control departments have been criticised for not fully understanding the carbon implications of some of the designs on the table, for example allowing too much biomass in inner-city areas (which creates issues with supply, delivery and emissions).

Sean Lockie is a director at Faithful+Gould