Building regulations in the UK require all new houses to undergo an energy efficiency calculation using SAP, This calculation generates the energy efficiency rating provided with all new houses on sale in the UK.
SAP is a catchall spreadsheet primarily designed for conventional houses with traditional heating. It assesses 2 temperature zones within a house, calculating the energy required to achieve 21 degrees in the living areas and 18 degrees in other areas for 9 hours a day on week days and 16 hours a day at the weekend.
Passivhaus houses are modelled to a far more specific and exacting energy calculation, designed to maintain a constant internal temperature of 20 degrees throughout the property during all seasons. The energy required to heat a conventional house to the same standard would be exceptionally high.
SAP is, therefore, too simplistic to adequately assess the energy efficiency of a Passivhaus - indeed, the limitations of SAP can actually penalise Passivhaus design against a conventional building, despite clear evidence to the contrary.
It has been repeatedly demonstrated that new buildings assessed under SAP perform considerably worse than estimated during design. Conversely, a significant number of Passivhaus certified projects have been monitored and have demonstrated an excellent correlation between predicted and actual energy consumption.
It is not unusual for a certified Passivhaus to achieve a B or C energy efficiency rating according to SAP.
In 2015 the UK government abandoned it's commitment to make all new housing carbon zero by 2016. Greentithe has been producing 'carbon zero' houses (as defined by SAP) since 2011. However, with the shift in policy away from more energy efficient housing, Greentithe is focussing exclusively on producing low-energy houses to the far more robust approach of Passivhaus standards.
The conflicts that exist between the limitations of SAP and the detailed approach of Passivhaus assessment, make it counter-intuitive and cost prohibitive to produce houses that are both Passivhaus certified and SAP rated 'carbon zero'.
As a house builder who has produced 'carbon zero' homes, our houses built to Passivhaus principles are by far the most energy efficient homes we have built.