We use the Passivhaus Planning Package (PHPP) for energy calculations because of the performance gap in SAP calculations. REALcosting gives reliable energy performance using Passivhaus software and produces indicative build costs as well. For long term energy use accurate energy calculations do make a difference, this is where REALcosting can help.
In the UK energy calculations are widely available using SAP based methods because they are required by Building Control for new houses, however the construction industry recognises and frequently discusses a ‘performance gap’ where buildings do not meet the energy performance intended.
Four studies are shown below, the performance gap can be seen in the first 3 studies. No Performance gap is seen in the Passivhauses in the last two studies.
1) NHBC Study found actual energy could be double.
2) The Green Construction Board found that actual energy use could be double that of the EPC certificate.
3) LEAP and Newcastle University found that there was quite a performance gap using conventional software, but hardly any for Passivhauses. This is partly also due to the extra rigour of the Passivhaus certification process. Every Passivhaus must have a calculation in PHPP. There are 60,000 Passivhaus Projects worldwide and these save 80-90% of the heating energy.
4) The passivhaus Institute also found that even though there was a range of results the average measured consumption matched the PHPP prediction well.
SAP methods are quite basic and do not usually include climate data, measurements of windows (in rdSAP) or accurate u-values for the windows. Most energy assessors use SAP based software for their energy calculations, however SAP was not intended to accurately represent the energy use of a building in use, only to allow reasonable comparison of consumption for home buyers and in any case quite a lot of the energy consumption is specific to the occupier, and what temperature they like to live at.