Mind the Gap/SAP – Go for Passivhaus

There is a gap between real energy consumption and consumption calculated by SAP (standard assessment procedure), known as the performance gap. In spite of this sometimes enormous gap it still used by the majority of engineers to size heating loads for homes, boiler sizes and everything else related. The savings made by adding Solar thermal and solar electric are then put forward to show what massive reduction in energy use is going to be delivered.

In the paper cited in the last blog (A Comparative Study of the Effects of Thermal Mass in New Dwellings in Scotland by Janice Foster) two identical dwellings, one of high mass and one of low mass had their energy consumption monitored and then compared to their SAP calculations. Both buildings had identical architectural designs (design layout, U-values, etc.) and were in the same location and orientation and so SAP predicted the boiler heating load for both would be 20 kWh/m2/annum. The actual consumption was 67 kWh/m2/annum for the high mass building and 89 kWh/m2/annum for the low mass.

Apart from showing how thermal mass can benefit modern, highly insulated buildings by reducing heating requirements by up to 20% it showed the monumental discrepancy between calculated and simulated energy consumption. This is very worrying given that the output of SAP is pivotal in a lot of domestic scale decisions regarding heating and lighting and also policy decisions from government.

I recently looked at the SAP (2009) calculation for the Silverton Passivhaus building and again found a very significant difference between the actual and calculated energy performance of the building. However, this time the building was using at least 30% less energy than calculated rather than the norm of using significantly more. I say at least 30% less as SAP doesn’t include cooking in it’s calculations whereas the total energy figures for the house do.

Maybe it’s time we switched over to using rather more complex modelling tools, such as PHPP (the spreadsheet used to model Passivhaus buildings) which will actually reflect the real performance of a building in most cases. The situation we are currently in appears to be that in most cases SAP does not represent the energy consumption likely.

Admittedly the reasons for this are largely due to the generally poor build quality of UK housing combined with massively variable occupant behaviour. The build quality of Passivhaus certified buildings is much higher than the norm as without this they will not achieve the levels of airtightness required. Also, the simplicity and attention to detail with which they are designed makes building them much easier from the outset.

If we are to really push our 21st century housing in to the 21st century we need to design it, model it and build it properly. It is time we stopped being apathetic about the way our housing is built and demand houses that perform as designed and don’t waste energy. Afterall, it is as important to make the volume house builders build homes that don’t waste energy as it is to demand that the energy used to heat and light them is not exorbitant.

If you’d like to read more about (including the SAP calculations) the Silverton Passivhaus then see the case study here.

Chris Brookman
Chris Brookman