We need accurate planning tools SAP performance gap solved by post-processing to align results with PHPP

19 November 2023 by Jason Quinn

The UK’s first net zero energy bills have been introduced by several English councils as part of their response to the climate emergency. That’s good. These requirements need to be measured so they can be enforced. SAP 10 (Standard Assessment Procedure) is what they use to show building code compliance in England and Wales, for new dwellings and major renovations. 

The issue is that when SAP 10 results are matched to measured heating energy consumption, there’s a large performance gap. Buildings aren’t in reality performing nearly as well as the tool predicts they will. Contrast this with PHPP, which real-world studies have shown to be accurate. This is thanks to conservative assumptions and boundary conditions, as well as mandated measured infiltration rate tests (ie blower door testing). Together, these impose quite a bit of quality assurance on every Passive House project.

The workaround for the English councils is a correction tool for SAP to make it more like PHPP. It would have been easy, accurate and cheaper to just use PHPP. But as I pointed out in relation to ASHRAE, consenting authorities or professional bodies are reluctant to prescribe the use of proprietary software. Even when they trust PHPP enough to pay to build a correction tool for the Code tools they use instead.

UK Passive House consultant Chris Worboys goes into some detail here about how that happened. He explains that to enforce the policies these councils had introduced, they needed a way for applicants to use SAP 10 calculations to produce realistic design stage estimates of operational energy use.

“We have been working with the councils over the past few months to develop an Excel-based tool that post-processes SAP 10 results to produce design stage predictions of space heating demand, total energy use, and solar generation, which are similar to PHPP. The tool is now complete and is currently being used for new planning applications in Cornwall.”

Bottom line is we need ambitious policies and accurate tools to comply with them, whatever those tools are.

From the Bath and North East Somerset Council announcement:

The new housing development policy will ensure the energy use of any proposed development is measured and meets a specified target — setting a limit on the total energy use and demand for space heating. It will also require sufficient on-site renewable energy generation to match the total energy consumption of the buildings — ensuring the development is 100% self-sufficient.

New policies will also address building emissions such as a policy to limit carbon emissions resulting from the materials used in the construction of large-scale developments. These ‘upfront’ embodied carbon emissions will be limited to 900kgCO2e/m2.

The council will also impose net zero operational carbon standards for new major non-residential development.

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