PHPP vs. SAP: The Path to Low Energy Excellence
When it comes to assessing the energy efficiency of homes in the UK, two primary methodologies are often discussed, PHPP vs. SAP. the Standard Assessment Procedure (SAP) and the Passivhaus Planning Package (PHPP). While both aim to evaluate a building’s energy performance, they differ significantly in their approaches, applications, and outcomes.
SAP: A low Bar for Energy Efficiency
SAP is the UK government’s approved method for assessing and comparing the energy and environmental performance of dwellings. It is primarily used to demonstrate compliance with Building Regulations and to produce Energy Performance Certificates (EPCs) for new homes. SAP calculations consider a variety of factors such as construction materials, heating and hot water systems, insulation levels, and air leakage. To ensure consistency across assessments, the methodology relies on a range of standardised assumptions. These include average UK climate data, such as typical temperature ranges, solar radiation, and weather conditions, which are used to model heating and cooling demands. SAP also assumes typical occupancy patterns, estimating the number of occupants, their energy usage habits, and the amount of internal heat gains from lighting, appliances, and other household activities. Furthermore, it assumes standard levels of ventilation and insulation, based upon standard details. While these assumptions help create a consistent framework for assessment, they may not fully reflect local microclimates, unique building designs, or the specific behaviours of individual occupants. As a result, SAP’s generalised assumptions can sometimes lead to discrepancies between predicted and actual energy performance.
Why PHPP is the Game-Changer in Low-Energy Building Design
In contrast, PHPP is a detailed design and certification tool developed by the Passivhaus Institute in Germany. It is used globally to design ultra-low energy buildings that adhere to the rigorous Passivhaus standard. PHPP provides an in-depth analysis of a building’s energy balance, considering local climate data, building geometry, thermal bridging, and specific component performance. This meticulous approach allows for precise predictions of actual energy consumption and indoor comfort levels. PHPP is particularly favoured by designers aiming to optimise energy efficiency and achieve superior building performance. If you want learn more about Passive house check out our first ever blog post.
The Path to Low Energy Excellence: PHPP vs. SAP
One crucial distinction is that SAP assessments are legally required when selling or renting a home in the UK, as they produce the Energy Performance Certificate (EPC), which is mandatory for compliance with building regulations. This regulatory requirement can make PHPP seem redundant, especially since it is not a legal necessity. However, for homeowners and developers who want to optimise their building’s energy efficiency beyond the minimum standards, PHPP is an invaluable tool. Unlike SAP, which focuses on compliance, PHPP enables precise energy modelling to minimise heating demand, improve indoor comfort, and reduce running costs. Developers targeting low-carbon or Passivhaus-certified homes will use PHPP to refine insulation levels, airtightness, and ventilation strategies to achieve the highest possible performance. In essence, while SAP is necessary for regulatory approval, PHPP is essential for those who want to go beyond compliance and create truly energy-efficient, comfortable, and cost-effective homes.
What the SAP Reform Could Mean for Sustainable Building
It is safe to say that not long after the release of SAP10 in 2012, a large section of the sustainable construction and decarbonisation community began calling for a reform of the policy related to EPC ratings. The Retrofit Hub wrote an excellent report outlining the eight steps for change they would like to see. It was supported and backed by many key players in the UK construction sector, including consultants, designers, academics, community organisation members, contractors, installers, and developers, to name a few. The report concludes with a national campaign and rebrand, aiming for EPCs to provide a more holistic view of building performance and health. In December 2024, the UK government initiated a consultation to reform SAP. The consultation will be completed by March 2025. The primary aim is to enhance the accuracy and relevance of energy assessments, aligning them more closely with the nation’s net-zero emissions target by 2050. Proposed changes include updating EPC metrics to focus on aspects such as fabric performance, heating systems, and smart readiness, as well as refining the requirements for Energy Performance Certificates (EPCs) and Display Energy Certificates (DECs). These reforms could lead to more comprehensive and informative energy assessments, providing homeowners and developers with better insights into building performance and potential improvements.
There is growing discussion about integrating more accurate and performance-based assessment tools into energy ratings. The PHPP has been developed over the last 30 years using data from monitored built examples to inform and improve the accuracy of the results. This ensures a level of accuracy in the energy balance that the Passivhaus Institute claims is within +/- 0.5 kWh! This of course accounts only for buildings which have been certified. Significantly, this eliminates what is known as the ‘performance gap’ when actual real-life energy usage exceeds the predicted figures.
Etude Sustainability engineers have done extensive work with Cornwall Council to identify the gap between SAP and real-life energy use compared to PHPP. For example, SAP assumes higher internal heat gains from occupants and appliances, which can lead to a lower estimated heating demand. In contrast, PHPP uses more conservative estimates for internal gains, resulting in predictions that align more closely with actual energy usage.
One study looked at space heating energy use predicted by SAP versus measured energy use for 50 new dwellings. The measured energy use was, on average, 27.8 kWh/m²/yr more than predicted by SAP. Whislt another measured energy use was on average 62 kWh/m2/yr more than predicted by SAP. Etude has created a Methodology for converting SAP to more realistic expected energy use, bridging the gap between theoretical assessments and actual performance. It explains in details the difference between PHHP and SAP for energy use calculation.
While PHPP provides a more detailed and precise energy model, especially for low-energy and Passivhaus-standard buildings, it would require significant adjustments to align with the regulatory structure. If reforms aim to incorporate more dynamic and fabric-first approaches, there could be a case for allowing PHPP assessments as a valid alternative or supplementary tool.
Resolve’s Role in Bridging the Energy Performance Gap
Resolve values the role of EPCs in providing a standardised and accessible way to compare the energy efficiency of homes quickly and easily. However, we acknowledge that EPC ratings, based on SAP calculations, are not always accurate predictors of actual energy use and are often misleading regarding running costs. Instead of SAP’s broad, cost orientated assessment approach. We appreciate the emphasis on actual energy consumption produced by PHPP, ensuring a more meaningful assessment of a building’s sustainability.
For many homeowners or developers whose goal is not to achieve Enerphit or Passive house certification. EPC rating provide their only indication for comparison. Therfore, we use our own conversion prediction to give an indication of what EPC rating a building might receive. This is best done when an EPC is available prior to a project, allowing a PHPP assessment to be made on both the pre- and post-refurbishment energy performance. We do see a time in the future where a PHHP will become a valid and recognised alternative to SAP. However, these will likely be certified projects.
