Energy Performance of Buildings (Scotland) Regulations 2025: stock model research

2 Introduction

The domestic energy landscape in Scotland will shortly be undergoing a significant transformation. The current methodology for assessing the energy performance of residential properties, known as the 'Standard Assessment Procedure' (SAP), is scheduled to be replaced in 2026 by the new 'Home Energy Model' (HEM).

The Home Energy Model will be adopted across the UK, leading to changes to EPC calculations and ratings being introduced by the UK Government and Scottish Government through new regulations. Both Governments have proposed a new rating to measure the fabric energy efficiency of domestic properties. In Scotland, this new rating will be known as the Heat Retention Rating (HRR), and it will become the primary benchmark for legislation on energy efficiency in homes, superseding the existing SAP rating.

This report, commissioned by the Scottish Government, provides an analysis of the projected impacts of this transition. By using a representative dataset of Scottish homes and our proprietary energy modelling tools we have simulated the outputs of the new HEM methodology. Our analysis directly compares the results from the new model with those from the current SAP/RdSAP methodology, offering critical insights into how the new metrics will affect the energy ratings of different housing archetypes and tenures. The findings of this research are designed to inform policies that will allow a smooth and effective transition to the new system while maximising continuity in energy performance assessment.

2.1 Objectives

The objectives of this project are:

• To develop and validate a robust methodology for large-scale energy modelling. This includes the preparation and ingestion of a suitable dataset, and to build a robust methodology for conversion of RdSAP data for use with HEM within a system capable of batch processing of calculations.
• To assess the impact of the new Home Energy Model (HEM) on the energy performance ratings of Scottish housing stock. This involves a direct comparison of the new HEM ratings with the existing EPC ratings and metrics derived from SAP, identifying how homes will be affected by the change in assessment methodology.
• To propose a threshold value for the Heat Retention Rating (HRR) that aligns with the current energy rating bands. The goal is to find a threshold that maximises equivalence between the current SAP rating bands and the proposed HRR bands, thereby minimising disruption for homeowners, landlords, and policymakers.
• To identify and document key assumptions and limitations of the modelling process. This ensures transparency and provides a clear understanding of the confidence levels associated with the findings, particularly in relation to the use of proxy data and the ongoing development of the HEM.

2.2 Overview

In this report, we first establish the Research Background for this project by providing information about the transition from the current SAP/RdSAP assessment methodology to the new Home Energy Model (HEM).

We then detail the project Methodology, from preparing and ingesting the Scottish housing data, building a converter to write it into a format suitable for the new HEM/HEM wrapper model, and performing large-scale batch processing. The Results section presents our key findings as summarised in the Executive Summary.

Finally:

• Appendix: preliminary additional Scottish HEM wrapper ratings and indicators offers some observations on the outputs of the Scottish EPC wrapper for consideration in the design of secondary ratings and other metrics which may feature in the final Scottish EPC design.
• Appendix: RdSAP to FHS Assumptions provides full details of the assumptions and technical issues encountered during the modelling process.