HEM vs SAP Explained: What It Actually Means for Builders

⏱️ ~ 10 min read

Another regulation brings about another acronym, but this one is important. The introduction of the Home Energy Model (HEM) signals a significant change in how the energy performance of UK homes is assessed. HEM is set to replace the current methodology the Standard Assessment Procedure (SAP) and aims to give a more accurate picture of how homes actually perform, not just how they are designed to perform. So, what does this mean for builders? The short answer is that, once finalised, this model will affect how homes are constructed.

SAP Vs HEM: What are the key differences?

For many years, the Standard Assessment Procedure (SAP) has been the go-to methodology when it comes to assessing the energy performance of new residential homes. SAP ensures compliance with Building Regulations (Part L) and takes into account various elements such as insulation, heating systems, lighting, ventilation, and renewable energy technologies. However, SAP is based on standardised assumptions and uses monthly averages, which means the methodology relies on expected performance as opposed to actual results. SAP has also been criticised for not truly reflecting the performance benefits of renewable technologies.

To tackle this, the Department for Energy Security and Net Zero has introduced the Home Energy Model (HEM) to provide a more accurate model of energy use in the home and the impact of construction quality. Rather than focus on monthly averages, HEM calculates energy use every half hour, which offers better accuracy and relies less on standard assumptions.

This shift in approach aligns with the wider strategy of the Future Homes Standard, that sets out to close the performance gap between design and reality.

What the Government is really saying

The HEM document is extensive and is still being refined, however, the Government wants to focus on three main elements:

1. A stronger focus on the building fabric. HEM is designed to reflect real world homes, which means closer scrutiny on how the building fabric including walls, roofs and junctions actually perform.

2. Greater accuracy and accountability. Although changes have been made to SAP periodically since its introduction in 1993, it has been generally only used at the technical design stages and not reassessed post construction.

HEM is designed to include and accurately reflect the performance of renewable technologies such as heat pumps and solar power to improve “accuracy, robustness, and ensure it is fit for net zero.”

3. Less room for poor detailing. The switch to HEM will reduce relying on default assumptions, meaning a greater focus on the finer details such as junction performance will have a more significant impact on results.

What this means in practice

In short, the HEM methodology raises the importance of how homes are constructed, not just how they are designed. The quality of construction is the key focus, which means every element of detailing is under closer scrutiny.

Junctions in particular, become more critical. Whether they’re wall-to-floor, wall-to-ceiling or reveals to windows and doors, junctions are a potential weak spot as this is where the building typically experiences heat loss. For contractors, HEM means effectual detailing and consistency in construction is imperative on site.

Why lintels are part of the conversation

Since HEM encompasses the entire building envelope, lintels become part of the wider conversation about thermal performance. Windows and doors are already considered as high-risk areas for heat loss, and if lintels are not correctly designed or are not made to the right specifications, they can create thermal bridges. These resultant weak spots can impact overall energy efficiency.

Supporting effective thermal performance

Improving thermal performance is key, especially at junctions and thermally broken lintels play a crucial role. The aim is to reduce the risks of weak points that often cause issues in the thermal performance of a building and raise standards without overcomplicating traditional construction methods.

Thermally broken lintels incorporate a low-conductivity material between the interior and exterior frame sections, which reduces the risk of heat transfer (thermal bridging) at junctions. They ensure insulation continuity, preventing gaps or weak points in the building envelope.

Catnic’s thermally broken lintels use two folded steel sections bonded to a high-strength, thermally efficient core. The inherent design minimises thermal loss and provides reliable effective detailing with quality of construction maintained. This achieves psi values no greater than 0.05 W/mK, minimising thermal loss along the junction.

Using Catnic’s thermally broken lintels supports better real-world performance, which in turn aligns with the goals of HEM.

It’s all in the detail

In short, the transition from SAP to HEM is driving towards a more accurate methodology that reflects real-world performance. For builders, it ultimately means there are clear

implications on site such as:

• The quality of the construction will directly impact compliance

• Effective junction detailing will be imperative

• Ensuring consistency in construction methods on site will be critical

Paying close attention to these areas is vital in meeting future standards and delivering net zero homes. For a limited time, both HEM and a version of SAP will be available, so now is the time to get familiar with HEM and be ready to discover what it means in practise.