SAP 10: What You Need to Know About Psi Values

With the arrival of SAP 10 drawing ever closer, it is crucial for clients to prepare for the upcoming changes. One of the key areas affected by these changes is thermal bridging, a critical component of SAP calculations. The new legislation aims to enhance energy efficiency in newly constructed dwellings, making it imperative for developers and assessors to understand the implications of Psi values.

What Are Psi Values?

Psi values measure linear heat loss across a junction between two thermal elements, such as the connection between a ground floor and an outer wall. Expressed in W/mK, Psi values quantify the efficiency of these junctions in limiting heat loss.

Thermal bridging occurs where two adjoining materials meet within a construction (e.g., mortar and blocks) or where different constructions connect (e.g., wall/floor junction). In addition to Psi values, U-values are also relevant, as they measure heat loss across a surface area rather than a linear junction.

Psi values are crucial because all SAP assessments must include a thermal bridging component. If Psi values are not manually calculated, default values are applied, which are often significantly worse than the actual figures. Relying on these default values can result in non-compliance with the Target Emission Rate (TER) and Target Fabric Energy Efficiency (TFEE), making it essential to ensure accurate calculations.

What Is Changing in SAP 10?

Currently, the heat loss assigned to all junctions in a dwelling can be determined using a combination of methods:

• Government-approved Accredited Construction Details (ACDs)
• Calculated Psi values, conducted by a thermal modeller trained in BR497
• Default Psi values
• A default Y-value of 0.15

To comply with the existing Part L of the Building Regulations, a mix of ACDs and calculated Psi values is often employed to reduce emissions and improve fabric performance.

However, SAP 10 introduces significant changes. The option to use ACDs will be removed, as they are now considered outdated and inaccurate. Additionally, achieving compliance using assumed values will become substantially more difficult. As a result, there will be a greater emphasis on accurately calculated Psi values to ensure compliance with regulatory standards.

Preparing for SAP 10

With the implementation deadline of June 15th approaching, it is vital for stakeholders to stay informed about the differences between the current version of SAP and SAP 10. Ensuring a clear understanding of the updated regulations will help developers and assessors avoid potential compliance issues.

By staying ahead of these changes, you can ensure compliance while optimising the energy efficiency of your projects.