Thermal Bridging: The Business Case for Early Assessment

By Alisha Pinheiro, Senior Building Physics Consultant

In the drive for greater energy efficiency, building fabric has taken centre stage. However, thermal bridging is often overlooked, leading to compliance issues and unnecessary costs.

For a typical UK dwelling, ‘cold bridges’ - the junctions where building elements like walls and floors meet - can account for 15–30% of total heat loss. When these junctions are assessed using generic, default psi-values (ψ-values), the calculated heat loss is often penalised, making compliance with Part L of the Building Regulations challenging

This blog makes the business case for a more precise approach, explaining why bespoke ψ-values are the best ‘invisible insulation’ you can specify, delivering savings in cost and carbon, whilst improving compliance, comfort and health. This blog also presents a simple, reproducible workflow which identifies the top heat-loss junctions in a project, explains how to model them accurately and demonstrates the significant resulting savings.

Understanding the Technical Details: U-values vs. ψ-values (psi values)

To appreciate the impact of thermal bridging, it's crucial to understand the two key metrics that govern a building's fabric performance: U-values and ψ-values.

U-values (W/m²K): This is a measure of thermal transmittance through a plane element of construction, like a wall, roof or floor. It indicates how much heat is lost per square metre of the element. A lower U-value means better insulation.
ψ-values (W/mK): A psi-value (ψ-value) quantifies the linear thermal transmittance at the junctions between elements or where the insulation layer is penetrated. It measures the extra heat flow that occurs at these specific lines, which isn't captured by the U-value alone.

The U-value represents the overall thermal performance and the ψ-values represent the heat lost through the edges and junctions of building fabric. In buildings, relying solely on good U-values without addressing the junctions is highly likely to result in poor predicted and actual performance.

The Problem with Defaults: Why Generic ψ-values Fall Short

Historically, designers could rely on government-approved schemes like Accredited Construction Details (ACDs) to provide a set of standardised ψ-values. However, with the introduction of the 2021 update to Part L and the forthcoming Future Homes Standard, these ACDs have been withdrawn. The regulations now push project teams towards calculating project-specific ψ-values.

Why has this changed? Default values, whether from old schemes or the conservative back-stops in the Standard Assessment Procedure (SAP), come with significant drawbacks:

Default values are inherently conservative: Default values assume a worst-case scenario to ensure a margin of safety, often overestimating the actual heat loss at a junction. This penalises your energy model, particularly in the crucial Fabric Energy Efficiency Standard (FEES) calculation in SAP.
Default values can reduce innovation: Default values are based on outdated, generic construction methods and don't reflect the high-performance details and materials used in modern designs. This ‘one-size-fits-all’ approach can lead to thicker, more expensive wall build-ups as designers overcompensate with insulation to meet targets which pushes up build costs and carbon intensity.
Default values create a performance gap: Relying on inaccurate, generic values leads to a disconnect between the designed performance and the as-built reality, contributing to the well-documented performance gap in UK buildings.

The Business Case for Bespoke: More Than Just Compliance

A bespoke thermal bridging assessment is a strategic and relatively modest investment that can deliver significant and disproportionate returns. For architects, developers and contractors the benefits include:

Significant Cost Savings: Accurate thermal modelling allows for value engineering of the building fabric. Bespoke ψ-value calculations can lead directly to:
- Reduced insulation thickness: By proving a junction performs better than the default, the amount of insulation required in the main elements can be reduced, saving on material costs.
- Avoiding expensive compensatory measures: Carrying out thermal modelling early in a design’s development can reduce the likelihood of failing SAP assessment at a late stage. This avoids adding costly technologies like photovoltaic (PV) panels or Mechanical Ventilation with Heat Recovery (MVHR) at a late stage simply to achieve compliance. Improving the fabric performance with accurate ψ-values can be a more cost-effective, fabric-first solution.
- Value engineering specifications: In some cases, improvements from bespoke ψ-values have been significant enough to relax other specifications, such as removing the need for triple-glazing, reducing insulation or relaxing air permeability targets, delivering huge cost savings across a development.
Greater Design Flexibility: A precise understanding of a building’s thermal performance can empower creative design. An understanding of how key junctions perform provides greater freedom with a design avoiding penalisation by conservative default values. This can allow for more innovative and complex architectural solutions that are also highly energy-efficient.
Enhanced Performance & Occupant Comfort and Health: Minimising thermal bridging has direct benefits for the end-user.
- Reduced risk of condensation and mould: Thermal bridges create cold spots on internal surfaces. When warm, moist internal air comes into contact with these surfaces, condensation can form, leading to mould growth - a serious health concern and a potential cause for costly defects and potential liability claims. Thermal bridging analysis includes a condensation risk assessment (calculating the fRsi value) to ensure designs are robust.
- Improved comfort: A consistent building envelope with no cold spots provides more comfortable and stable internal environment for occupants, reducing draughts and improving overall wellbeing.
Future-Proofing & Robust Compliance: With Building Regulations tightening and the Future Homes Standard on the horizon, a fabric-first approach is likely to become best practice. Bespoke thermal bridging analysis provides a robust, evidence-based route to compliance with Part L. It demonstrates a commitment to quality and ensures buildings are designed to perform as intended, future-proofing them against upcoming regulatory changes.

A Simple Workflow for Success

Adopting bespoke ψ-values is a simple process which can be integrated seamlessly into the design workflow, particularly around RIBA Stage 3-4, before construction details are finalised.

A typical workflow includes:

Identification of Key Junctions: Review the architectural drawings to identify the repeating junctions that make up the bulk of a building's thermal bridges (e.g. ground floor/wall, window jamb, eaves, intermediate floor).
Model & Calculate: A building physics specialist uses 2D thermal modelling software to calculate the specific ψ-value and fRsi value for each of these key junctions based on precise drawings and material specifications.
Optimise the Design: If a junction is underperforming or showing a condensation risk, the model allows the testing of different solutions, such as adjusting insulation placement or using thermal break products, before they become expensive problems on site.
Input into SAP: The calculated, bespoke ψ-values are then used in the SAP calculations, replacing the default values and providing a more accurate picture of the building's energy performance.
Bank the Savings: The improved SAP results often create enough performance margin to value-engineer other parts of the build, as detailed above.

The information below summarises common problem areas and how to approach them.