The introduction of Part L 2021 in England has placed greater emphasis on thermal bridging heat losses and how they are calculated.

Compliance calculations, carried out using the Standard Assessment Procedure (SAP), have accounted for thermal bridging since 2006. The way the Part L 2021 requirements makes it harder than ever before to ignore accurate thermal bridging detailing.

Since 2006, accredited construction details (ACDs) have been available to assist in using accurate psi values (calculations of heat loss through thermal bridges) as part of the SAP calculations. Those ACDs have now been withdrawn, putting more emphasis on using other detail libraries or obtaining project-specific psi value calculations.

Why do national building regulations factor in thermal bridging?

A thermal transmittance, or U-value, is used to express heat loss through building elements such as walls, floors and roofs. Specifically, it is the amount of heat energy transfer through one square metre of the element, per degree of temperature difference between the warm side and the cold side.

Measuring heat losses by U-values alone does not account for all of the heat lost through the building fabric. At junctions between elements – such as where the ground floor meets the external walls, or at the eaves where a roof meets the external walls – the patterns of heat loss are different and the result is a linear thermal bridge.

Traditionally, much of the heat loss at junction details could be attributed to a lack of continuity in the thermal insulation from one element to another. Awareness of the need for a continuous thermal envelope has generally improved, though it is not achieved consistently.

Even where it is achieved, however, thermal bridging heat losses need to be accounted for. Simple changes in geometry of a building act as a thermal bridge too, because of differences between the internal and external surface areas of the building, and the way different construction materials interact with one another.

How have attitudes to thermal bridging changed?

Thermal bridging heat losses were introduced into national building regulations because it was recognised that, as U-values improved, psi values made up a greater proportion of overall heat loss. That trend has continued as subsequent versions of energy efficiency regulations, like Part L in England, have sought better and better building performance.

ACDs were also introduced in 2006, with the aim of showing how continuous insulation and airtightness could be achieved in common junction details. Junctions for different construction types – masonry, timber frame and steel frame – were included.

Each ACD included a checklist of key points for designers and installers so that the intended performance would be delivered on site. The given psi value for the junction could then be included in the SAP calculations.

Despite this, some project teams see thermal bridges and psi value calculations as an imposition or an unwanted extra cost. Others don’t understand the benefit of calculating psi values, even though they have been a feature of mainstream construction for a decade-and-a-half.

The nature of the compliance calculations means it has always been possible to use default psi values in SAP, compensate elsewhere in the specification, and still achieve compliance. Often, this turns out to be more expensive than getting psi value calculations carried out, but it has been an option. With the levels of performance required by Part L 2021, we may finally be seeing an end to any culture of ignoring thermal bridging.

Part L 2021 and the removal of ACDs

Over the course of the last fifteen years, ACDs have not been updated. It’s recognised they’re no longer fit for purpose in terms of the performance we want and need from our buildings, and they have been withdrawn as a result.

Other libraries of junction details have been produced over time, with the intention of supporting designers, specifiers and installers in achieving compliance. Many trade associations and product manufacturers have produced psi values as part of promoting the use of their products.

Where standard details don’t cover a junction as designed, bespoke psi value calculations, carried out by a competent person, remain an option.

Further emphasising the importance of thermal bridging in compliance, Part L 2021 includes more technical guidance on addressing junction details than has previously been the case. Read more about what it says on continuity of insulation at ground floor edges.

Polyfoam XPS has recently released new CAD details showing best practice junction detailing for ground floors, flat roofs and basements. Supporting the launch of our Below DPC insulation board, we have also produced a white paper that includes details and calculated psi values for cavity wall foundation details that meet the moisture resistance requirements of Part L 2021.

Alternatively, contact us to discuss how extruded polystyrene can help you to achieve compliance on your residential or commercial project.