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Why Detailing Matters in Part L 2021 and Section 6 2021

England, Wales and Scotland have all updated the sections of their Building Regulations and Standards handling the energy efficiency of buildings (contained within Part L for England and Wales and Section 6 (Energy) in Scotland). In addition to setting lower carbon emissions targets for buildings, these updated regulations also look to tackle the so-called 'performance gap' between the designed and actual energy performance of properties. Research has suggested the average home may use up to 40% more energy than anticipated. In addition to raising running costs, this presents a serious barrier to constructing net zero carbon buildings. 

One of the most notable changes included to address this issue is the removal of the Accredited Construction Details (ACDs), a set of standard junctions designed to address thermal bridges in a construction. The backstop values in SAP and SBEM have also been worsened, meaning projects which rely on these values will have to make significant improvements elsewhere in their specification. Additionally, installers will need to provide compliance reports including photography of arrangements at key junctions. These changes mean project teams will need to pay much closer attention to details to reach compliance

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Why do we need to address thermal bridges?

Thermal bridges are areas of the building envelope which are more conductive to heat than the surrounding construction. These commonly occur where there is a gap in, or penetration through, the insulation layer and act as a fast-track for heat to escape out of a property. Thermal bridges can account for as much as 30% of the total heat loss from a property. This is why the Passivhaus Standard requires all buildings to be effectively thermal bridge free. 

Addressing this type of heat loss is increasingly important as we transition to low carbon heating technologies such as air source heat pumps. These operate at a lower temperature than conventional gas boilers and have a lower coefficient of performance in cooler months, meaning they’re less efficient at heating during these periods. It is therefore critical that properties retain heat as effectively as possible.  

By addressing key junctions, and developing accurate workable details, it is possible to achieve substantial heat savings and ultimately ensure buildings perform as expected on paper.

Why were the ACDs removed?

The ACDs were first developed back in 2002 as the energy performance requirements within the Building Regulations began to ramp up. They covered a range of different constructions including steel frame, timber frame, masonry cavity walls and solid wall insulation. In 2010, Scotland published its own set of ACDs which were further updated in 2015.

Whilst the ACDs provided a useful reference point for designers, it was increasingly felt that these details were out of date and that the construction industry was better placed than regulators to develop and maintain up-to-date and effective details. 

Other key changes to thermal bridging in SAP and SBEM

In addition to using the ACDs, designers previously had the option to use a default Psi-value for an individual junction (contained in table K1 of SAP). These were typically double the Psi-value achieved when using the ACDs. Alternatively, they could assume a global Y-value for the property. Both of these options were designed to assume high levels of heat loss at junctions, encouraging designers to consider junction design.

In the latest version of SAP and SBEM, which came into force with the new energy efficiency requirements in 2022, the default Psi-values have been worsened. Where a default Y-value is available, this has also been worsened.  This means that other areas of the specification may have to be significantly improved to reach compliance when using these values.

For example, in the illustration below we have used the new global Y-value within SAP to determine what upgrades would be needed for a three-bedroom property to achieve compliance under the Part L 2021 requirements for new homes in England.

Element                                                                                             

Specification                                                                                           

Walls (W/m2K)

0.10

Roof (W/m2K)

0.10

Floor (W/m2K)

0.10

Windows (triple glazed) (W/m2K)

0.90

Door (W/m2K)

0.80

Air permeability (m³/m²/Hr@50Pa)

1.0

Ventilation

Mechanical Ventilation with Heat Recovery (MVHR)

Heating appliance

Gas Combi with TTZC and a weather compensator

Waste Water Heat Recovery (WWHR)

Yes

Photovoltaics

1 Peak kW photovoltaics

Even with a specification that is significantly upgraded over the notional building, including lower U-values, triple-glazed widows, reduced air-leakage and an MVHR unit, the home would still only just meet the higher Target Fabric Energy Efficiency rate. The use of PV and WWHR, is also essential to meet the carbon emissions and Primary Energy targets. This means using the default Psi-values or the new global Y-value may significantly increase the cost of reaching compliance.

What are the alternatives to using ACDs?

There is a raft of manufacturer-created and industry-created details and guidance now available reflecting actual build-ups and performance, complete with pre-calculated Psi-values. These include reputable non-government databases containing independently assessed thermal junction details. In practice, however, these will not cover every scenario so it may be necessary to produce bespoke details with calculated Psi-values.

Learn more about the benefits of bespoke detailing.

When developing these, it is important that all junction details are reflective of real constructions, with good thermal performance to minimise losses, but also represent good practices and are buildable.

Click here to download indicative psi-values and standard details for our products.

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  • What is Thermal Bridging?
  • What are Psi-values and Y-values
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In this article we look at what thermal bridging is, where it occurs and how to prevent it.

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We look at the importance of Psi and Y-values of a construction.

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What are the Benefits of Bespoke Detailing?

We look at the benefits of bespoke detailing in calculating linear or geometric thermal bridges

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