Cutting details down to psi

Jonathan Ducker from Kingspan Insulation GB explores how the industry has developed a range of details to assist specifiers pursuing higher thermal performance 

As part of the recent updates to the energy efficiency requirements in the Building Regulations and Standards in England, Scotland and Wales, there has been a notable effort to close the performance gap between the design and actual energy performance of buildings. This gap can be significant, with testing showing that total energy use from some new homes can be as much as 250% higher than expected. In particular, the new regulations look to raise practice around addressing thermal bridges – implementing a range of measures to encourage the use of designed insulation details for junctions and to ensure that the original specification is carried through into the final building.

Why thermal bridges matter

Thermal bridges (also known as cold bridges) are areas where materials which are more conductive to heat than the insulation, are allowed to form a ‘bridge’ between the internal and external parts of a construction. These can act as a fast-track for heat to escape and can account for as much as 30% of total heat losses from a property. This means addressing them is a key step to ensuring a good level of fabric performance.

Thermal bridges typically occur because of gaps or inconsistencies in the insulation layer and are common in areas such as junctions between the wall, floor and roof; junctions around openings such as windows; and at penetrations such as pipework.

The heat losses at these junctions are measured as psi-values (Ψ). Heat losses at penetrations through layers can also be measured as chi-values (χ). A lower value represents lower expected heat losses. A global y-value can also be calculated, representing the percentage of overall heat losses from a building associated with thermal bridging. 

What’s new in the regulations?

The updates to Part L of the Building Regulations in England and Wales, and Section 6 (Energy) of the Building Standards in Scotland implement a number of key updates designed to raise practice around junctions and thermal bridges. In the design phase, these changes encourage the use of details and associated psi-values where possible. 

As under the previous regulations, when modelling a property within SAP or SBEM, energy assessors still have the option to use a default psi-value for a junction where no detail is provided. However, this default value has been notably worsened. Similarly, where a default global 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.

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.

It is also important to note that the Accredited Construction Details (ACDs) have now been removed in England and Wales. These were first developed back in 2002 and provided standard details and psi-values for a range of common constructions. However, it was felt these had become outdated and were no longer fit for purpose, as the details modelled were for less well insulated constructions. Scotland has a separate set of ACDs with modelled psi-values for each junction, which are more up-to-date and have been retained. 

Reaching compliance

There is a raft of manufacturer-created and industry-created details and guidance now available, reflecting actual build-ups and performance, complete with associated 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. When developing these, it is important that all junction details are reflective of real constructions, with good thermal performance to minimise losses, but that also represent good practices and are buildable on site.

It’s also worth noting that the use of bespoke or industry-created details may help to reinforce specifications in England and Wales. This is due to the new requirement for project teams to submit design stage and as-built compliance reports. These reports include the energy modelling carried out in SAP/SBEM and require any changes from the original specification used in the design stage report to be clearly detailed. 

As factors such as the insulation product’s thermal conductivity are incorporated into psi-value calculations, this can prevent insulation products being substituted with lower performing alternatives.

An eye for details

Closing the performance gap is a key priority for everyone within the construction industry. By utilising bespoke or industry-generated details and associated psi-values, it should be possible to limit heat losses at building junctions. This can help to support a best practice fabric-first approach, allowing properties to use heat efficiently and therefore limiting both their operational carbon demand and heating costs. 

Jonathan Ducker is head of regulatory affairs at Kingspan Insulation GB