CPD 14 2021: Bridging the performance gap

This Kingspan Insulation sponsored CPD looks at the need to reduce heat loss through thermal bridges in the light of coming changes in statutory energy performance requirements. Deadline: 7 January 2022

In 2022, England, Scotland and Wales are to introduce changes to the energy performance requirements in their building regulations and standards. These revisions will set tougher targets for carbon emissions and energy demand and aim to address the so-called “performance gap” between the designed and actual energy performance of buildings.

In particular, they look to raise standards in the detailing and installation of insulation at key junctions in the building envelope to prevent thermal bridging. These bridges commonly occur where there are gaps or inconsistencies in the insulation layer and can account for as much as 30% of heat loss from buildings.

This CPD will look at thermal bridges, measuring heat losses through them, changes to regulations and the latest performance requirements.

Thermal bridges can commonly occur at junctions between building elements

Introduction

At present, designers are able to use standardised details known as the accredited or approved construction details (ACDs) to represent heat loss in areas likely to be affected by thermal bridging. However, under the proposed changes to the Building Regulations, these will now be removed. This is because the details and the heat losses associated with them are not reflective of modern insulation levels.

In addition, default heat loss values for junctions (for use when no details are provided) are set to be worsened. This means, if accurate details are not provided, other areas of the specification will need to be significantly improved to reach compliance. These changes aim to encourage the use of bespoke detail calculations or good-quality details provided by manufacturers or via industry libraries.

Installers in England and Wales will also be required to provide photography of junctions to building control authorities to prove the installed measures match the original details and specification. This means it is essential that all parties are aware of the changes and ensure their practices are up to standard ahead of 2022.

What are thermal bridges and what issues do they pose?

Thermal bridges (also known as cold bridges) occur where materials that are better conductors of heat than the insulation layer(s) are allowed to form a bridge between the inner and outer face of a structure. As buildings are insulated to a higher level, these bridges take on more importance, effectively acting as a fast track for heat to escape out of a property, undermining its performance.

This issue takes on even greater significance as properties will increasingly transition to heat pumps in the coming years, with the sale of gas boilers being phased out by 2035. Heat pumps operate most efficiently at lower flow temperatures than boilers (typically 55°C or below for heat pumps compared with 65°C to 75°C for gas boilers). Additionally, air-source heat pumps have to work harder to produce heat during cooler months, as air temperatures are lower. If homes with heat pumps allow high levels of heat loss through thermal bridges, heating bills will be significantly increased, pushing some occupants into fuel poverty. This could also place significant strain on the national grid, potentially increasing carbon intensities as fossil fuel sources are needed to meet demand.

Thermal bridges will also result in cold spots in the construction. This can lead to condensation forming either on the face of the construction (surface condensation) or inside the construction (interstitial condensation). If not addressed, this can result in issues such as damp and mould, which not only present health risks but could also damage the building structure (for example, causing rot in floor joists). It is therefore essential to carefully address these bridges.

There are four main categories of thermal bridge:

  • Repeating thermal bridges occur as regular interruptions in the building fabric, such as studs or wall ties.
  • Point thermal bridges are typically single penetrations in the building’s thermal envelope and can include fixings or fasteners, brackets for a canopy or in a rainscreen, a flue passing through a wall, or isolated steel beams or columns.
  • Linear (non-repeating) thermal bridges – these occur where there are gaps in the insulation layer around openings such as windows or doors or where a more conductive material penetrates or bridges through the insulation layer.
  • Geometrical thermal bridges – these occur where two or three different planes meet, such as junctions between the different building elements. These typically occur where the heat loss area is greater than the internal surface such as corners.

How are heat losses through thermal bridges measured?

Repeating thermal bridges are typically considered as part of the thermal performance (U-value) calculation for the element. Heat losses through point thermal bridges are typically determined through calculating a chi-value, which is typically considered as an adjustment in the U-value calculation.

Heat losses through linear and geometrical thermal bridges are estimated using a separate calculation known as a psi-value. This measures heat loss through the linear joint. By multiplying the psi-value by the junction length, we can estimate the heat loss for an entire junction. A lower psi-value indicates lower heat loss through the junction.

The sum of all linear thermal transmittances multiplied by the length of all details is known as the HTB (heat loss due to thermal bridging), and by dividing this by the total area of external elements (excluding party walls), you can determine the percentage of overall heat loss from a building associated with thermal bridging. This is known as a y-value.

Alongside the thermal heat loss calculations, the internal temperatures at the coldest points of junctions are assessed. This is to avoid problems of surface condensation or mould growth. The calculated temperatures can be compared against critical temperature factors, with risk dependent on typical humidity levels for different building types.

How can thermal bridges be prevented?

There are several ways to address thermal bridges. During the design stage, this can include:

  • Adjusting the insulation specification to improve the thermal performance of an element (either by increasing the thickness or using a more thermally efficient material)
  • Changing the positioning of the insulation, for example, by adjusting the position of window units to increase the overlap with cavity closers or altering the arrangement of insulation materials to increase the heat flow path
  • Moving or removing the material identified as causing the bridge from the design
  • Changing the materials causing the thermal bridge for ones that are less conductive.

Building details should also consider the potential for future retrofit improvements to the property. For example, insulation should ideally only be used to isolate the sill and jamb on one side of a window construction. This allows the unit to be removed and replaced in the future, potentially with more efficient units, without having to rip out the insulation layer.

It is also important to ensure details are buildable, safe and do not become overly complicated. Time is a major factor on all construction sites, so the more complex and time-consuming a detail is to achieve, the more likely it is that steps may be missed. Installers have an important role to play in ensuring that the finished installation matches up with the details – this includes avoiding substituting materials.

For example, while rigid insulation boards look similar, their thermal performance can vary considerably. This means substitution or mixing of different materials can easily introduce bridges into a construction.

To support best practice on site, the detail should be provided with a clear process sequence. This can be a simple bullet list, setting out the order in which the materials should be installed and providing clarity on areas such as how far different elements should overlap and how to address any gaps. The process should also address the air barrier to ensure best practice on airtightness considerations.

In addition to calculating bespoke details, it is also possible to obtain calculated details with accompanying process sequences from manufacturers and from reputable detail libraries.

How will detailing change under the updated Building Regulations?

As mentioned, the key change put forward in the consultations for Part L of the Building Regulations in England and Wales (with separate documents in each region) and Section 6 (Energy) in Scotland is that the ACDs will be removed. This is because it was felt they had become increasingly out of date and that the construction industry is better placed than regulators to develop and maintain up-to-date and effective details.

In Scotland, the introductory section for the ACD document (which sets out best practice on dealing with thermal bridges) will be updated and retained as a reference point.

In addition to using the ACDs, designers currently have the option to use a default (usually worst-case) psi-value for an individual junction. Alternatively, they can assume a global y-value for the property. Both these options are designed to assume high levels of heat loss at junctions, encouraging designers to consider junction design.

In the latest version of the SAP (Standard Assessment Procedure, for domestic dwellings) and the SBEM (Simplified Building Energy Model, for non-domestic buildings), which will come 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. As a consequence of those changes, other areas of the specification may have to be significantly improved to reach compliance when using these values.

Table 1: SAP modelling for a typical three-bedroom semi-detached house

How will poor detailing affect compliance?

To see what impact this will have in practice, SAP modelling has been carried out for a typical three-bedroom semi-detached house. The second column in the table above shows the proposed notional dwelling specification for Part L 2021 in England which will be used to calculate the target emissions and energy rates for a property.

This notional building assumes a reasonably good level of junction detailing, using a “global” junction detailing y-value of 0.05. The third column shows a specification that meets the basic compliance criteria where the default psi-values have been used for all junctions.

Clearly, the specification for the building using default psi-values has had to be significantly upgraded over that of the notional dwelling specification to make up for the poorer junction detailing. The fabric package and air leakage rate for the property are at Passivhaus levels (or beyond), although obviously without their high levels of attention to detailing and consideration of junction losses, and the property also requires the addition of an MVHR unit.

Even with these costly changes, the property only narrowly complies with the target fabric energy efficiency rate under Part L 2021. The use of photovoltaics and wastewater heat recovery is also essential to meet the carbon emissions and primary energy targets (these do not affect the fabric energy efficiency performance). This means that failure to use details is likely to significantly raise the cost and complexity of developments.

New compliance reports

In addition to encouraging the adoption of bespoke details, the English and Welsh consultations also set out a requirement for project teams to fill in compliance reports. These will be known as the Building Regulations England Part L (BREL) and Building Regulations Wales Part L (BRWL) compliance reports.

The BREL and BRWL reports will require key aspects of the SAP or SBEM model for the property to be documented and signed off by an energy assessor. This includes the details and psi-values for key junctions within the building such as sills, jambs and eaves. Installers will then be required to photograph these junctions before they are covered up to ensure that the materials fitted match the specification and have been installed per the detail. The finished report will be submitted to building control authorities and deviations could lead to a building being found to be non-compliant, resulting in costly remedial work.

In Scotland, a working group has been set up to develop its own compliance plan. This will be an in-depth process, considering various aspects of building design and construction to set new requirements. It is therefore likely that installers will need to meet similar, or possibly even more stringent, requirements on Scottish projects once this work has been completed. Contractors in all regions should take steps now to ensure the work they are carrying out fully matches up with the supplied details.

Architects should also consider how this change can help to strengthen their own specification if bespoke details are supplied. Factors such as the insulation product’s thermal conductivity and emissivity are incorporated into the psi-value calculation for these details. As a result, the value is non-transferable to a different material, preventing product substitution.

Accuracy on paper and on site

The updated regulations should be fully in force for both domestic and non-domestic buildings by the end of 2022. With the new focus on detailing, they will provide a more robust measure to cut the carbon emissions of completed buildings, and ultimately ensure they provide good value for homeowners over the long term. All parts of the project team will need to collaborate to deliver this, ensuring details are accurate and workable and that they are completed correctly on site.#