A Beginner’s Guide to Thermal Bridging

Principal Engineer, Emma Mathias-Jones, discusses the principles, uses, and importance of Thermal Bridging.

What are Thermal Bridges and why are they important?

“Thermal bridges can occur at any junction between building elements or where the building structure changes” BRE Information paper 1_06.

Fabric Heat Losses

Building fabric heat losses are divided into two categories: heat losses through elements such as walls, floors, and roofs, and heat losses through thermal bridges. Heat losses through the walls, floors, or roofs are accounted for within the U-value (the transfer rate of heat through a structure, divided by the difference in temperature across that structure) measured in W/m2.K, i.e. per area or m2.  Heat losses through thermal bridges are accounted for within Psi values measured in W/m.K, i.e. per linear length.

The lower (better) the U-value gets, the more critical the thermal bridges become. They typically account for ~10-15% of the building’s total heat loss but could be much higher depending on the type of construction.

 

Types of Thermal Bridge

“Thermal bridges can occur at any junction between building elements or where the building structure changes” BRE Information paper 1_06.

There are two main types of thermal bridges:

 

Repeating thermal bridges, e.g. timber joists and mortar joints, are typically accounted for within the U-value calculations. However, non-repeating thermal bridges, e.g. ground floor-to-wall junctions, require a thermal bridge model to calculate the additional heat loss.

There are two types of non-repeating thermal bridges, linear Psi (Ψ) values (W/m.K) and also point Chi (χ) values (W/K), e.g. corners of buildings or pipes penetrating the insulation layer.

Figure‑1: Examples of Repeating thermal bridge (Brick wall), non-repeating (Linear/Psi examples in the middle and Point/Chi example on the right) Image sources: Mabel Amber- Pixabay and Étienne Beauregard-Riverin- Unsplash and Pawel Czerwinski Unsplash

Thermal bridges typically require modelling in 2D to assess the heat loss associated with the junction. 3D modelling is sometimes required to calculate the Psi value (e.g., junctions incorporating steel) and is always required for the Chi value.

Other Determining Factors

Surface temperature condensation risk

Often, the thermal bridge model can be easily adapted to also calculate the surface temperature condensation risk, i.e. the likelihood of mould that could occur at a junction. Different boundary conditions to those used in the thermal bridge calculation are required to calculate the surface temperature condensation risk.

BRE Information paper 1/06 recommends a critical temperature factor (FCRSI) of 0.75 which is the minimum temperature factor (FRSI), to avoid condensation in dwellings, residential buildings, and schools.

Conventions – Geometry

The same thermal bridge model can be used for both SAP calculations, Part L calculations, and Passivhaus/AECB calculations, although the measurements are taken from different points. For SAP and Part L calculations, the measurements are taken from the inside face (green). For Passivhaus and AECB calculations, the measurements are from the outside of the insulation layer (Red), which is often an overestimate of the heat losses associated with the junction. In contrast, SAP and Part L calculations are often an underestimate. The different measuring methodologies give rise to different thermal bridge coefficients, Ψint and Ψext, which correct for these under and overestimates, respectively.

 

Figure -2: SAP Conventions (left) and Passivhaus Connections (right)

Naming Conventions

SAP has set naming conventions for non-repeating thermal bridges, e.g. E3- Sills, E4- Jambs, E5- Ground Floors. SAP and Part L also sometimes use the Y-value to account for thermal bridges. The Y-value is the sum of the individual junction heat losses divided by the total exposed surface area of the dwelling. The Y-value is a way of including the thermal bridges as an equivalent U-value for the dwelling rather than individual linear lengths.

‘Thermally Bridge Free’

Figure ‑3: Zero Carbon Hub, Thermal Bridging Guide image.

Thermal bridges can often be improved, e.g., the image above shows the difference low-density block can make to the Psi value. Even when the insulation is continuous at a junction, which is often described as “Thermally Bridge Free”, it can be worth calculating the Psi/Chi values as sometimes, depending on the geometry, this can reduce the calculated heat loss; this mostly occurs in Passivhaus projects. NB To include negative Psi/Chi values in Passivhaus calculations, all thermal bridges need to be calculated.

In Passivhaus projects, all non-repeating thermal bridges should be avoided. This is not only to reduce energy loss but also to improve thermal comfort as well as to avoid risk to the structure due to condensation around thermal bridges. A Psi value of less than 0.01W/(m.K) can typically be described as thermally bridge free.

Figure -4: “As overall heat losses are squeezed; thermal bridges must also be eliminated to avoid cold spots.” Image and description from The Passivhaus Trust Retrofit Masterclass course, Introduction, Mark Elton

The image above shows how as the U-values are reduced (grip tightness), the thermal bridges (gaps) become more and more prominent and therefore need addressing.

Thermal Bridges and Building Regulations

New Part L Requirements Volume1: Dwellings

As the above guidance shows, the thermal bridges need to be calculated either a) for the project, b) from a reputable source, c) a mix of calculated and default, or d) the default Y-value of 0.2(W/m2.K) must be used. To put this into context, if you were targeting a U-value of 0.1(W/m2.K), the Y-value would be double this, so it is worth noting.

If the calculations are from a reputable source, the junctions need to be identical. Temperature factors also need to be calculated.

The default assumes the thermal bridges are very poor. Therefore, we recommend calculating some or all the thermal bridges.

SAP 10.2 (Residential Buildings

 

 

Figure -5: SAP Thermal Bridge data sources

 

New Part L Requirements Volume 2: Buildings other than dwellings

As the above guidance shows, the thermal bridges should be a) calculated, including details of the construction sequencing, or b) defaults can be used as per BRE information paper 1/06. Temperature factors also need to be calculated. NB The default figures for the thermal bridges could be poor; therefore, we recommend calculating some or all the thermal bridges.

Part L 2021 (Commercial Buildings)

 

Example Building Thermal Bridges

The table below has a list of thermal bridges which you might expect to have based on the simple house image.

 

Figure ‑6: House Icon Symbol, Janjf93- Pixabay

 

 

 

 

 

 

 

 

 

 

 

Real world examples (Images – QODA Consulting):

Watch points:

• Thermal bridges are challenging to predict.
• SAP/Part L default thermal bridges are poor.
• SAP/Part L – it may be more cost-effective to calculate the thermal bridges rather than opting for improvements elsewhere, e.g. PV.
• For Passivhaus projects, if thermal bridges with a negative value are included, then all thermal bridges need to be calculated, i.e. no assumptions.
• Thermal bridges should be considered in heat loss calculations.

Conclusion

To effectively mitigate heat losses and reduce the risk of surface temperature condensation, thermal bridges are extremely important to consider and, where possible, eradicate. Thermal bridges have become of interest more recently due to the changes to Part L.

Further information/Guidance:

• BR443- Conventions for U-value calculations
• BS ENISO 6946 Building components and building elements – Thermal resistance and thermal transmittance – Calculation methods
• BS EN ISO 10211 Thermal bridges in building construction. Heat flows and surface temperatures. Detailed calculations
• BR 262 Thermal Insulation: Avoiding Risks. Third Edition [2002]
• BR 497 Conventions for Calculating Linear Thermal Transmittance and Temperature Factors. Second Edition [2016]
• Information Paper 1/06 Assessing the Effects of Thermal Bridging at Junctions and around Openings in the External Elements of Buildings [2006]
• BS EN ISO 14683:2017 Thermal bridges in building construction – linear thermal transmittance – simplified methods and default values
• Zero Carbon Hub – Thermal Bridging Guide
• Understanding Passivhaus, A Simple Guide to Passivhaus Detailing and Design by Emma Walshaw
• Design To Perform: An Illustrated Guide to Delivering Energy Efficient Homes by Tom Dollard
• Thermal Bypass Risks, A Technical Review by Mark Siddall
• Passivhaus Institute Component Database (https://database.passivehouse.com/en/components/)

 

Written by Emma Mathias-Jones