Understanding Thermal Bridging and PSI Values

In this post we will explore thermal bridging, how it impacts buildings and the different types of thermal bridges. We will then look at Psi values and what they are used for. Finally we will share information about our recent work on calculating our own detail junctions.

[Scroll to the end of the page to download this article as a handy PDF guide]

01 Detail Library Lighting Key Cropped
A Psi value (Ψ) is a measure used in building construction to understand how much heat travels through a particular part of a building that’s prone to thermal bridging.

What is thermal bridging?

Thermal bridging refers to an area within the buildings’ envelope where the insulation is reduced or not as efficient, resulting in increased heat loss or gain compared to the surrounding insulated areas.

A thermal bridge commonly occurs at junctions or connections between different building elements, like walls to floors or roofs, or openings like windows and doors.

There are several factors that contribute to thermal bridging in buildings. The main cause is often the presence of a denser or more conductive material, such as metal or concrete (usually structural components) that transfer heat more easily than insulation materials. As we strive to achieve improved thermal efficiency and u-value targets become more stringent, it is more important than ever to ensure we consider the interface between structure and insulation to prevent thermal bridging.

Other common causes of thermal bridging are breaks in insulation layers, inadequate installation on site during construction, structural elements penetrating the building envelope, amongst other issues.

01 Thermal bridge examples

While it’s not always possible to completely remove thermal bridges we can aim to reduce them as much as possible. When thermal bridges are present, areas of the assembly become susceptible to increased heat loss or gain which can result in areas of condensation, mould growth and deterioration.

Types of Thermal Bridging

Thermal bridges can be categorised into three main types; repeating thermal bridges, non-repeating thermal bridges and random thermal bridges.


Repeating Thermal Bridges

A repeating thermal bridge occurs at regular intervals within the building structure. These are typically seen in situations like a timber frame wall where the timber studs bridge the layer of insulation. As we know that this is a regular occurrence, we can account for the thermal bridge when we calculate our u-values. 


Non-Repeating Thermal Bridges

A non-repeating thermal bridge refers to a thermal bridge that occurs sporadically, and is not part of a regular pattern of the building. Examples include junctions, where walls intersect with roofs or floors, openings like windows and doors.

The effect of this type of cold bridge can be condensation forming on the internal surface of the building assembly, resulting in mould growth, and degradation of the materials. 

A non-repeating thermal bridge can sometimes be difficult to avoid completely, but we can take steps to reduce thermal bridges with careful detailing. 


Random Thermal Bridges

A random thermal bridge occurs unpredictably and is not part of any pattern or regularity. These can be a result of construction errors, material defects or other unforeseen structural issues. 

Examples include a gap or void in the insulation layer, poorly executed junctions, or unexpected changes in the building geometry.

For this post we will be exploring non-repeating thermal bridges only.

What are Psi Values?

A Psi value (Ψ) is a measure used in building construction to understand how much heat travels through a particular part of a building that’s prone to thermal bridging. It helps us figure out how much extra heat is lost through these areas compared to the rest of the building’s insulation. The higher the Psi value, the more heat is lost, so lower Psi values are better for keeping buildings warm and energy-efficient.

A Psi value (Ψ) is expressed in units of watts per metre Kelvin (W/mK).

Our Detail Calculations Experiment

We decided to run a thermal bridge (Psi value) calculation experiment on a selection of our details so we can better understand the performance of the junctions.

We worked with SW Thermal Analysis to review and calculate the Psi values on a selection of junctions for our 150mm masonry cavity wall details and our CLT details. In addition, SW Thermal Analysis also calculated the fRsi values – more on that later.

02 Detail Library Thermal Calculation Examples Page

The full results can be downloaded from our Resources section demonstrating the results for each junction (Free to download for all members). We are delighted that the details generally performed well.

Along with the Psi value results for each junction, we received a temperature distribution diagram of each detail. These diagrams are powerful tools used in construction to visualise thermal bridging effects within the building assembly. The diagram employs a rainbow colour scale to represent temperatures, allowing us to identify areas of heat loss or gain more intuitively. The colour scale ranges from red to blue, which represent maximum to minimum temperatures.

By analysing these diagrams we are more easily able to pin point weak areas and they can help us make informed decisions to mitigate thermal bridging and improve the overall energy performance of our designs. 

An example below demonstrates the difference between specifying a thermally broken lintel vs a standard insulated lintel.

03 Lintel comparisons

The image on the left shows the standard lintel, while the right image shows a thermally broken lintel. The temperature diagram shows the impact of the thermally broken lintel clearly, and the Psi value results demonstrate the difference in performance.

Having access to these images and results is a great way to make improvements to the detail and have a much better understanding of how the junction will perform.


* A note about our thermal calculations

These are for information purposes only and cannot be used for your own SAP Assessment Reports. If you would like any calculations carried out for your own projects, please refer to SW Thermal Analysis and they will be very happy to help. Make sure you mention the Detail Library when you get in touch 🙂

How Psi values are used in SAP

Standard Assessment Procedure (SAP) is the current approved procedure for assessing the performance of dwellings in line with Approved Document Part L of the Building Regulations. 

It is worth noting that SAP is currently under review and likely will be replaced with the Home Energy Model. You can read more about the ongoing consultations and plans here.

At present, all new dwellings are required to provide SAP calculations in order to comply with Approved Document Part L of the Building Regulations. The SAP assessment is a way to demonstrate thermal performance and energy efficiency of the project. 

The regulations set out a notional dwelling requirement for the Psi values of different junctions. You can read the full requirements of SAP here.

To summarise the requirements according to SAP and the notional dwelling, you can see the Psi values below along with the guidance for junction labelling.

04 SAP 10.2 Reference Values of Psi for Junctions

Table from SAP 10.2

05 Junctions for Psi Values

Standard Junctions for Psi Value Calculations – Source Unknown – please contact us for credit

Historically it was possible to submit default figures according to the Accredited Construction Details (ACD’s) but ACDs have now been removed and it is becoming more common that junctions are needing to be independently assessed in order to provide Psi values for the SAP Assessment. If independently assessed figures are not used, a default figure will be used that is often higher than the actual value, essentially making it more difficult to pass the SAP Assessment and encouraging the independent calculation of junctions. 

By independently calculating junctions, it is possible to adjust the detail and improve thermal performance if necessary. For this reason, it’s important to review the details as early as possible in order to have time to make necessary amendments. 

Some manufacturers do provide their own Psi value calculations including timber frame companies or insulation providers so be sure to check those out if you are looking at an off the peg construction build up.


What is fRsi?

In simple terms, fRsi, or the surface temperature factor, tells us how warm or cold a surface in a building gets compared to the indoor air temperature. It’s important because it helps us understand if a surface might feel uncomfortably cold to touch, which can affect how comfortable people feel inside a building. If the fRsi value is low, it means the surface might feel quite cold, while a higher value means it’s closer to the indoor temperature, which is generally more comfortable.

The surface temperature factor (fRsi) is closely linked to the possibility of condensation in buildings. When a surface inside a building gets too cold compared to the indoor air temperature, it increases the risk of condensation forming on that surface. This happens because the colder surface can cause moisture in the air to turn into water droplets when it comes into contact with it.

So, if the fRsi value is low, indicating that the surface temperature is significantly cooler than the indoor air temperature, there’s a higher chance of condensation occurring on that surface. This can lead to issues like mould growth, dampness, and damage to building materials over time. Therefore, maintaining appropriate surface temperatures throughout a building is crucial for preventing condensation and maintaining a healthy indoor environment.

fRsi factor ranges from 1 to 0. fRsi=1 means that the internal surface temperature at the thermal bridge is the same as the rest of the house. fRsi=0 means the internal surface temperature at the thermal bridge is the same as the outside temperature, this is a bad result. 

The best result to obtain for fRsi is closest to 1. For dwellings, fRsi should be maintained above 0.75 to avoid the risk of mould growth. 

Our details calculation experiment also provides the fRsi values for each junction, which can be seen in the full report in our Resources section. Free to all members. 

Explore the Calculations

All monthly and annual members have access to our calculation experiment, Detail Library Thermal Calculation Examples.

06 Detail Library Thermal Calculation Examples

Please head over to our resources section to download the document.

We are really interested to know if our members would like to see more of these on the library, so please provide any feedback you may have. Just drop us an email hello@detail-library.co.uk we would love to hear what you think.

Download the Guide

30 Understanding Thermal Bridging and PSI Values


Written by Emma Walshaw, Architectural Technologist. Emma is the founder of First In Architecture and the Detail Library. She has written a number of books on construction and detailing.