Understanding Thermal Bridges

What are thermal bridges, and why are they important?

Most people in the construction world, both new build and retrofit, should be familiar with the concept of U-values. A U value defines the rate at which heat transmits through an element. So consider, for example, a floor in a new build with a U value of 0.13W/m²K. This is considered to be a well-insulated element and will lose relatively little heat. On the other hand, let’s look at a roof in an existing house with very little insulation, the U value here could be as high as 2W/m²K. Therefore the takeaway is that the lower the U value, the more efficient the element is at retaining heat.

The role of junctions

As mentioned before, most people in the industry are familiar with this concept. However, what is less understood is what happens at the junctions where those elements meet.

themal bridge modelling in retrofit

In essence, you can build a new property or retrofit an existing one with elements that have low U values, but if you do not pay attention to the details between those elements, you can have considerable heat losses at the junctions or where the structure itself changes. In fact, the more insulation you add to your property, the more important these thermal bridge losses become, as they represent a larger percentage of the total heat loss, sometimes as much as 30%.

Not only will the property lose significant amounts of heat at these junctions, but the contrast in warmer elements with the colder junctions can result in an increased risk of mould & condensation.

In retrofit specific situations, PAS2035 guidance states the following;

9.1.15 Construction details shall be configured to maintain the continuity of the three-dimensional insulated envelope and the integrity of any air-tightness barrier, in order to eliminate thermal by-pass (i.e. the uncontrolled penetration of cold air to the warm side of any insulation layer), minimize thermal bridging and maintain an appropriate or specified standard of air-tightness. Construction details published as part of industry best practice guidance can be used; alternatively, if other details are used it shall be shown by calculating it in accordance with IP1/06 Assessing the effects of thermal bridging at junctions and around openings [N2] that the temperature factor (f Rsi) of each details is not less than 0.75.

PAS2035 guidance

It is, therefore, vital that we model these specific junctions to show that the temperature factor does not drop below 0.75 when new works are completed.

The risks of thermal bridge losses

The image below shows a party wall detail that has been modelled. One property has had External Wall Insulation (EWI) added to the external wall, and the neighbouring property has not. The warmer areas in the construction are shown in red/yellow, and the cooler areas in blue/green (shown in the small bar graph in the top left from 0-20°C). You can clearly see the improvement that the EWI insulation has had on the external wall, but at the junction, with the party wall, you will notice the band of cooler colours, this is the heat loss at this junction, and this is what we are concerned with.

The importance of modelling thermal bridges

When modelling a thermal bridge, we are looking for 2 results. First is the Psi Value ( ψ–value ), which shows how much heat loss occurs at the particular thermal bridge, and second is the fRsi value which dictates if mould & condensation is likely to be present.

Looking at the example above, you will see that the internal temperature of the property in question has a value of 16.91 °C after the EWI has been added, which in this model equates to a fRsi value of 0.85.

The Psi value for this particular model is 0.56 W/mK.

What this shows us is that while it is unlikely there will be a risk of mould and condensation in the corner of this retrofitted property, the heat loss at this junction is quite substantial.

If the neighbouring property was to also have EWI insulation as well we could remodel the junction as shown below.

Here you can see that the heat loss (ψ–value) drops significantly, and the internal temperature at that junction rises considerably.


Whether designing a new build or whether working on energy-efficient upgrades to an existing property, it is critical to pay close attention to the details at junctions and have thermal bridges modelled to limit the risk of condensation and mould and to limit the heat losses from the property.

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