A ‘thermal bridge’ occurs when an element or a structure penetrates the external envelope of a building, causing a break in insulation.
A ‘thermal break’ is a load-bearing thermal insulation element that has a lower thermal conductivity than the material surrounding it. Thermal breaks are fitted in any localised area of the building envelope which has a significantly higher thermal conductivity than the surrounding areas and can therefore result in an increase of the thermal efficiency within the structure. They are placed as an insulation assembly and reduce the flow of thermal energy between conducted materials.
There are many potential points for thermal bridging to occur around the external envelope and they contribute to a significant amount of heat loss throughout the building. But their effects can be alleviated with the use of thermal breaks in any areas where there is a penetration through the outside of the building.
Examples of thermal bridging
As an example, thermal bridging happens at the connection of the balcony or canopy, where there is higher heat transfer in the assembly. This area would have thermal breaks fitted to oppose the effects of thermal bridging at the junction between the balcony and the inside of the building.
A cantilevered balcony has critical thermal bridging and studies have suggested that they can account for 20-30% of heat loss out of a building. With the drive for more sustainable construction, it is imperative that everything should be done to provide effective thermal breaks to reduce heat loss, and by extension, the carbon footprint of a building in use. Not providing one leads to many problems:
Condensation on cold surfaces, which in turn leads to corrosion of metals and rotting of timber
Mould growth, leading to health concerns for residents
Damage to the structure
Overall further reduction of thermal performance.
It also means that a higher amount of energy is needed to cool or heat the building. All of this leads to non-compliance of building regulations.
Calculating thermal bridging
There are two types of thermal bridging: linear that occur at junctions between elements, and point where there are single penetrations through the external envelope. Each type of thermal bridge’s transmittance is measured in a different way. Linear is measured as a psi value and point is measured in chi value. The calculation of the overall heat loss of a building is set within the government-led model and all the values combined to give the overall energy efficiency that an architect can then measure against the requirements of that particular build.
If you look at a building as a whole, and especially a high-rise residential building, there are lots of thermal breaks that are small connections holding cladding up, in windows, doors and on balconies. In a large residential project, the thermal loss for balconies alone can be 20-30% of the overall. Combine that with weight and projection considerations that affect the size of connections and mitigating for heat loss becomes quite complicated.
Detailed calculations need to be made to comply with building regulations. Standard Assessment Procedure (SAP) calculations need to be made for heat loss and condensation and mould control. The BRE paper 1/06 provides information on the effects of thermal bridging and how design can maintain internal surfaces to prevent damage such as condensation or mould.
Rigidity vs thermal performance – striking the balance
The nature of balconies, with their active use, furniture and potential snow loadings mean that their structure needs to be robust enough to support the loads, whilst also keeping serviceability requirements within allowable limits, as set out by Building Standards.
Reducing the number of penetrations in a façade and placing a thermal break on each penetration reduces heat loss and improves thermal transfer, but also means it reduces rigidity and strength. Consideration should also be given to condensation and mould control at each location. So, there is a fine balance to make and choosing the right thermal break element to manage and accommodate the structural requirements as well as the right thermal properties is critical.
In the UK it is mandatory to have thermal breaks that comply with building regulations. For sustainability purposes, they reduce energy loss over the lifetime of a building, and in turn reduces the carbon footprint of the building in use. Overall, they are a small cost to pay upfront compared to the extra energy and refurbishment costs end users will need to consider.
“Reducing the number of penetrations in a façade and placing a thermal break on each penetration reduces heat loss and improves thermal transfer.”
This article is based on an interview with Simon Hill, Product and Marketing Manager, Schöck Ltd UK. Simon has a background in structural engineering and has been working in the field of construction for 20 years. He has worked for Schöck for six years in different managerial positions.