Matthew Evans, Director of Technical and Regulatory Affairs at Kingspan Insulation, answers questions on thermal bridges.

Thermal bridges are commonly found at junctions between different building elements and openings around windows.

What are thermal bridges?
Thermal bridges are points or areas in the building envelope which allow heat to pass through more easily than the surrounding construction. They occur where materials which are better conductors of heat are allowed to form a ‘bridge’ between the inner and outer face of a construction. This commonly happens where there is a gap in the insulation layer or where an element, such as a joist, penetrates the construction.

Where do they typically occur?
While commonly found at junctions between elements and openings around windows, thermal bridges can arise in a number of areas. There are several different types of thermal bridges which designers need to consider. Repeating thermal bridges typically occur at regular interruptions in the building fabric, such as studs or wall ties. Linear or non-repeating thermal bridges arise around openings, such as windows or doors, or where a more conductive material penetrates or bridges through the insulation layer. Geometrical thermal bridges are found where two or three different planes meet, such as junctions between different building elements. These typically arise where the heat loss area is greater than the internal surface, such as corners. Finally, point thermal bridges are found at penetrations through the building envelope, including pipework and flues, isolated steel beams and columns, and fixings and fasteners.

What are the main risks associated with thermal bridges?
Heat loss is the most obvious risk. As we insulate buildings to a higher level, losses from thermal bridges will account for a larger proportion of their total heat loss. Research suggests they can account for as much as 30 per cent of total losses. This can undermine efforts to insulate a property and contribute to the performance gap between the expected and actual energy demand of the building. As thermal bridges will be colder than the surrounding construction, they can also cause condensation to form on the internal face of the construction, or interstitially within it. This can lead to a range of issues, including mould growth and damage to structural elements.

Do rooflights need regulatory approvals?
Rooflight installation on existing roofs normally falls under permitted development rights, which are granted automatically. However, listed buildings or those in conservation areas will most likely be subject to an Article 4 direction made by the local authority, and the rooflights will require planning permission.

What are the key steps specifiers should take in designing out thermal bridges?
The key is to maintain a continuous layer of insulation across the building envelope. This means paying particular attention to any junctions. Common strategies for designing out thermal bridges include extending wall insulation below the top of the perimeter insulation for floor slabs, or using a flexible insulation at the eaves of a pitched roof to close any gap between the roof and wall insulation. Many manufacturers now provide details with pre-calculated psi-values covering junctions where bridges commonly occur. However, these will not cover every scenario so it may be necessary to create bespoke details and calculate psi-values. It is important to ensure these are reflective of a real construction and will be practical for site teams to install.

What are thermal breaks and where are they used?
Thermal breaks are materials with low thermal conductivities which are placed between materials with high thermal conductivities. As their name suggests, these materials help to act as a ‘break’ on heat losses that would otherwise occur through these areas. Thermal breaks are commonly used on, or integrated into the design of, metal elements, such as balconies and window frames.

What is the best way of identifying existing thermal bridges on retrofit projects?
Aside from obvious visible signs, such as surface condensation or mould growth, thermal bridges are typically identified with thermographic cameras. These will help identify areas of a construction where there is a significant temperature differential with the surrounding construction.

How difficult and costly is it to rectify thermal bridges on existing buildings?
This will depend on the cause of the bridge, where it is located, and the construction of the building. For example, gaps in loft insulation can be readily accessed and rectified. In most cases, however, the work required to rectify a thermal bridge in an existing building is both complex and costly. It may require areas of a construction to be stripped back both to correctly address the cause of the bridge and deal with potential related issues, such as rotten timbers caused by interstitial damp. This is why it is so important to address thermal bridges during the design and construction phase.

Is there any regulatory guidance or best practice advice specifiers can follow for both new-build and retrofit projects?
There are a number of manufacturer and industry-created details and guidance now available reflecting actual build-ups and performance, complete with pre-calculated psi-values. These include reputable non-government databases containing independently assessed thermal junction details. Many of these include process sequences, which provide step-by-step guidance for site teams. This provides improved assurance that the details you specify will be accurately achieved in the final building.

For further information, please visit the Kingspan website.