Thermal bridges (cold bridges) represent a critical challenge in building construction and energy efficiency. These thermally weak junctions allow for significantly higher heat transfer than the surrounding materials, acting as conduits between the interior and exterior of a building. Examples include window jambs, where the path of least resistance is created for heat transference. Surprisingly, thermal bridges can be responsible for up to 30% of a building’s total heat loss. This impacts energy consumption and costs. It also poses a risk for internal surface condensation and mould growth, which can have detrimental health implications.

Understanding the types of thermal bridges is essential for addressing these challenges effectively. There are four principal categories, each with unique characteristics and impact on building thermal performance.

Repeating thermal bridges

Repeating thermal bridges are those that occur systematically within the structure of a building, creating a pattern of thermal weakness that allows heat to transfer more readily than through the surrounding materials. These bridges are typically integrated into the building fabric during construction. They result from continuously using materials or structural elements with a higher thermal conductivity than the surrounding insulation materials.

In buildings, repeating thermal bridges commonly occur in:

• Brick Mortar Joints: Where the mortar creates a continuous path for heat flow, bypassing the insulating properties of the bricks.
• Wall Ties: Metal ties stabilising masonry walls can conduct heat across the insulation barrier.
• Studs in Timber or Metal Frame Construction: The frame acts as a thermal bridge, transferring heat through the wall’s insulation layer.

These elements are necessary for the structural integrity and construction of the building but inadvertently create paths of least resistance for heat to escape. The U-Values for these components are critical, as they represent the heat transfer rate through a building element. The lower the U-value, the better the material’s insulating properties. However, in the case of repeating thermal bridges, the U-Values are elevated, indicating a reduction in thermal efficiency.

Addressing repeating thermal bridges

To mitigate the heat loss through repeating thermal bridges, various strategies can be employed:

• Improved Design: Design the structural system to minimise thermal bridging using materials with lower thermal conductivity or alter the structure to reduce direct paths for heat flow.
• Enhanced Insulation: Installing additional or more effective insulation materials around the thermal bridges to reduce heat flow.
• Thermal Breaks: Incorporating materials that act as thermal breaks within the structure, such as insulative wall ties or studs with a lower thermal conductivity, can significantly reduce the heat flow.

Linear (non-repeating) thermal bridges

Linear (non-repeating) thermal bridges represent a significant aspect of building design and energy efficiency that requires careful consideration. Unlike repeating thermal bridges, characterised by their regular occurrence within the building structure, linear thermal bridges are irregular and typically found at junctions where insulation is interrupted or diminished. These thermal bridges are crucial because they can significantly impact a building’s thermal performance, leading to increased energy consumption and potential discomfort for occupants.

Linear thermal bridges often occur in areas such as:

• Windows and Doors: The frames of windows and doors can create significant thermal bridges if not adequately insulated or designed. The interface between the frame and the building structure can allow heat to bypass the insulation.
• Junctions Between Different Building Elements: At points where a roof meets a wall or at the junction of a floor and an external wall, the continuity of the insulation can be compromised, creating a pathway for heat flow.

The impact of these thermal bridges is quantified using Psi-Values, which measure the linear thermal transmittance of a building component. The Psi-Value indicates the heat flow per unit length of the thermal bridge per degree of temperature difference between the inside and outside of the building. High Psi-Values indicate poor thermal performance, meaning more heat is lost through these bridges.

Mitigation

Mitigating the effects of linear thermal bridges requires thoughtful design and construction practices:

• Thermal Breaks: Incorporating thermal breaks within the frames of windows and doors can significantly reduce heat flow. These materials have low thermal conductivity and interrupt the path of heat transfer.
• Improved Insulation Techniques: Ensuring that insulation is continuous and extends over the areas where thermal bridges might occur can help minimise heat loss. This may involve special insulating materials or techniques between different building elements at junctions.
• Detailing and Design: Careful detailing and design can prevent the formation of linear thermal bridges. This includes designing junctions and interfaces between different building elements to minimise direct paths for heat transfer.

Geometrical thermal bridges

Geometrical thermal bridges occur where the shape or geometry of the building itself leads to an increased area of heat transfer. These bridges are often overlooked but play a significant role in the thermal efficiency of a building. The unique aspect of geometrical thermal bridges is that they are not solely dependent on the materials. Instead, they are dependent on the design and layout of the building’s structure.

Geometrical thermal bridges are typically found in areas such as:

• External Corners: The point where two external walls meet can often create a larger surface area exposed to the outside temperature than the interior surface area. This disproportion leads to higher heat loss in these areas.
• Eaves and Roof Overhangs: Where the roof extends over the walls, the change in geometry can create areas where heat loss is more significant than in other parts of the building.
• Balconies and External Projections: Any part of the building that projects externally can form a geometrical thermal bridge. The thermal bridging effect is due to the increased external surface area compared to the internal boundary.

The thermal performance of these areas is measured using Psi-Values, similar to linear thermal bridges. These values quantify the linear thermal transmittance and help assess the impact of geometrical thermal bridges on a building’s overall energy performance.

Mitigation

Mitigating the effects of geometrical thermal bridges involves considering the building’s design from an early stage:

• Thermal Breaks in Balconies and Projections: Incorporating thermal breaks in the design of balconies or other external projections can significantly reduce the thermal bridging effect. These breaks should be designed to minimise the path of heat flow from the interior to the exterior.
• Optimised Design: Adjusting the design and layout of the building to reduce the number of external corners and projections can naturally decrease the occurrence of geometrical thermal bridges. Smooth transitions and minimised external surface areas can help in this regard.
• Enhanced Insulation: Applying extra insulation at points susceptible to geometrical thermal bridging can help offset the increased heat loss. This might include insulating materials specifically designed to address these challenges.

Point thermal bridges

Point thermal bridges are localised areas within the thermal envelope of a building where heat flow is disproportionately high due to the penetration of elements with high thermal conductivity. These bridges are often the result of necessary structural or service elements penetrating the insulation layer. These elements create a direct path for heat to transfer from inside to outside a building. Despite their small size, point thermal bridges can have a significant impact on the overall thermal efficiency of a building, leading to increased energy consumption and potential for condensation and thermal discomfort.

Examples of point thermal bridges include:

• Flues and Chimneys: These are necessary for venting but create a path for heat to escape if not properly insulated.
• Fixtures and Fastenings: Metal fasteners can conduct heat through the insulation.
• Utility Penetrations: Openings for pipes, cables, and ductwork can breach the insulation, forming a thermal bridge if not sealed correctly.
• Stanchions and Beams: Structural elements that penetrate the building’s insulation layer can significantly increase heat transfer if made of materials with high thermal conductivity.

The impact of point thermal bridges is quantified using Chi-Values (χ-values). Chi-values which measure the thermal transmittance due to localised penetrations in the thermal envelope. A high Chi-Value indicates a significant thermal bridge and potential for substantial heat loss.

Mitigation

Mitigating the effects of point thermal bridges involves careful planning and construction techniques:

• Insulation of Penetrations: Where possible, flues, pipes, and cables should be insulated.
• Use of Thermal Breaks: Incorporating materials that act as thermal breaks around penetrations can significantly reduce heat flow. This can include special grommets or collars for pipes and cables or insulated backing plates for fasteners.
• Selection of Materials: Choosing materials with lower thermal conductivity for structural elements that must penetrate the insulation layer can help reduce the impact of point thermal bridges.
• Sealing and Caulking: Properly sealing around penetrations with materials limiting air and heat flow can minimise thermal bridging.