Understanding Thermal Bridging

Thermal bridges are areas of the building envelope where heat flows more easily than through the surrounding construction. Understanding and minimising thermal bridging is essential for energy-efficient buildings.

Types of Thermal Bridges

1. Repeating Thermal Bridges

These occur regularly through the building element and are accounted for in the U-value calculation. Examples include:

• Timber studs in a timber frame wall
• Mortar joints in masonry
• Wall ties in cavity walls
• Metal fasteners through insulation

2. Non-Repeating (Linear) Thermal Bridges

These occur at junctions and edges, characterised by psi (ψ) values. Examples include:

• Wall/floor junctions
• Wall/roof junctions
• Window and door reveals
• Corners (internal and external)
• Balcony connections

3. Point Thermal Bridges

Discrete points where heat loss is concentrated, characterised by chi (χ) values:

• Structural fixings through insulation
• Services penetrations

The Combined Method

For elements with repeating thermal bridges (like timber frame walls), the combined method calculates a weighted average U-value:

Common Psi Values

Typical linear thermal transmittance values from Accredited Construction Details:

Impact on Heat Loss

Thermal bridges can account for 20-30% of total fabric heat loss in well-insulated buildings. As U-values improve, the relative impact of thermal bridges increases.

Timber Frame Example

In a typical 140mm timber stud wall with mineral wool insulation:

• Insulation-only U-value: 0.21 W/m²K
• Timber stud U-value: 0.76 W/m²K
• Stud fraction: 15%
• Combined U-value: 0.29 W/m²K

The timber studs increase the U-value by approximately 38%.

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