knowledge-base

What are Thermal Bridges?

Imagine your building envelope – the walls, roof, and floor – as a thermal barrier designed to keep heat in during cold weather and out during warm weather. Thermal bridges are localized areas within this barrier where heat flows much more easily than through the surrounding materials. Think of them as weak points in your building’s insulation.  

These areas of higher heat transfer arise due to two main reasons:

  • Geometry: Complex shapes and junctions where different building elements meet, such as corners, eaves, window and door frames, and wall-to-roof or wall-to-floor connections, can create concentrated pathways for heat flow. A simple corner where two walls meet, for example, inherently has more surface area exposed to the outside than a flat section of wall.  
  • High Conductivity Materials: The presence of materials with high thermal conductivity, like metal lintels above windows, steel beams embedded in walls, fasteners, brackets, and even mortar joints, can act as efficient conduits for heat transfer. Even if these materials are small components, their higher conductivity compared to insulation allows significant heat to pass through.  
  • Discontinuities in Insulation: Gaps, compressions, or interruptions in the continuous layer of insulation at junctions or around penetrations (like pipes or electrical conduits) create direct pathways for heat to escape or enter.

Why are Thermal Bridges a Problem?

The consequences of neglecting thermal bridges can be significant:

  • Increased Heat Loss (and Gain): Thermal bridges act as escape routes for heat in winter and entry points for heat in summer. This leads to a substantial increase in the overall heat loss (or gain) of a building, forcing heating and cooling systems to work harder and resulting in higher energy bills. In energy-efficient buildings, thermal bridges can account for a surprisingly large proportion – sometimes 15 to 30% – of the total heat lost through the building fabric. The extra heat flow through linear thermal bridges is quantified using the linear thermal transmittance (Ψ-value), while individual penetrations are assessed using the point thermal transmittance (χ-value).  
  • Reduced Internal Surface Temperatures: The increased heat flow at thermal bridges leads to localized colder (in winter) or warmer (in summer) internal surface temperatures.  
  • Surface Condensation and Mould Growth: These colder surface areas are more prone to surface condensation when the indoor air contains moisture and comes into contact with the cold surface. If this condensation persists, it can lead to dampness and the growth of mould, which can severely impact indoor air quality and the health of occupants. The temperature factor (fRsi) is a critical indicator of this risk; a lower fRsi value signifies a higher likelihood of condensation

The Role of BR 497 in Addressing Thermal Bridges Effectively 

Fortunately, thermal bridges can be effectively mitigated through careful design, material selection, and construction practices. Accurate numerical modelling (both 2D and 3D) is crucial for understanding and quantifying heat loss and surface temperatures at these critical areas. BR 497 provides valuable guidance and conventions for undertaking these calculations.  

Here are key strategies for addressing thermal bridges:

  • Careful Design of Junctions: Prioritize simple, well-insulated junction details. Design junctions to ensure continuity of the insulation layer and minimize the use of highly conductive materials at these points.
  • Incorporating Thermal Breaks: Introduce thermal breaks made of materials with low thermal conductivity within structural elements and components like balconies, canopies, and steel beams to interrupt the direct flow of heat.  
  • Specifying High-Performance Products: Utilize building products specifically designed to minimize thermal conductivity, such as insulated lintels, thermally broken window and door frames, and thermally broken fixings.  
  • Accurate Material Property Data: Use reliable thermal conductivity values for all materials included in your thermal models. Refer to reputable sources like BRE Report BR 443, CIBSE Guide A, and manufacturer-provided data.
  • Correct Modelling Techniques: Adhere to the conventions outlined in BR 497 for accurate modelling. This includes:
    • Defining appropriate model extents and boundary conditions: Accurately represent internal and external temperatures and surface resistances.
    • Handling repeating thermal bridges: Explicitly include repeating thermal bridges close to a junction in the model or account for their effect through an equivalent thermal conductivity. For 2D models, flanking elements should typically extend at least one metre or three times their thickness from the thermal bridge.
    • Mesh Refinement: Ensure the numerical model has a sufficiently fine mesh to capture the temperature gradients and heat flow accurately.
    • Considering Air Spaces: Properly account for the thermal resistance of air spaces within the construction, considering their dimensions, orientation, ventilation, and the emissivity of the surrounding surfaces.
  • Addressing Special Cases: Pay particular attention to junctions connected to the ground, such as party wall/ground floor junctions and inverted ground floors. These often require 3D modelling and careful consideration of ground heat transfer