Glossary · Two-word term
Thermal Bridging
In short
areas within a building's envelope where heat can more easily transfer
Thermal bridging occurs where conductive materials penetrate a building's insulation, creating pathways for heat loss. In UK cladding systems, this typically involves metal fixings and junctions, requiring careful design and thermal breaks to meet Building Regulations.
Last updated
On this page
Thermal Bridging
Definition and Fundamental Concepts
Thermal bridging refers to areas within a building’s envelope where heat can more easily transfer from the interior to the exterior, or vice versa. This phenomenon occurs when a material or component with higher thermal conductivity than the surrounding insulation penetrates the thermal envelope. In the context of cladding and external wall systems, these bridges act as a path of least resistance for thermal energy, bypassing the intended resistive layers.
Thermal bridges are categorised into three primary types. Repeating thermal bridges occur at regular intervals, such as through timber studs or metal rails within an insulation layer. Non-repeating thermal bridges occur at specific junctions, such as where a floor slab meets an external wall or around window reveals. Geometrical thermal bridges occur where the geometry of the building creates a larger external surface area relative to the internal surface area, such as at external corners.
Thermal Bridging in External Wall Systems
In modern external wall systems (EWS), thermal bridging is a critical consideration during design and remediation. Cladding systems typically require a support structure, often consisting of metal brackets (commonly 'helping hand' brackets) and rails fixed back to the building substrate. Because aluminium and steel have significantly higher thermal conductivity than mineral wool or PIR insulation, these fixings can create significant thermal pathways if not properly addressed.
The impact of these bridges is quantified through the 'U-value' of the wall assembly. A U-value calculated without accounting for thermal bridging will significantly overestimate the thermal performance of the building. Consequently, technical assessments often involve complex calculations to determine the 'effective' U-value, ensuring that the heat loss via mechanical fixings is factored into the overall energy profile of the facade.
UK Regulatory and Process Context
In England and Wales, the control of thermal bridging is primarily governed by Building Regulations, specifically Approved Document L (Conservation of fuel and power). These regulations mandate that the building fabric be constructed so that there are no reasonably avoidable thermal bridges. The guidance emphasises the importance of continuity in the insulation layer to limit heat loss and prevent internal surface temperatures from dropping to levels where moisture might condense.
During cladding replacement or remediation projects, the local authority building control or an Approved Inspector will typically require evidence that the new system meets current thermal standards. This involves demonstrating that the design minimises thermal bridging at junctions and through the fixings. The use of 'thermal breaks'—non-conductive spacers or pads placed between metal brackets and the building substrate—is a common method used to satisfy these requirements. Professionals use software based on BS EN ISO 10211 to model these junctions and ensure they meet the specific 'Psi-values' (linear thermal transmittance) required by the project specification.
Significance for Building Management and Owners
For building owners, Resident Management Companies (RMCs), and Right to Manage (RTM) directors, understanding thermal bridging is essential when evaluating the long-term performance and maintenance of a building. While the term is technical, its effects are tangible. Persistent thermal bridging can lead to localised cold spots on internal wall surfaces. When warm, moist internal air meets these cold spots, it can lead to surface condensation and subsequent mould growth, which may be mistaken for external water ingress.
When a building undergoes a facade survey or an EWS1 assessment, the presence of thermal bridges may be noted as part of the wider review of the building's technical state. If a building is undergoing cladding remediation, the design team must balance thermal performance with other requirements, such as structural integrity and fire performance. Managing agents should be aware that changes to the cladding system—such as increasing insulation thickness—may require a corresponding redesign of the bracketry to ensure that thermal bridging does not negate the benefits of the additional insulation.
Thermal Bridging
Common Misunderstandings
A frequent misunderstanding is the belief that insulation alone determines the thermal efficiency of a wall. It is possible to have a very thick layer of high-performing insulation that is rendered ineffective by a high density of metallic penetrations. Another misconception is that thermal bridging only affects energy bills. While heat loss is a major factor, the structural implications of interstitial condensation—where moisture forms inside the wall cavity due to temperature drops caused by thermal bridges—can be more significant over time, potentially leading to the corrosion of metal components or the degradation of organic materials.
Furthermore, there is often confusion regarding the difference between a 'point' thermal bridge and a 'linear' thermal bridge. A point bridge is a single penetration, such as a bolt, whereas a linear bridge occurs along a length, such as a lintel or a balcony connection. Both must be accounted for in a professional thermal assessment, yet they require different mitigation strategies during the repair or replacement phase.
Technical Terminology and Related Concepts
The study of thermal bridging involves several specific metrics and components used by facade engineers and surveyors.
| Term | Definition |
|---|---|
| U-value | The rate of heat transfer through a structure, divided by the difference in temperature across it. |
| Psi-value (ψ) | A measure of linear thermal transmittance at junctions or bridges. |
| Thermal Break | An element of low thermal conductivity placed in an assembly to reduce the flow of heat. |
| Interstitial Condensation | Condensation occurring within the layers of the building envelope rather than on the surface. |
| Point Transmittance (χ) | The heat flow associated with a point thermal bridge, such as a single fixing. |
Closely related to thermal bridging is 'thermal looping', which occurs when air circulates behind insulation boards, effectively bypassing their thermal resistance. Both phenomena are addressed through rigorous site quality control and adherence to the 'motive of continuity' regarding the thermal envelope. When cladding systems are repaired, ensuring that the insulation is tightly butted and that all penetrations are thermally broken is standard practice to maintain the designed thermal integrity of the building.
Thermal Bridging