What is a Thermal Bridge?
A thermal bridge, also known as a cold bridge, is a part of a building’s structure where heat transfer is significantly higher than in the surrounding materials. This occurs due to gaps in insulation, insufficient insulation, or when insulation is interrupted by materials with higher thermal conductivity. For example, a thermal bridge can form when a lintel spans the inner and outer leaf of a cavity wall, creating a path for heat to escape from inside the building to the outside environment. In colder climates like the UK, this leads to considerable heat loss.
As energy efficiency regulations become stricter and awareness increases, addressing thermal bridges in walls, roofs, and floors becomes essential. Research shows thermal bridging can account for up to 30% of a building’s heat loss.
E10 Eaves (insulation at ceiling level), source ERS Consultants Ltd.
Heat Loss and Energy Performance
In modern, well-insulated homes, the impact of thermal bridging is more pronounced. This heat loss, measured as linear thermal transmittance or Ψ-value (pronounced ‘psi-value’), can significantly affect a building’s overall thermal performance. To comply with building regulations, new builds must pass SAP (Standard Assessment Procedure) calculations, which account for both U-values and Ψ-values.
Types of Thermal Bridges
- Repeating Thermal Bridges: These occur regularly within a building, such as where timber studs bridge insulation in a cavity wall. These are factored into the ‘U-value’ calculation, which measures a material’s insulating effectiveness.
- Non-repeating Thermal Bridges: These occur at junctions between building elements and around openings, measured as Ψ-values. These are more complex and require detailed thermal modelling.
Surface Temperature, Mould Growth, and Health
Assessing thermal bridges also involves calculating the surface temperature factor (f), which helps determine the risk of mould growth. Mould can produce allergens, irritants, and toxic substances, causing allergic reactions like sneezing, runny nose, red eyes, and skin rashes. Mould can also trigger asthma attacks and other respiratory conditions.
Thermal Bridging Guidance and Ψ-Values
The push for energy-efficient buildings, driven by changing legislation, has increased the focus on reducing thermal bridging through improved architectural detailing and construction practices. Minimizing heat loss through the building fabric is the first step. Non-repeating thermal bridging, measured as Ψ-values, plays a crucial role. These values are calculated using specialist thermal modelling software, which simulates the construction detail and material conductivities to determine heat loss at junctions.
Assessment
Thermal bridges occur in the building fabric where heat flows are two or three-dimensional due to geometry or high conductivity materials. Simple calculations are often insufficient, requiring numerical modelling for accurate thermal performance assessment. Software packages allow for detailed modelling, specifying geometry, materials, and boundary conditions in two or three dimensions.
The BRE BR 497 publication (Conventions for Calculating Linear Thermal Transmittance and Temperature Factors) provides guidelines for thermal modellers. These conventions ensure consistent, reproducible results, supporting efforts towards energy efficiency and healthier buildings. The guidelines pertain to using thermal modelling software to evaluate the ‘as designed’ thermal performance of building junction details, products, or elements.
Modelled E10 Eaves (insulation at ceiling level), source ERS Consultants Ltd.
Legislation
The BR 497 guidelines are referenced in the England and Wales Building Regulations Approved Document Part L, and the Scottish Technical Standards Section 6 (Energy). The construction industry relies on accurate thermal bridging values from credible sources to ensure these figures are incorporated into the National Calculation Method tools like SAP and SBEM (Simplified Building Energy Model).
By effectively addressing thermal bridging, we can significantly improve energy performance and indoor health, meeting regulatory requirements and enhancing overall comfort. These calculations additionally improve Part L compliance significantly where typical details from manufacturers cannot be utilised in the SAP or SBEM calculations.
We would be happy to assist with your Thermal bridge and PSI Value calculation, so please do contact us today on 01865378885 or through the Contact Us form.