Thermal bridging significantly contributes to heat loss in buildings, occurring where insulation is interrupted or penetrated by more conductive materials. This phenomenon undermines the overall energy efficiency of structures, making it essential to address thermal bridging in building design to comply with Building Regulations Part L.
Read on to discover the impact of thermal bridging on building design and how Accredited Construction Details (ACDs) can enhance energy performance and ensure compliance with Part L Building Regulations.
What Is Thermal Bridging?
A thermal bridge, also known as a cold bridge or thermal bypass, is a point in a building’s structure that is more thermally conductive than the surrounding materials. It occurs when two exposed thermal elements meet, allowing heat to transfer from inside to the outside world.
Common thermal bridging examples include:
- The ground floor meets an external wall
- An external wall meets the roof
- A party wall meets an external wall or floor
- Around doors & windows (lintels, sills & jams)
- Balconies & cantilevered structures
- The use of metal studs instead of wood
- Other penetrations (e.g., plumbing vents & anchor bolts)
Thermal Bridge vs. Thermal Bridging – What’s the Difference?
The terms "thermal bridge" and "thermal bridging" are essentially synonymous. They both refer to areas in a building where heat can easily flow through, bypassing the insulation and leading to heat loss.
While "thermal bridge" often refers to a specific physical element or path of heat transfer, "thermal bridging" describes the phenomenon of heat flowing through that path.
The Impact of Thermal Bridging on Building Performance
Thermal bridging allows the wasteful transfer of heat through areas of high conductivity in building envelopes, significantly reducing energy efficiency. By requiring more energy to maintain a comfortable indoor temperature, thermal bridging can lead to higher heating costs, greater strain on heating systems, and increased carbon emissions.
Additionally, the temperature of the internal surfaces of these elements will differ from better-insulated internal surfaces. This can cause condensation problems, including mould growth and material damage, which can be harmful and costly over the long term.
Given that Part L Building Regulations aim to enhance the energy performance of new and existing buildings in the UK, addressing thermal bridging is crucial for achieving compliance.
Addressing Thermal Bridging in Building Design: Part L Building Regulations
Thermal bridging is a key consideration in building design, influencing energy efficiency, thermal comfort, and structural integrity. Calculating thermal bridging involves estimating heat loss across junctions (where two thermal elements meet) within the planned building, measured in Psi values.
Thermal bridging calculations are usually part of calculations for the Standard Assessment Procedure (SAP) and the Simplified Building Energy Model (SBEM). SAP and SBEM assessments are a legal requirement under the Building Regulations Part L, providing the framework for calculating the overall energy efficiency of all new buildings in the UK.
To help buildings pass SAP and SBEM calculations and meet compliance with Part L Building Regulations, the Accredited Construction Details (ACDs) offer standardised junction details designed to reduce heat loss through thermal bridges.
Reducing Thermal Bridging with Accredited Construction Details (ACDs)
Accredited Construction Details (ACDs) are standardised building details developed in 2002 to minimise thermal bridges. These details provide architects and builders with guidance on how to construct specific junctions, such as wall-to-foundation connections, to maintain insulation continuity and reduce heat loss.
However, under SAP 10 (Part L Building Regulations 2021), ACDs cannot be used as the default option with credit simply gained by ticking a box within the software. To benefit from using “good” details, specific empirical evidence and calculations must be included as part of the SAP calculation. These calculations must accurately reflect the actual details being built.
Accredited Construction Details Checklist: What’s Included?
The ACDs provide a complete description of how to construct specific junctions, providing checklists for designers, constructors, and building control bodies to ensure all provisions have been made to eliminate all reasonably avoidable thermal bridges. Generally, the more complex a building is, the more thermal bridges it will have.
The details are grouped by generic construction type, covering:
- Steel frames
- Timber frames
- Masonry cavity wall insulation
- Internal walls
- Masonry external wall insulation
The ACDs have now been archived. However, they are still available to download from the national archive.
Using Thermal Bridging Calculations to Quantify Thermal Performance
The Accredited Construction Details (ACDs) work hand-in-hand with thermal bridging calculations to accurately estimate the rate of heat loss through the junctions planned in the building design stage and ensure optimal performance and compliance at completion.
To calculate the Psi value of a detail, a set of drawings and a specification of materials are required. This assessment must be performed by a competent person using approved software, in accordance with BS EN ISO 14683 standards.
Each thermal bridge junction is assigned a Psi value, which, when multiplied by the length of the bridge, provides a heat loss value for SAP or SBEM. This process is crucial for demonstrating benefits such as improved EPC ratings, lower emission rates, and reduced energy demand for the dwelling.
Achieve Part L Building Regulations Compliance with Stroma
Your thermal bridging calculations and analysis should take place at the design stage as part of your SAP or SBEM assessments, helping to ensure Part L Building Regulations compliance from the outset.
Our experienced assessors can carry out Psi value calculations in accordance with BS EN ISO 14683 standards and, where necessary, advise on measures to reduce thermal bridging.
Visit our Energy Performance Services page or get in touch to learn more about our services and how we can assist you.