Thermal Bridging and Accredited Construction Details

What This Requirement Covers

Thermal bridges are localised areas of the building envelope where the insulation is reduced or bypassed, creating paths for heat loss. Common thermal bridges occur at wall-floor junctions, around window and door openings, at corners, and at the junction of walls and roofs. Approved Document L requires thermal bridging to be minimised and its impact to be included in the energy performance calculation.

Key Requirements

What Is a Thermal Bridge?

1. A thermal bridge is any location where the thermal resistance of the building envelope is significantly reduced compared to the general area
2. Linear thermal bridges occur along junctions and edges (measured as psi-values in W/mK)
3. Point thermal bridges occur at discrete locations such as steel fixings through insulation (measured as chi-values in W/K)
4. Thermal bridges cause increased heat loss, lower internal surface temperatures, and risk of condensation and mould growth

Accredited Construction Details (ACDs)

1. The Government publishes Accredited Construction Details that provide standardised junction details with pre-calculated psi-values
2. ACDs are available for common construction types: masonry cavity wall, timber frame, steel frame, and insulated concrete formwork
3. Using ACDs allows the SAP assessor to use the published psi-values without individual thermal bridge calculations
4. If the actual construction matches the ACD drawings, the psi-values are deemed to be achieved

Individually Calculated Psi-Values

1. Where the construction does not match an ACD, or where better performance is sought, psi-values must be individually calculated
2. Calculations must be carried out to BS EN ISO 10211 by a competent thermal modeller
3. Individually calculated psi-values can demonstrate better performance than ACDs, improving the SAP result
4. A thermal bridging report must be provided to the SAP assessor

Default Psi-Values

1. If neither ACDs nor individually calculated psi-values are used, SAP applies default psi-values that assume poor thermal detailing
2. Default values significantly increase the calculated heat loss and make compliance with Part L more difficult
3. Using default values is strongly discouraged; they represent the worst acceptable thermal performance

Common Problem Areas

1. Window reveals: The insulation must return around the window reveal to minimise the thermal bridge at the frame edge
2. Ground floor edge: Insulation must extend to the edge of the floor slab and connect to the wall insulation without gaps
3. Eaves and verge: Roof insulation must continue to the wall line without gaps, particularly at the eaves
4. Lintels: Steel lintels without thermal breaks create significant thermal bridges; specify insulated lintels
5. Balcony connections: Concrete balconies connected to the internal floor slab create major thermal bridges; use proprietary thermal break connectors

Practical Compliance Tips

1. Specify ACDs or individually calculated psi-values at the design stage; do not rely on defaults
2. Use insulated lintels to minimise thermal bridges at window and door heads
3. Ensure insulation is continuous at all junctions; a small gap can create a disproportionate thermal bridge
4. Check that the as-built construction matches the ACD or thermal bridge calculation drawings
5. Request the thermal bridging calculation report from the designer and include it in the building file
6. Thermographic surveys (infrared imaging) can identify thermal bridges in completed buildings
7. Pay attention to cavity closers and insulated cavity barriers at openings; these details are critical for both thermal performance and fire safety