A thermal bridge is created where materials that are relatively poor insulators are in contact across an insulation barrier. This creates a low resistance path for heat to escape from in an otherwise well insulated construction.
Limiting thermal bridges is becoming increasingly important with higher environmental standards such as Code Level 4 and the impending 2013 Part L1a Building Regulations. Considering these bridges in the design process can have a significant effect on reducing the annual CO2 emissions of a dwelling.
The Governments Standard Assessment Procedure For Energy Rating of Dwellings (SAP), which is used to demonstrate compliance to Building Regulations, considers two kinds of thermal bridging; repeating and non-repeating.
Repeating thermal bridges, such as timber roof trusses and the mortar between building blocks, are included in the normal elemental U-values. Non-repeating thermal bridges occur at junctions where building components, such as metal lintels, cross from the building interior to exterior with little or no intervening insulation. This creates a ‘bridge’ for heat to transfer along. The magnitude of heat loss through a thermal bridge depends on the particular construction detail.
Non-repeating thermal bridges typically occur at junctions between plane building elements and around openings where the continuity of the insulation is interrupted.
Non-repeating bridging in SAP is dealt with separately from the main heat loss calculation which relies on U-values. Instead a ‘y-value’ for the particular building is calculated. The SAP worse case default y-value is 0.15 W/m2K. Only in considering the length of every detail through a fixed calculation process can this be improved upon. Therefore limiting thermal bridging will become increasingly important as more energy efficient building are made a requirement of Building Regulations and higher levels of the Code for Sustainable Homes.
The Accredited Construction Details (ACD) produced by The Department for Communities and Local Government provide guidance on methods to limit heat losses through thermal bridging resulting in the y-value being reduced by up to 50%. ACD was based on the following principles:
- Minimise the number of various types of construction within the thermal envelope
- Pay careful attention to the design of junctions between building elements to ensure continuity of the insulating barrier.
- Minimise the number of penetrations of the thermal envelope.
- Use of low thermal conductivity materials where thermal bridges are likely to occur
It is possible to go beyond ACD with further reductions in heat losses from non-repeating thermal bridges. A thermal bridging calculator available to SAP assessors requires the specific Psi-Value for manufacturers products to be inputted. A combined total of the Psi-Values will then hopefully result in improved y-values, and therefore reduced CO2 emissions. It could of course have an cumulative result worse than the default, y-value but this is unlikely and would be a sign that details must be reconsidered.