Thermal bridging in buildings

A thermal bridge (sometimes referred to as thermal bridging, a cold bridge or thermal bypass) describes a situation in a building where there is a direct connection between the inside and outside through one or more elements that are more thermally conductive than the rest of the building envelope.

As a result, there will be wasteful heat transfer across this element, its internal surface temperature will be different from other, better insulated internal surfaces and there may be condensation where warm, moist internal air comes into contact with the, potentially cold, surface. This condensation can result in mould growth.

Thermal bridges are common in older buildings, which may be poorly constructed, poorly insulated, with single skin construction and single glazing.

In modern buildings, thermal bridging can occur because of poor design, or poor workmanship. This is common where elements of the building penetrate through its insulated fabric, for example around glazing, or where the structure penetrates the building envelope, such as at balconies.

However, as buildings have become better insulated, with increasingly strict regulation, so thermal bridges that might previously have been considered insignificant in terms of the overall thermal performance of a building, can actually be the cause of considerable thermal inefficiency. There is the potential for such inefficiency at every opening and every junction (even in party walls).

Thermal bridges can be categorised as 'repeating' for example where wall ties regularly bridge the cavity, or 'non-repeating' such as a wall junction or lintel.

The Approved Documents to Part L of the building regulations (Conservation of fuel and power) state that 'The building fabric should be constructed so that there are no reasonably avoidable thermal bridges in the insulation layers caused by gaps within the various elements, at the joints between elements and at the edges of elements such as those around window and door openings.'

They require that where unaccredited construction details are used, generic linear thermal bridge values must be increased by levels (depending on the building type) set out in the Approved Documents for the calculations of building emission rates (BER) or dwelling emission rates (DER).

Thermal bridges in completed buildings can be revealed with thermal imaging cameras (see Thermographic survey), but they can be very difficult to rectify, particularly if they are repeated throughout a building.

NB Assessing risks in insulation retrofits using hygrothermal software tools, Heat and moisture transport in internally insulated stone walls, by Joseph Little, Calina Ferraro and Beñat Arregi, published by Historic Environment Scotland in 2015, defines thermal bypass as: ‘Heat transfer that bypasses the conductive or conductive-radiative heat transfer between two regions. It may well be a major contributor to the performance gap that appears to exist between predicted and actual thermal performance. Bypass mechanisms include air leakage, thermal looping, wind washing etc.’

[edit] Related articles on Designing Buildings

• Accredited construction details ACDs.
• Assessing the effects of thermal bridging at junctions and around openings.
• Building fabric.
• Cavity wall.
• Co-heating test.
• Computational fluid dynamics.
• Conventions for calculating linear thermal transmittance and temperature factors.
• Emission rates in the building regulations.
• Floor insulation.
• g-values.
• Insulation for ground floors.
• Insulation specification.
• Interstitial condensation.
• Mould growth.
• Roof insulation.
• Reducing thermal bridging at junctions when designing and installing solid wall insulation FB 61.
• Solid wall insulation.
• Thermal bridging and the Future Homes Standard.
• Thermal comfort.
• Thermal imaging to improve energy efficiency in building design.
• Thermographic survey.
• U-values.