Thermal Bridging: How Cold Bridges Reduce Building Performance and Energy Efficiency

A thermal bridge—also called a cold bridge or thermal bypass—describes a direct connection between the inside and outside of a building through elements that conduct heat more readily than the surrounding building envelope. These weak points in the thermal barrier allow wasteful heat transfer, lower the internal surface temperature of affected areas, and frequently lead to condensation where warm, moist indoor air meets a cold surface. That condensation can, in turn, result in mould growth.

Thermal bridges are widespread in older buildings, which may be poorly constructed, poorly insulated, or built with single-skin construction and single glazing. In modern buildings, thermal bridging typically arises from poor design or poor workmanship—especially where elements penetrate the insulated fabric, such as around glazing, or where structural members penetrate the building envelope, such as at balconies.

As buildings have become better insulated and energy regulations increasingly strict, thermal bridges that might previously have been considered insignificant in terms of overall thermal performance can now be the cause of considerable thermal inefficiency. There is potential for such inefficiency at every opening and every junction, even in party walls.

How Thermal Bridging Occurs

Heat transfer occurs through three mechanisms: convection, radiation, and conduction. A thermal bridge is an example of heat transfer through conduction. The rate of heat transfer depends on the thermal conductivity of the material and the temperature difference experienced on either side of the bridge. When a temperature difference is present, heat flow follows the path of least resistance through the material with the highest thermal conductivity and lowest thermal resistance—that path is the thermal bridge.

Thermal bridges can be categorised as repeating—for example, where wall ties regularly bridge the cavity—or non-repeating, such as at a wall junction or lintel. Both types undermine the U-value and overall thermal resistance of the building envelope.

Common Locations for Cold Bridges

Several building details are particularly susceptible to thermal bridging:

Impact on Energy Performance and Building Standards

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 set out in the Approved Documents for the calculations of building emission rates (BER) or dwelling emission rates (DER). This regulatory framework reflects the growing recognition that thermal bridges can significantly undermine compliance with energy performance targets, particularly in Passivhaus and KfW-Effizienzhaus projects.

Identifying Thermal Bridges: Thermography and Automation

Surveying buildings for thermal bridges is performed using passive infrared thermography (IRT) according to the International Organization for Standardization (ISO). Infrared thermography of buildings can reveal thermal signatures that indicate heat leaks, detecting thermal abnormalities linked to the movement of fluids through building elements and highlighting variations in the thermal properties of materials that cause a major change in temperature.

One accuracy challenge is the drop shadow effect, where the surrounding environment casts a shadow on the facade, leading to inconsistent sun exposure. An alternative analysis method, Iterative Filtering (IF), can be used to solve this problem. In all thermographic building inspections, thermal image interpretation is performed by a human operator, involving a high level of subjectivity and expertise. Automated analysis approaches, such as laser scanning technologies, can provide thermal imaging on three-dimensional CAD model surfaces and metric information to thermographic analyses, enabling surface temperature data in 3D models to identify and measure thermal irregularities of thermal bridges and insulation leaks.

Thermal imaging can also be acquired through the use of unmanned aerial vehicles (UAV), fusing thermal data from multiple cameras and platforms. The UAV uses an infrared camera to generate a thermal field image of recorded temperature values.

Thermal Bypass: A Related Phenomenon

Thermal bypass is defined as heat transfer that bypasses the conductive or conductive-radiative heat transfer between two regions. It may 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, and wind washing. This phenomenon underscores the importance of continuity in both insulation and air barriers.

Mitigation Strategies

Thermal bridges in completed buildings can be revealed with thermal imaging cameras, but they can be very difficult to rectify, particularly if they are repeated throughout a building. Prevention through design is therefore critical. Strategies to reduce or prevent thermal bridging include limiting the number of building members that span from unconditioned to conditioned space and applying continuous building insulation materials to create thermal breaks.

Specialist products such as structural thermal break materials—often used in masonry shelf angles, balcony connections, and column bases—interrupt the conductive pathway while maintaining structural integrity. Detailed attention to junctions, openings, and penetrations during the design and construction phases is essential to minimise the risk of thermal bridging and ensure that the building envelope performs as intended.

For further reading on related topics, see how moisture moves through building materials and energetic renovation strategies.