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.

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[edit] Related articles on Designing Buildings Wiki

• Accredited construction details ACDs.
• Air tightness in buildings.
• Building fabric.
• Cavity wall.
• Co-heating test.
• Computational fluid dynamics.
• Condensation.
• Conduction.
• Conventions for calculating linear thermal transmittance and temperature factors.
• Defects in dot and dab.
• Emission rates in the building regulations.
• Emissivity.
• Flashing.
• Floor insulation.
• g-values.
• Humidity.
• Insulation for ground floors.
• Insulation specification.
• Interstitial condensation.
• Mould growth.
• Rising damp.
• Roof insulation.
• Solid wall insulation.
• Thermal comfort.
• Thermographic survey.
• U-values.