Thermal Bridging - The Basics

Thermal bridging describes a design or construction detail with 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.

The heat transfer rate depends on the material's thermal conductivity and the temperature difference on either side of the thermal bridge. When a temperature difference is present, heat flow will follow the path of least resistance through the material with the highest thermal conductivity and lowest thermal resistance, creating the thermal bridge.

As a result, where warm, moist internal air comes into contact with the potentially cold surface, condensation can result in mould growth and durability issues.

Despite insulation requirements specified by various national regulations, a building's envelope remains weak in the construction industry. It is only regulated in a small number of cold-climate construction markets. Thermal losses are likely greater in practice than anticipated during the design stage and are potentially one of the main sources of unquantified heat losses in better-performing building envelopes.

Thermal bridges can occur at several locations within a building envelope. Most commonly, they occur at junctions between two or more building elements, including:

• Floor-to-wall or balcony-to-wall junctions, including slab-on-grade and concrete balconies or outdoor patios that extend the floor slab through the building envelope

• Roof/Ceiling-to-wall junctions, especially where full ceiling insulation depths may not be achieved

• Window-to-wall junctions

• Door-to-wall junctions

• Wall-to-wall junctions

• Wood, steel or concrete members, such as studs and joists, are incorporated in the exterior wall, ceiling, or roof construction.

• Recessed luminaries that penetrate insulated ceilings

• Windows and doors, especially frame components

Structural elements remain a weak point in construction, commonly leading to thermal bridges that result in high heat loss and low surface temperatures on the interior side of the building envelope.

Construction Thermal Bridges

External walls are often structurally supported by timber or steel partitions. Such partitions typically run through the thermal control layer and represent (the combination of frame plates and noggins) between 10 – 15% of the overall area of a standard full-height wall. These are known as construction thermal bridges.

As a result of this thermal bridging effect, they increase the overall thermal transmittance of the wall and need to be considered. Steel wall frames, with high thermal conductivity, are particularly poor performing and may need to consider non-combustible thermal breaks to ensure a continuity of the thermal control layer.

Geometric Thermal Bridges

Unlike construction thermal bridges, geometric thermal bridges are less frequent. Typically non-repeating, they occur where the geometry of the thermal control layer causes increased heat loss. Indeed, geometric thermal bridges are often unavoidable and subject to the complexity of building envelope form.

Geometric thermal bridges do not form a literal bridge in the way construction thermal bridges do and can occur where continuity of insulation is maintained. Typically a geometric thermal bridge is where the external heat loss area is greater than the corresponding internal area of the thermal envelope. External wall corners, eave junctions, around window and door openings and at the ground floor and external wall junctions are all good examples.

Hybrid Thermal Bridges

Hybrid thermal bridges combine features of different thermal bridges, such as construction and geometric thermal bridges. An external wall corner is a good example of a typical geometric thermal bridge but will also have an additional structure that creates a construction thermal bridge. A good example of combined thermal bridges is corners with metal studs with high thermal conductivity. In this case, increased heat flow through this detail is caused by the intense heat transfer through the metal studs and the corner shape effect.