Thermal bridging - The Basics

Thermal bridging describes a design or construction detail 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.

The rate of heat transfer depends on the thermal conductivity of the material 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 a weak spot in the construction industry and is only regulated in a small number of cold climate construction markets.  Thermal losses are likely greater in practice than that 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, 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 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 junction are all good examples.

Hybrid Thermal Bridges

Hybrid thermal bridges combine features of different types of thermal bridges, such as construction thermal bridges and geometric thermal bridges. An external wall corner is a good example of a typical geometric thermal bridge but will also tend to have an additional structure that also creates a construction thermal bridge. A good example of combined thermal bridges are 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, as well as by the corner shape effect.