TYPES OF THERMAL BRIDGES   

Thermal bridges can be classified into three main types:

• Repeating or quasi-homogeneous thermal bridges.
• Non-repeating or linear thermal bridges.
• Geometrical thermal bridges.

REPEATING

Repeating thermal bridges usually follow a regular pattern and are evenly distributed over an area of the thermal envelope. Typical examples include:

• Ceiling joists in cold pitched roofs that are insulated at ceiling level.
• Ground floor joists in an insulated suspended timber ground floor.
• Timber studwork and I-beams in timber frame construction.
• Mortar joints in an insulating block inner leaf.
• Steel wall ties in masonry cavity external wall construction.

Repeating thermal bridges

Mortar joints in insulated blockwork and stainless steel wall ties are common examples of repeating thermal bridges

Repeating thermal bridges can have a significant effect on heat loss, and are required to be taken into account in the calculation of U values (see Calculating Thermal Bridges).

NON-REPEATING

Non-repeating thermal bridges are intermittent and occur at a specific point in the construction. They are often caused by discontinuities in the thermal envelope. These discontinuities may be a result of the construction method used or may be due to changes in materials over the thermal envelope. They commonly occur around openings and other instances where materials of different thermal conductivities form part of the external envelope. Typical examples include:

• Around windows, doors and rooflights.
• Around loft hatches.
• Where internal walls or floors penetrate the thermal envelope.
• Where steel I-beams have been used to support timber roofs.

Non-repeating thermal bridges

Box lintel results in a non-repeating thermal bridge through the external wall at the window head

Use of top hat lintels results in a number of non-repeating thermal bridges at the window head

GEOMETRICAL

Geometrical thermal bridges, as the name suggests, are a result of the geometry (or shape) of the thermal envelope. They can be 2-dimensional (where 2 planes intersect) or 3-dimensional (where 3 or more planes intersect). The occurrence of geometric thermal bridging is likely to increase the more complex the building geometry. Typical examples include:

• At the corner of an external wall.
• At wall/roof junctions.
• At wall/floor junctions.
• Junctions between windows and doors and walls.
• Junctions between adjacent walls.

Geometrical thermal bridges (assumes insulation placed at ceiling level)

The corner of the external wall and the junction between the wall and the roof both result in a geometric thermal bridge