DEFINITIONS   

THERMAL BRIDGE

A thermal bridge (or cold bridge) is an area of the building fabric which has a higher thermal transmission than the surrounding parts of the fabric, resulting in a reduction in the overall thermal insulation of the structure.

It occurs when materials that have a much higher thermal conductivity than the surrounding material (i.e. they are poorer thermal insulators) penetrate the thermal envelope or where there are discontinuities in the thermal envelope. Heat then flows through the path created – the path of least resistance – from the warm space (inside) to the cold space (outside).

The higher thermal transmission of this part of the fabric results in a reduction in the thermal performance (an increase in U-value) as heat flows through the fabric, and the surfaces of the interior side of the bridge become cooler.

The use of the term’ thermal bridge’ is somewhat misleading as it implies that the thermal envelope must be ‘bridged’ in some way for a thermal bridge to occur. This is in fact not the case. Thermal bridges can occur in unbridged construction where discontinuities exist in the thermal envelope.

Thermal bridges occur due to:

• Geometrical effects, such as corners.
• Penetrations through the thermal envelope, for instance windows and doors.
• Junctions between different elements (wall and floor) and different components (around windows and doors).
• Poor construction practice (gaps or discontinuities in thermal insulation).

The occurrence of thermal bridges can be attributable to:

• Issues associated with the design of the thermal envelope.
• Buildability and construction issues; or as is often the case
• A combination of both of the above points.

PSI (ψ) VALUE

The linear thermal transmittance, or psi (ψ) value is the rate of heat flow per degree temperature difference per unit length of a thermal bridge. It is measured in W/mK and is a property of the thermal bridge. ψ values are used to calculate the heat loss or gain through a non-repeating or geometrical thermal bridge.