A building can have high levels of insulation and high-performance windows, yet still experience additional heat loss at the junctions between building elements.
These areas are known as thermal bridges.
What is a thermal bridge?
A thermal bridge occurs where heat can pass more readily through part of the building envelope than through the surrounding construction.
They commonly occur where materials, components or structural elements interrupt the continuity of the insulation layer.
Why do thermal bridges matter?
As the thermal performance of walls, roofs and windows improves, the relative importance of junctions increases.
Poorly resolved thermal bridges can contribute to:
- increased heat loss
- lower internal surface temperatures
- increased condensation risk in vulnerable conditions
- reduced overall building-envelope performance
This is why building performance needs to consider not only individual product U-values, but also how those products connect to the surrounding construction.
What is a Ψ-value?
Heat loss through a junction is commonly described using linear thermal transmittance, or a Psi-value (Ψ-value).
A Ψ-value is expressed in W/mK and represents the additional heat flow associated with a linear junction.
In simple terms, the lower the Ψ-value, the lower the additional heat loss associated with that junction.
Thermal bridging around windows and doors
Window and door openings are particularly important because they interrupt the surrounding wall construction.
The frame, structure, insulation and sealing layers all have to meet at the perimeter of the opening.
Window position can therefore influence the thermal performance of the junction. Positioning a frame within, or further towards, the insulation layer can help create greater continuity between the window and the building's thermal envelope.
But position is only one consideration.
Structural support, insulation continuity, airtightness and external weather protection all need to be considered as part of the complete installation.
Find out more
We have explored thermal bridging at window openings and the principles behind thermally optimised window positioning in more detail in the Lynvale Technical Hub.
Read: Why Window Position Matters – Reducing Thermal Bridging with Triotherm+
