Thermal Bridges: Psi-Value (Ψ-value) and R-Value Explained

R-Value

The R-value measures the thermal resistance of a material or assembly, indicating how well it resists heat flow. The higher the R-value, the better the insulation performance. R-values are used to rate the insulation performance of walls, roofs, floors, and other building elements. They are crucial for determining the overall thermal resistance of these elements.

It is typically expressed in units of (m²·K)/W.

Psi Value (Ψ-value)

The Psi (Ψ) value measures the linear thermal transmittance of thermal bridges. It indicates the amount of heat that passes through a linear thermal bridge (such as a wall-to-floor junction) compared to the adjacent insulated areas. Ψ-values are used to quantify the additional heat loss due to thermal bridges in a building. They are important for accurately calculating the overall heat loss of a building envelope.

It is expressed in units of W/(m·K).

Relationship between Psi Value (Ψ) and R-Value

While the R-value focuses on the thermal resistance of homogeneous or planar elements (like a wall or a roof), it does not account for heat loss through linear features such as joints, edges, or corners.

The Psi (Ψ) value complements the R-value by providing a measure of the extra heat loss that occurs where different building elements intersect or where there are material changes.

How They Work Together

When calculating the thermal performance of a wall, for instance, you use the R-values of the wall materials to determine the overall thermal resistance of the wall assembly.

To get a more accurate picture of the building's thermal performance, you also need to include the effects of thermal bridges. Ψ-values are used to quantify the additional heat loss along linear features such as corners, edges, and junctions.

Example of Combined Use

Imagine you have a well-insulated wall with a high R-value. If this wall has a poorly insulated window frame, the area around the frame will have a higher rate of heat loss.

The R-value gives you the thermal resistance of the wall itself, while the Ψ-value of the window frame junction tells you how much additional heat is lost due to this specific thermal bridge.

By combining both the R-values of the main elements and the Ψ-values of the thermal bridges, you can more accurately assess the overall thermal performance of the building.

This holistic approach ensures that all aspects of heat transfer are considered, leading to better energy efficiency and comfort in buildings.