Psi Values

Thermal Bridging and psi (ψ) values

The evaluation of thermal performance of both elements and whole constructions is becoming increasingly important.

Thermal bridges occur at junctions; for instance where walls meet, or where a wall meets a floor or a wall meets a roof, window or door. Generally speaking, they occur in a building where a material with poor insulating properties passes through an insulation system causing a ‘bridge’ for heat to escape.

There are two main types of thermal bridge: A repeated thermal bridge, such as the studs in a timber frame wall, which are dealt with in a conventional U-value calculation. There are also non-repeated thermal bridges, for example around openings, using a section of steel in a timber frame wall, or the junction between different elements types, such as a wall and a floor.


What is a ψ-value?

The heat loss associated with the non-repeated thermal bridge is called linear thermal transmittance or psi-value or ψ-value (pronounced ‘si’). This is the rate of heat flow per degree per unit length of the thermal bridge that is not accounted for in the U-value of the plain elements.

The Psi value has become the industry standard to define the thermal weakness at the intersection of building elements such as walls and floors. They much calculate the thermal weakness . The Psi value defines the weakness that results from the junction of the floor and the wall and the higher the value the greater the losses or weakness.

When known, the Ψ-value for each detail in the dwelling design can be multiplied by its length (l x Ψ) to establish the rate of heat loss through that specific bridge. The heat loss of the dwelling due to all thermal bridges (HTB) is then the sum of (l x Ψ) for all the construction details that include thermal bridges, as defined in the following equation:

HTB =Σ(l x Ψ)

Where HTB is the total heat loss due to non-repeating thermal bridges – known as thermal bridge transmission heat loss.

Calculation of ψ-values and the relevant standards?

This is done with the use of computer programs. The CTS has used primarily THERM ,BISCO and TRISCO. All these programmes are adequate in estimating the heat flow through different types of junctions, in both simple (2D) and the more complex (3D) cases.

Why is it becoming important now?

The Approved Documents L1A and L2A (sections 5.12a and 5.7a) addressed the issues of thermal bridging, introducing the concept of a scheme for quality assurance of thermal bridging details. The context of this scheme is yet to be determined and in the meantime designers, energy assessors and specifiers tend to depend on the default values of the existing accredited construction details. These conservative values are higher and less accurate than those that can be achieved by performing bespoke calculations, and they don’t cover all types of construction. Consequently, the heat loss estimated in a SAP calculation in such cases may be higher than expected based on the details used.

How can we help you?

As an alternative to using default values individually calculated psi values can be used, but building control officers still need to satisfy themselves that the psi-values used have been created by a person with suitable expertise and experience. To assist designers, CTS and Litecast have prepared a Constructive Detail Sheet based on our own XT product range. These values can be used rather than the less accurate default values.

BRE document SAP 2012 – Appendix K (Junctions with an external wall and ground floor (normal) E5 has an Approved ψ-value of 0.16 W/mK and a default value of 0.32W/mK.

Our calculated ψ-values below improve on these figures.

XT20, XT18, XT15 or XT12

(EPS 190 - 300mm thick)

Beams parallel to junction

Starter or end blocks ≥ 270mm

Beams perpendicular to junction
Wall U values (Wm²/K) Ψ Value (W/mK) Temperature factor Ψ Value (W/mK) Temperature factor
0.26 ≤ U≤ 0.30 0.063 0.93 0.089 0.90
0.21 ≤ U ≤ 0.25 0.060 0.94 0.083 0.91
U ≤ 0.20 0.059 0.94 0.077 0.91








or for our deeper panels

XT10, XT08

(EPS 301 - 400mm)

 Beams parallel to junction

Starter or end blocks ≥ 270mm

Beams perpendicular to junction
Wall U values (Wm²/K) Ψ Value (W/mK) Temperature factor Ψ Value (W/mK) Temperature factor
0.26 ≤ U≤ 0.30 0.064 0.93 0.091 0.91
0.21 ≤ U ≤ 0.25 0.061 0.94 0.083 0.91
U ≤ 0.20 0.058 0.95 0.077 0.92


Please click here for more information on Thermal Bridging Constructive Details



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