When we talk about insulating buildings, the focus is often on the U-value. This is essentially the sum of the thermal resistance of all the layers that make up a construction, providing an estimate for how it will prevent heat loss. Whilst U-value calculations take into account a number of factors, one aspect of heat loss they don’t consider is so-called linear and geometric thermal bridges. These occur around openings such as windows and doors, or at junctions between the different building elements – providing a path of least resistance for heat to escape. In practice, thermal bridges can account for a significant amount of heat loss from a property, particularly where the construction elements are insulated to a high level, so it is important to carefully address them.
Learn more about thermal bridging.
To determine how well a designed junction can prevent heat loss, we have to carry out additional calculations to generate what are known as Psi-values and Y-values.
What is a Psi-value?
A Psi-value (also written as a ψ–value) is the measure of how much heat is expected to be lost through a particular junction. Unlike a U-value, which measures heat loss across the surface area of an element, Psi-values are calculated for the line of a junction – whether it be the joint between a wall and floor or around a window cavity. This heat loss is measured in Watts per metre Kelvin (W/mK).
The Psi-value calculation is complex and considers several factors, including heat flow simulation of the junction and the impact of U-values in the surrounding or adjoining building elements. The output value is multiplied across the total length of a junction to create a total junction Psi-value.
What is a Y-value?
The Y-value represents the total thermal bridging heat losses for a property. To calculate this, we first multiply the Psi-value for each junction (Ψ) by its corresponding length (l) and add the resulting figures together. This is known as the HTB (heat loss due to thermal bridging). By dividing this by the total area of external elements (excluding party walls), we can determine the percentage of overall heat loss from a building associated with thermal bridging. This is known as a Y-value.
How are Psi-values and Y-values used within the Building Regulations and Standards?
Both Psi and Y-values are important factors in determining whether a property complies with the energy efficiency requirements of the Building Regulations and Standards (contained within Part L of the Building Regulations in England and Wales and Section 6 (Energy) of the Building Standards in Scotland). This is calculated using the Standard Assessment Procedure (SAP) methodology for domestic buildings and the Simplified Building Energy Model (SBEM) for non-domestic.
For each junction, designers can choose to:
• obtain calculated details from a suitably competent person following the guidance in The Building Research Establishment’s BR 497, and the temperature factors set out in the Building Research Establishment’s Information Paper 1/06;
• use pre-calculated PSI-values supplied by manufacturers or online databases; or
• use default Psi-values given within SAP table K1 or SBEM.
Designers can choose to use a mixture of the above options to cover the various junctions in a property. It is also possible to use a global backstop Y-value for the entire property. Both the default Psi-values and the global backstop Y-value represent a high level of heat loss to encourage designers to use accurate details.
Under the current Building Regulations, it is also possible to use Accredited Construction Details (ACDs). These are standard junctions designed to simplify the process of calculating Psi-values for a property. However, it is now considered that the ACDs are outdated and as part of the changes to Part L and Section 6 in England, Wales and Scotland, (due to come into force in 2022), they are set to be removed from SAP.
Learn more about the changes to detailing in Part L 2021 and Section 6 2021.
The Y-value is then calculated based on the supplied Psi-values.
In order to produce a total fabric heat loss value, the total U-values for the different elements are first multiplied by their respective element areas. Next, the Y-values are multiplied by the total exposed surface area of the building. The resulting figures are then added together to produce the overall value.