What are PSI values?

PSI values play a pivotal role in understanding heat loss in buildings, particularly at intersections where different building materials and geometries converge. So, what are PSI values? Essentially, these values quantify the heat loss when the building fabric changes direction or its geometric structure.

For building designers and specifiers, PSI values aren’t just a theoretical metric. They’re a critical tool for adopting a fabric-first approach in projects. Proper junction detailing, combined with precise psi value calculations and accurate on-site construction, paves the way for buildings with high energy efficiency, ensuring optimal comfort and health conditions.

AutoPSI’s dynamic thermal modelling software is BRE-accredited and meets the ISO requirement. Read on to learn more about what PSI (Ψ) values are.

What Is a PSI Value Calculation?

A PSI value calculation involves measuring the total heat loss in a building. These calculations take place during the design of new-build construction projects and are crucial in determining where amendments need to be made to reduce heat loss and improve energy performance.

The measurements are taken through the junctions of a building. These junctions include meeting points like the point between the ground and an external wall. PSI values help identify where buildings can be made more energy efficient by indicating areas where there is a loss of energy between two thermal elements. Essentially, it measures the linear thermal transmittance in a building.

Previously, building designers worked off predetermined PSI values and used Accredited Construction Details to meet building regulations. However, these predetermined junction designs are no longer allowed to be used to meet building regulations. This is why accurate PSI calculations are more important than ever for construction projects.

The Difference Between U-Values & PSI Values

PSI values are often mistaken for U-values because they calculate a similar output. However, there is a significant difference between a U-value and a PSI value.

U-values measure heat loss through thermal elements but cannot calculate the total heat loss. Where there are junctions in a building, one will find additional heat loss, which are non-repeating thermal bridges.

Taking into consideration non-repeating thermal bridges, construction PSI values help calculate the total heat loss even when there are meeting junctions, whereas a u-value calculation cannot achieve this. 

PSI values are measured in Watts per metre Kelvin (W/mK), indicating the loss of energy per metre between thermal elements. SAP calculations must contain a thermal bridging component to successfully consider temperature factors and higher thermal conductivity in a building. If they are not calculated properly, default PSI values will be used in the SAP assessment, which can lead to a lack of energy efficiency.

Why Are PSI Values Calculated?

A PSI value calculation plays an important role in energy performance and meeting building regulations.

To minimise heat loss from the interior, a building’s insulation (known as the thermal envelope) should be continuous. Continuous insulation is most at risk at junctions between two thermal elements, such as where the ground floor meets an external wall or at the eaves where the walls and roof meet. This is usually because structural considerations take precedence.

Take a cavity wall construction, for example. The inner leaf masonry must continue down to the foundations to transfer the loads it is bearing into the ground. The ground floor insulation stops at the face of the masonry and cannot be continuous with the insulation layer in the wall cavity.

By interrupting the continuity of the thermal elements, the masonry acts as a thermal bridge. The heat loss through that thermal element is greater than the relatively well-insulated construction on either side of it. 

There are ways to reduce the thermal bridging effect, but it must still be accounted for as part of establishing the whole building’s thermal performance.

How Are PSI Values Calculated?

Wondering how to calculate the PSI value? PSI values are calculated easily using building thermal modelling software. 

The construction junction is copied from the drawings and incorporates the construction materials. It’s important to note that if the materials change or the efficient building design itself changes, the PSI value will change, too, and a new PSI value calculation will have to be completed.

There are several considerations in a PSI value thermal bridge calculation, such as how much heat is flowing in the junctions and the impact of U-values in the thermal elements.

You’ll need to input all of these factors into the PSI value calculator software. The value resulting from PSI calculations is multiplied across the length of junctions, forming the total junction PSI value. This shows the overall heat loss at thermal bridges.

The results of the calculation for PSI values of common junctions are entered into a Standard Assessment Procedure (SAP) calculation.

Why Do We Need PSI Value Calculations?

Understanding PSI values and the critical component they play in low-energy buildings indicates the need for thermal bridging solutions.

The thermal envelope or insulation of a building aims to prevent heat loss from its interiors. This envelope’s continuity faces challenges at junctions, like where a ground floor converges with external walls or where walls and roofs intersect at eaves. Often, structural factors dominate these junctions.

Consider cavity wall constructions: the inner masonry leaf continues to the foundation, transferring the loads it bears. However, the floor insulation ends at the masonry face, disrupting continuity with the wall cavity insulation. 

This discontinuation results in what we call a ‘thermal bridge’, where the masonry causes increased heat loss compared to the surrounding well-insulated sections. Even though methods exist to mitigate this thermal bridging effect, its impact on overall building energy performance must be factored in.

Reduce building emissions and build greener with ease by calculating bespoke PSI values with AutoPSI. Try AutoPSI today.

How Do PSI Values Measure Heat Loss?

Linear thermal bridging defines the heat loss type described above, occurring along junction lengths. To determine the PSI value, software modelling calculates the heat loss for one metre of the said junction. This PSI value, when multiplied by the entire length of the junction in the design, provides the cumulative heat loss for that specific junction. This modelling and calculation process extends to different junction types.

The challenge? Historically, linear thermal bridging was often overlooked because building details could vary from one site to another. Although standard detail libraries and PSI values emerged, they couldn’t fit every situation, necessitating bespoke calculations. Unfortunately, this wasn’t always executed, leading to worst-case value assumptions and making project compliance more challenging.

AutoPSI: Simplifying the Complex World of PSI Value Calculations

Recognising the intricacies of PSI value calculations and junction detailing, we introduced AutoPSI. Designed to make streamlined, bespoke PSI value calculations, AutoPSI is an online tool that simplifies the modelling of custom junction details. 

Its user-friendly drag-and-drop interface lets users effortlessly add or remove components, facilitating easy exploration of different configurations. AutoPSI stands out by computing new PSI values swiftly, assisting in identifying optimal solutions.

In a nutshell, if the realm of PSI value calculations seemed intricate, tools like AutoPSI are ushering in a new era of simplicity and precision. Dive into the world of enhanced building performance with accurate PSI values today!

Interested in learning more about PSI values? To find out more about AutoPSI and how it could help you, request a demonstration.

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