PSI Value Calculations for Thermal Modelling

Types of Junctions

To do a residential heat loss calculation, you need to understand different building junctions that require PSI values (Ψ-values). When two exposed elements meet, such as the corner where the ground floor meets an external wall, thermal bridging occurs. This creates linear thermal transmittance values. To measure heat loss in thermal bridges, you use thermal bridging calculations with PSI values.

Junctions can be walls, floors, roofs, windows, and doors. Our specialist thermal modelling software has advanced features that simplify PSI value calculations for various junctions.

Here are the three main kinds of junctions considered in SAP energy rating calculations that you can calculate with AutoPSI:

External Wall Junctions

External wall junctions include all floor junction details, such as the ground floor, basement floor, etc. This includes places where a window thermal bridge meets other construction materials, like sills and jambs in windows and doors. Both door and window thermal efficiency are important in construction, as poor energy performance results in higher heating and cooling costs.

Party Wall Junctions

Party wall junctions include other kinds of ground and exposed floors, as well as intermediate floors. This category includes the roof wall junction detail, where the insulation is usually in the ceiling.

Roof Junctions

Roof junctions include eaves, gables, flat roofs, balconies, and corners. This can also include party walls.

There is a separate category for roof junction details known as “roof and room-in-a-roof” that includes roof windows, lights, and more. Window thermal performance is also important in calculating the thermal performance of roofs.

Get an approved thermal bridging scheme for your building with AutoPSI. Choose your plan and discover how our PSI calculation software can simplify your PSI value calculations.

PSI Value Calculations – FAQs

Have a few questions about PSI value calculations for thermal modelling construction? Take a look at our frequently asked questions below.

What is a PSI value?

PSI values (written as Ψ-values) measure heat transmittance and total heat loss between two thermal elements throughout accredited construction junctions. A PSI value is measured in watts per metre kelvin (W/mK) and indicates how much energy is lost between thermal elements. They are used in SAP energy calculations to show how the building is performing thermally.

How do you calculate PSI?

With PSI values indicating total heat loss between thermal elements, you need to use specialised thermal modelling software to do calculations. Historically, this was calculated manually by multiplying the output U-value by the length of a junction. With advanced software, you can copy drawn junctions to measure PSI values for all junctions. These are utilised in new-build SAP calculations for building control.

If the external PSI value (thermal bridge loss coefficient Ψe) of a thermal bridge is lower or equal to 0.01 W/mK, you won’t need to calculate it.

Why are PSI values important?

A PSI calculation is important for reducing heat loss in your building, ensuring efficient building design. With an accurate value, you can limit energy use, build strong insulation and have accurate SAP calculations for building regulations.

In SAP calculations for a new build property, there are different kinds of ceiling, floor, and wall junction details, along with thermal bridging details with default PSI values. While default PSI values were used historically, they are no longer accepted for SAP calculations, as they are often inaccurate.

Can I use a U-value instead of a PSI value?

Many believe that U-values are sufficient for heat loss calculations. However, U-Values can only calculate the heat loss through building fabric. PSI values can calculate heat loss in a non-repeating thermal bridge, which often has higher thermal conductivity, making them more accurate than a U-value calculation.

What is a thermal bridge?

Whenever the external or internal dimensions in a building change, it can create thermal bridges. The fabric’s surface area is different, which creates an overall heat loss that cannot be discovered by using U-values. Thermal bridging are weaker points in construction indicating heat loss in your site’s junctions.

What is a non-repeating thermal bridge?

When buildings have two different fabrics that meet, it forms non-repeating thermal bridges. These buildings have insulation replaced by other building materials, which causes higher overall heat loss.

If you have more questions about PSI value calculations, reach out to our team.

AutoPSI: Innovative Technical Software Designed For Efficiency

AutoPSI understands the need for accurate PSI values in the construction design industry.  We know how difficult it can be to control your site’s energy performance while meeting building regulations. We developed our software to make calculating PSI values more efficient.

Our software is formulated to help you pass SAP assessments and simplify PSI value calculations. Our online tool is ideal for discovering heat loss, improving thermal performance, and constructing low-energy buildings.

AutoPSI’s technical services team has worked on many construction projects over the years. As a result of our PSI value calculation software, businesses have been able to improve on the previous methods using accredited construction details (ACD).

Make bespoke PSI value calculations using our fast, fully online tool. Try AutoPSI’s specialist software today to simplify your PSI value calculations.