What is the Surface Temperature Factor Calculated With a PSI Value?

The surface temperature factor (fRSI) provides extra information about the building fabric performance of a linear thermal bridge. While the calculated PSI value for a junction detail gives a value for heat loss that can be input into building regulation compliance calculations, the surface temperature factor indicates the internal surface temperature at the junction detail.

fRSI has a variety of applications, including helping to assess what contribution junction details might make to internal comfort for building occupants. It predicts the likelihood of condensation and mould formation and can also be part of demonstrating compliance to ensure higher standards beyond building regulations.

Read on to learn about the role of surface temperature factor (fRSI) in calculating PSI values.

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Why is the Surface Temperature Factor Useful?

U-values (thermal transmittance) and PSI values are important measures of building fabric heat loss. They measure very different things and, together, help provide a complete picture of a building’s thermal envelope.

Construction industry professionals and clients are much more likely to have an idea of what a ‘good’ U-value is. Many people know the benchmarks they need to aim for in terms of the main building elements because U-values are generally better understood than PSI values.

Despite the growing importance of calculating PSI values – evidenced by their enhanced role within the Part L 2021 requirements and guidance – many people looking at the heat loss value for a particular junction detail won’t understand whether it represents good performance, bad performance, or meets improved energy efficiency requirements.

As an additional measure, using a thermal modelling calculator to calculate fRSI can provide a useful indication of whether the performance of the junction aligns with the overall goals for the performance of the building.

The Importance of fRSI Values in Thermal Bridging Calculations

A thermal bridge has a higher transmittance of heat than other materials and can make thermal conditions unacceptable. 

This can cause a total temperature drop, high humidity or more extreme conditions. Once the building temperature falls below the dew point, condensation forms on internal surfaces, creating damp and other wet areas. If the internal humidity is high, thermal bridges can create interstitial condensation. For colder surface temperatures, thermal bridges cause mould formation.

With an fRSI value calculation, you can identify areas with thermal bridges in airtight buildings and swimming pools. You can ensure your structure has acceptable temperature conditions, prevents the risk of mould formation and condensation, and passes SAP calculations.

The actual surface temperature (fRSI value) depends on external and internal temperatures. These temperatures work independently of the absolute conditions, regardless of different design situations.

What Surface Temperature Factor Should a Junction Detail Achieve?

Although the terminology is not appropriate, there is a reason that thermal bridges – including linear ones – are sometimes described as ‘cold spots’. A thermal bridge is often responsible for more heat loss than the building fabric immediately around it and, therefore, feels colder to the touch.

When warm, moist air comes into contact with a colder surface, the inside and outside air temperature cools and any moisture that it can no longer hold is deposited as condensation. A cool, damp surface creates conditions in which resultant mould growth can start to form, with a corresponding increase in the risk of respiratory problems for building occupiers.

In residential buildings, a 0.75 or greater is the surface temperature factor required to ensure that condensation will not occur. This is easier to understand than trying to contextualise the performance represented by a PSI value.

The surface temperature factor formula is a critical calculation. The value will depend greatly on the building’s consequent internal relative humidity.

The surface temperature factor puts comfort front and centre. Awareness of it is a benefit when designing to the Passivhaus standard and seeking to maintain a minimum internal surface average temperature of 17 deg.C throughout the building or retail premises.

BRS Technology’s online modelling heat loss tool AutoPSI offers the ability to calculate PSI values quickly and simply, all by relevant standards, with our online heat loss calculator. 

Using a drag-and-drop interface, you can create bespoke, customisable junction details, with both the PSI value and surface temperature factor calculated instantly on our thermal modelling software. 

To find out how AutoPSI can help you, request a demonstration.

AutoPSI: Heat Loss Calculations Made Simple

AutoPSI was founded in 2019 to develop innovative heat loss software tools for the construction design industry. We understand how complicated the surface factor formula is.

Our efficient online PSI value calculator performs the surface temperature formula and PSI calculation for you. You can prevent mould and condensation and meet BR497 and building regulations. Now you can evaluate temperature surface tension and the recognised construction details easily. 

After learning about the surface temperature factor (fRSI) in PSI calculations, discover the benefits of using AutoPSI’s software to simplify thermal bridging calculations. 

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