fRSI values for your project, in minutes
PAS 2035 requires that thermal bridging should be assessed wherever insulation continuity is broken. AutoPSI lets you model the detail you actually surveyed and get its fRSI straight away: no specialist, no queue.
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How to get an fRSI for a junction
The same model gives you both numbers: fRSI for surface temperature, and the Ψ-value for heat loss.
Start from a template, or draw it
Begin with the template library where a detail matches, and build it layer by layer in Free Draw where it does not, including the odd materials, with your own K-values.
Set the boundaries
Internal and external conditions, and the U-value arrows for the flanking elements.
Calculate
AutoPSI solves the temperature distribution through the junction and finds the coldest point on the internal surface.
Read the fRSI
That minimum surface temperature becomes the fRSI, ready to check against your criterion and attach to the project record.
Retrofit rarely follows the standard detail
Existing buildings bring variation. Construction can differ from what was expected at survey, and details often evolve as the building is opened up and the retrofit design develops.
AutoPSI gives you a fast starting point with retrofit templates, while making it easy to adapt or create details when the actual construction is different.
- Retrofit templates for common junctions
- Free Draw for non-standard construction
- Bespoke materials for existing and unusual build-ups
- Minutes per junction, making property-specific modelling practical
What fRSI actually is
fRSI is the internal surface temperature factor. It expresses how warm the coldest internal surface of a junction stays, relative to the difference between inside and outside:
fRSI = (Tsi − Te) / (Ti − Te)
Because it is a ratio rather than a temperature, it describes the junction itself rather than the weather on the day it was assessed, which is what makes it usable as a design criterion and comparable between details. The method sits in BS EN ISO 13788, and 0.75 is the figure normally quoted for dwellings, though the right criterion depends on building type, occupancy and assessment methodology.
The formula is trivial. Finding Tsi, the minimum internal surface temperature, is not. Only a thermal model of the junction tells you where the coldest point actually is. That is the part AutoPSI does, and the reason fRSI cannot be read off a table.
For the full explanation see what fRSI is and why 0.75 matters.
Insulation can make a cold spot worse, not better
Insulating an element raises the internal surface temperature across most of it. But wherever that insulation is interrupted (an intermediate floor, a party wall, a window reveal, a structural element), the bridge remains. Heat concentrates through what is left, and in relative terms the cold spot gets colder while the wall around it gets warmer.
A cold bridge causes condensation when the internal surface falls below the dew point of the air against it. Mould does not even need that: surface mould can establish wherever relative humidity at the surface stays high for long enough, which happens above the dew point.
So the U-value improves, the occupant reports damp in a corner, and nothing in the fabric calculation predicted it. A condensation risk assessment at junction level is what catches it, and fRSI is the number that expresses it.
Improving the insulation is not just about making the warm areas warmer. It is about making sure you have not made the cold spots critical.
Evidence for TrustMark audits
Retrofit delivered under PAS 2035 is lodged with TrustMark, and TrustMark audits registered businesses against the work they have lodged. Condensation and mould risk sits inside that scope, and it is one of the harder things to evidence after the fact, because the risk lives at individual junctions rather than in the headline U-value.
A modelled fRSI is worth having on file for exactly that reason. AutoPSI produces a calculated surface temperature factor per junction, with a report you can attach to the project record, so “how do you know this detail is safe?” has a documented answer rather than an assumption behind it.
Common questions
What does fRSI stand for?
The internal surface temperature factor. It expresses how warm the coldest internal surface of a junction stays, as a proportion of the difference between internal and external temperatures.
Is 0.75 always the right threshold?
It is the figure most commonly referenced for typical UK assessment situations, but the appropriate criterion depends on the building type, occupancy and assessment methodology. Treat 0.75 as the usual benchmark rather than a universal rule.
Can adding insulation make a junction worse?
It can make a cold spot more pronounced relative to its surroundings. Insulating the main body of a wall keeps that surface warmer, but where the insulation is interrupted (at an intermediate floor or a window reveal, say), heat concentrates through the remaining bridge. The element’s U-value improves while the junction becomes more critical.
Do I need thermal modelling to calculate fRSI?
To do it properly, yes. The formula is trivial; finding Tsi is not. Only a thermal model of the junction tells you where the coldest point on the internal surface actually is. See what fRSI is and how the PSI value calculator works.
Can thermal bridging cause condensation?
Yes. It is one of the most common causes of surface condensation in an otherwise dry building. The bridge creates a localised cold spot, and if that surface drops below the dew point of the air against it, moisture forms there. Mould can establish before full condensation, wherever surface humidity stays high for long enough.
How do you carry out a condensation risk assessment?
At junction level, by modelling the detail and reading the minimum internal surface temperature off it, then expressing that as fRSI. A whole-element U-value will not tell you: it describes average heat loss through the build-up, not the coldest point where condensation actually forms.
Does AutoPSI calculate Ψ-values as well as fRSI?
Yes. Both come from the same thermal bridge model, so you can assess heat loss and surface temperature performance together rather than running two exercises. The junction types it covers lists what you can model.