Condensation
Risk Analysis
fRsi temperature factor — BR 497 & Part L.

A Condensation Risk Analysis (also called a Surface Condensation Risk Assessment) determines whether the internal surfaces of building junctions — such as lintels, eaves, party walls and floor edges — remain warm enough to avoid surface condensation and mould growth. It does this by calculating the temperature factor (fRsi) at the coldest internal point of each junction and comparing it against the critical threshold defined in BRE IP 1/06.

The fRsi (temperature factor) is a dimensionless value between 0 and 1 that describes how warm the coldest internal surface of a junction is relative to the difference between internal and external design temperatures. The closer to 1, the warmer that surface is. For dwellings, BRE IP 1/06 sets the critical threshold at 0.75 — if the calculated fRsi at a junction is below 0.75, the surface is likely to fall below the dew point under typical occupancy conditions, leading to condensation and mould growth.

They are complementary but distinct assessments. Thermal Bridging Calculations (Psi-values) quantify the additional heat loss through a junction compared with the unbridged plane element — they feed into your SAP or SBEM model. Condensation Risk Analysis (fRsi / temperature factor) assesses whether the coldest point on the internal surface of that same junction is warm enough to avoid moisture. A junction can have an acceptable Psi-value for overall heat loss yet still fall below the fRsi threshold, meaning surface condensation will occur. Both assessments use 2D thermal modelling software, but they answer different questions.

You typically need one whenever Building Control requires confirmation that non-repeating junctions comply with Part L and BR 497. This most commonly arises on new-build dwellings and multi-residential developments, where any junction that departs from the standard Accredited Construction Details (ACDs) must be modelled. It is also requested where an architect has specified a bespoke detail — a green roof parapet, a structural glazing junction or a cantilevered floor edge, for example — that is not covered by published ACDs.

The primary references are: BR 497 (Conventions for Calculating Linear Thermal Transmittance and Temperature Factors, BRE 2016), which sets out the calculation methodology; BRE IP 1/06 (Assessing the Effects of Thermal Bridging at Junctions and Around Openings), which defines the critical fRsi threshold of 0.75 for dwellings; and Approved Document L (Conservation of Fuel and Power), which mandates compliance with these limits as part of the fabric performance route under the Building Regulations.

If the calculated fRsi for a junction falls below 0.75, we provide written remedial guidance — typically recommending an adjustment to the insulation position, thickness or continuity at that detail. Common fixes include extending insulation past the structural element, adding a thermal break, or adjusting the geometry of the junction. We re-run the model after each change so you have a confirmed-pass result to submit to Building Control.

We need the architectural junction drawings for each detail to be assessed — typically PDF or CAD format — along with the material specification (layer build-ups, conductivities, densities). For standard construction types, manufacturer datasheets are sufficient. We will confirm exactly what we need at quote stage, and flag any missing data before we start modelling.

The two assessments work closely together. Your SAP or SBEM model needs a linear thermal transmittance (Psi-value) for each non-repeating junction. We produce both the Psi-value and the fRsi temperature factor from the same 2D thermal model, so commissioning both together from us avoids duplication and keeps your compliance package consistent.