fRsi Condensation Risk Check (EN ISO 13788)

Find the coldest interior surface point of a detail and verify it against mould and condensation criteria.

What fRsi Is

Cold spots on interior surfaces — at thermal bridges, window reveals, corners — are where mould grows and condensation forms first. EN ISO 13788 assesses this with the temperature factor at the internal surface:

Because the factor is normalized, it characterizes the detail, not the weather: you compute it once and compare it against the minimum value f_Rsi,min required for your climate and indoor humidity. National regulations set practical thresholds — for example fRsi ≥ 0.72 in the Polish technical conditions (WT), and a critical factor of 0.75 is commonly used for dwellings in the UK. Always check the requirement that applies to your project.

Step 1 — Model the Detail

Draw or import the 2D cross-section exactly as for a ψ-value calculation: full layer build-ups, cut-off planes at least 1 m from the disturbance (EN ISO 10211 model rules apply to the temperature calculation too), materials from the EN ISO 10456 library.

Step 2 — Boundary Conditions for a Condensation Check

• Internal surface: design indoor temperature (typically 20 °C). For surface condensation assessment EN ISO 13788 uses an increased surface resistance R_si = 0.25 m²·K/W (0.13 for glazing) to represent furniture, curtains and corners reducing heat transfer.
• External surface: design exterior temperature for your location (e.g. 0 °C, or the national design value), R_se = 0.04 m²·K/W.
• Cut-off planes: adiabatic.

Like Ψ, fRsi is independent of the chosen temperature pair in a linear steady-state model — but the surface resistances do change it, so use the EN ISO 13788 values, not the heat-loss values.

Step 3 — Solve and Read fRsi

Mesh and solve. ThermX computes fRsi automatically and marks the critical point — the coldest interior surface location — directly on the canvas, and the Results panel shows the value with a pass/fail indication. Hover the canvas to read surface temperatures anywhere else.

Step 4 — Interpret the Result

• fRsi ≥ required minimum: the detail passes for the assumed humidity class — record the value, the critical point location and the assumptions in the report (the Sheets tab exports a PDF).
• fRsi below the minimum: mould or condensation risk. Typical fixes: continuous insulation across the junction, a thermal break in penetrating elements, insulating the reveal, or replacing a highly conductive layer.
• Re-run the check after each design change — 2D solves in ThermX take seconds, so iterating on the detail is cheap.