Feature | Surface Temperature Factor (fRSI) | Dew Point Calculation |
Primary Focus | Surface Condensation & Mould Risk | Interstitial Condensation Risk |
Location of Risk | Internal surfaces of a building junction. | Within the construction layers or voids. |
What it Measures | The thermal "strength" of a junction based on surface temperature. | The temperature (Tdp) where water vapor turns to liquid water. |
Key Variables | Internal/External air temperatures and minimum surface temperature (Tsi,min) | Vapor diffusion, relative humidity (RH), and material temperature profiles. |
Standard Threshold | Typically fRSI greater than or equal to 0.75 (for UK dwellings) to prevent mould. | Local material temperature must stay above the Dew Point. |
Requirement | Prevents mould growth (which starts at 75-80% RH). | Prevents liquid water buildup inside the wall assembly. |
AutoPSI Capability | Fully Supported: Models 2D heat flow to calculate psi-values and fRSI. | Not Supported: Requires separate hygrothermal or vapor diffusion software. |
AutoPSI assesses if your surface is cold enough for mould to grow. A full Dew Point analysis determines if a void is cold enough for water vapor to turn into liquid water.
If your project requires assessment of condensation risk within the layers of a detail (e.g., assessing a gap in a timber frame assembly), a separate hygrothermal or vapour diffusion calculation that includes relative humidity inputs is needed. AutoPSI is limited to fRSI (surface temperature) analysis and does not perform this separate Dew Point/Vapour Diffusion calculation.