Envelope U-Value
Add up the layers, add the air films, invert it: that is the U-value. The more useful output is the one underneath — the inside surface temperature. If it falls below the room’s dew point you get condensation, and mould starts growing a few degrees before that. This is why a thermal bridge causes damage that an average U-value never predicts.
Surface resistances are the ISO 6946 defaults — 0.13 m²·K/W inside, 0.04 outside. Leave a layer blank to skip it. This is a one-dimensional calculation: it does not see studs, ties, slab edges or window reveals, and those thermal bridges are exactly where condensation actually appears. The temperature factor fᵣₛᵢ is the standard mould-risk index — below about 0.75 an assembly is a problem in a temperate climate.
Design-stage estimates. A real project needs a full load calculation to the ASHRAE/CIBSE procedure, and ventilation rates are code-mandated — check the standard and jurisdiction that apply to your building. A helper, not a substitute for engineering judgment.