Wall Condensation Plane Temperature vs Dew Point
The hidden interface an IR reading of the room-side surface never catches: temperature drops across a wall in proportion to R-value, so T_plane = T_in - (R_inside/R_total)(T_in - T_out), and condensation forms wherever that plane sits at or below the indoor dew point. R-13.5 to the sheathing behind R-4 of cladding on a 70/20 F day puts the sheathing at 31.4 F, 13 degrees below the 44.6 F dew point - a wetting plane; continuous exterior insulation warms it above the dew point, the whole point of the ratio rule. 1-D steady-state screen, no vapor diffusion. A building-science aid, not a hygrothermal analysis.
Formula and source
R_total = R_inside + R_outside; T_plane = T_in - (R_inside/R_total)(T_in - T_out); T_dew = Magnus(T_in, RH); margin = T_plane - T_dew (<= 0 condensing).
The R-proportional through-wall temperature gradient and the Magnus dew-point comparison for a condensation-plane screen, standard building-science results, by name.
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