Wall Condensation Plane Temperature vs Dew Point
The hidden interface an IR reading of the room-side surface never catches.
Example
You enter
- R-value warm side to the plane 13.5
- R-value beyond the plane 4
- Indoor air temperature (°F) 70
- Outdoor air temperature (°F) 20
- Indoor relative humidity (%) 40
You get
- Condensation-plane temperature 31.4 F
- Indoor dew point 44.6 F
- Margin to dew point -13.1 F - CONDENSING
Details, formula, and sources
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.
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.
The R-proportional gradient and the Magnus dew-point approximation are public building-science results.
Estimate. AHJ and licensed professional govern.
Field names used by the API: r_inside, r_outside, t_in_f, t_out_f, rh_in_pct, t_plane_f, t_dew_f, margin_f
- Temperature gradient T_plane = T_in - (R_inside/R_total)(T_in - T_out), 1-D steady statebuilding science
- Dew point Magnus approximation of the indoor air's dew pointpsychrometrics
- Screen only no vapor diffusion or transient moisture; a Glaser/WUFI model is more completescope of this tile