Continuous vs Cavity Insulation Ratio for Condensation Control

The required ratio is nothing but a dew point calculation solved once per climate zone at assumed indoor conditions and turned into a table a plans examiner can use -- it rises with climate severity because a colder outdoor design temperature pulls the sheathing colder for the same split. Entering the indoor conditions and the design temperature here reproduces the physics behind the table, which matters because a house run at a higher indoor humidity than the table assumed needs MORE continuous insulation than the table gives. Two things this makes visible that the R-value alone does not. The first is the counterintuitive direction: adding CAVITY insulation to a wall with fixed exterior insulation makes the assembly WORSE from a moisture standpoint even though it improves the R-value, because it lowers the ratio and the sheathing gets colder. A deeper wall packed with more cavity insulation is a common energy upgrade and a moisture downgrade, and it is exactly the change a builder makes without thinking of it as an assembly change. The second is the inversion a builder actually asks for: not what ratio this wall achieves, but how much exterior insulation a given cavity demands, or how much cavity a given thickness of foam will support. Both are reported, because the design usually starts from one of them rather than from the ratio. The vapor retarder is the alternative rather than the partner, and the two trade against each other. Keeping the sheathing warm with exterior insulation lets the assembly dry inward; a low-permeance interior vapor retarder instead stops the moisture reaching the sheathing but also stops the wall drying that way, and a wall with both can be unable to dry in either direction. Which is permitted in lieu of which is a code question rather than a physics one. This screens a ratio at design conditions: it does not perform a hygrothermal analysis, account for air leakage carrying moisture into the assembly (which dwarfs vapor diffusion and is what actually wets most walls), address the drying potential of the materials, evaluate rain control or the water-resistive barrier, or determine what the code requires in any jurisdiction. The energy code in force, a hygrothermal analysis where the assembly is unusual, and the designer of record govern.

achieved ratio = R continuous exterior / (R continuous + R cavity) against a required ratio from the code table; R_ci,min = required x R_cavity / (1 - required) and R_cavity,max = R_ci x (1 - required) / required. The sheathing sits at T_out + ratio x (T_in - T_out) because only the continuous R lies between it and outdoors, so the ratio the physics requires is (dew point - T_out) / (T_in - T_out).

The continuous-insulation ratio for condensation control as the energy code expresses it, with the REQUIRED ratio entered from the applicable code table rather than reproduced, because the table is jurisdictional and changes between editions. The dew point uses the Magnus approximation.

One ratio, two rearrangements, and one published dew-point approximation.

Estimate. AHJ and licensed professional govern.

Field names used by the API: r_cavity, r_continuous, required_ratio, indoor_temp_f, indoor_rh_pct, outdoor_design_temp_f, achieved_ratio, r_continuous_min, r_cavity_max, dew_point_f, ratio_from_dew_point, sheathing_temp_f

• The required ratio is entered from the applicable energy code tablethe adopted code
• Design conditions, not hygrothermal analysis no transient or seasonal moisture accountinga hygrothermal model
• Air leakage is not modelled it dwarfs vapor diffusion and wets most wallsthe air barrier design