Assembly R-Value Calculator — nominal vs effective

See what a framed wall actually delivers once the studs bridge the insulation, using the parallel path method with a framing factor.

Assembly R-Value Calculator — nominal vs effective

Adding up the R-values on the labels gives you the number in the cavity, not the number the wall actually delivers. The studs are a thermal bridge, and this shows what they cost you.

Cavity insulation (R, per label)

Continuous exterior insulation (R)

Other layers, both paths (R)

Stud depth (in)

Framing factor (% of wall that is wood)

Include air films?

Heat takes the easy path. Wood conducts several times better than the insulation beside it, so the studs quietly hand back part of the R-value printed on the batt.

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Nominal (label math)	—
Path through cavity	—
Path through stud	—
Effective assembly R	—
Assembly U-factor	—
Lost to thermal bridging	—
Metric (RSI / U)	—

Method — parallel path (isothermal planes are not used here). U_assembly = f_cavity×U_cavity + f_framing×U_stud, then R_effective = 1 / U_assembly. Softwood framing is taken at R 1.25 per inch, so a 5-1/2" stud contributes about R 6.9 where the cavity beside it has R 21. Air films use the standard still-air interior 0.68 and moving-air exterior 0.17.

Framing factors are approximations for a whole wall including plates, corners and headers — a real wall varies. Continuous exterior insulation is the one layer the studs cannot bridge, which is why a small R on the outside often beats a large R in the cavity. Energy code compliance calculations have their own prescribed methods; use this to understand the assembly, not to submit it.