Wall Assembly R-Value
The whole-assembly R-value of a framed wall by the parallel-path method.
Example
You enter
- Cavity insulation R 13
- Stud depth (in) 3.5
- Framing factor (0-1) 0.25
- Air films R 0.85
- Finish layers R 1.05
You get
- R assembly 11.1
- R center 14.9
Checked against published worked examples from: Continuous Insulation Council (Timothy Ahrenholz).
Details, formula, and sources
ScopeWhere the studs short-circuit the cavity insulation -- the wall performs below its center-of-cavity R. Reports both.
Formula or tableU_assembly = framing factor x 1/R_framing-path + (1 - factor) x 1/R_cavity-path; R = 1/U; framing path adds stud depth x 1.25 R/in.
Source and editionASHRAE Handbook of Fundamentals parallel-path (isothermal-planes) method (by name).
Free accessASHRAE Fundamentals licensed; air-film and material R-values widely published free.
What governsEstimate. AHJ and licensed professional govern.
API field namescavity_r, stud_depth_in, framing_factor, air_films_r, finish_layers_r, r_assembly, r_center
- Framing factor ~0.25 at 16 in o.c., 0.22 at 24 in o.c.; softwood ~1.25 R/in (editable)ASHRAE / DOE