The R-value printed on insulation is only part of the story. Wood studs let heat through several times faster than insulation, and they make up about a quarter of a typical wall, so the whole wall rates well below the insulation alone. The calculator uses the parallel-path method the California Energy Commission uses.
How to use this calculator
- Choose 2x4 or 2x6 studs and their spacing.
- Choose the insulation in the stud cavities.
- Choose the sheathing, any rigid foam over it and its thickness, and the outside finish.
Worked examples
A typical 2x4 wall
Studs: 2x4 · Stud spacing: 16 in on center · Cavity insulation: Fiberglass batts · Sheathing: OSB, 7/16 in · Rigid foam over the sheathing: None · Outside finish: Vinyl siding
- Whole-wall R-value
- 10.9 R
- Through the insulation
- 13.6 R
- Through the studs
- 6.8 R
- U-factor
- 0.09 Btu/h·ft²·°F
A 2x6 wall with cellulose
Studs: 2x6 · Stud spacing: 24 in on center · Cavity insulation: Blown cellulose · Sheathing: OSB, 7/16 in · Rigid foam over the sheathing: None · Outside finish: Vinyl siding
- Whole-wall R-value
- 17.3 R
- Through the insulation
- 22.8 R
- Through the studs
- 9.3 R
- U-factor
- 0.06 Btu/h·ft²·°F
With an inch of foam outside
Studs: 2x6 · Stud spacing: 16 in on center · Cavity insulation: Fiberglass batts · Sheathing: OSB, 7/16 in · Rigid foam over the sheathing: XPS · Foam thickness: 1 in · Outside finish: Vinyl siding
- Whole-wall R-value
- 21.1 R
- Through the insulation
- 25.0 R
- Through the studs
- 14.3 R
- U-factor
- 0.05 Btu/h·ft²·°F
Closed-cell spray foam
Studs: 2x4 · Stud spacing: 16 in on center · Cavity insulation: Closed-cell spray foam · Sheathing: Plywood, 1/2 in · Rigid foam over the sheathing: None · Outside finish: Brick veneer
- Whole-wall R-value
- 14.6 R
- Through the insulation
- 23.7 R
- Through the studs
- 6.7 R
- U-factor
- 0.07 Btu/h·ft²·°F
R-values of wall layers
- Inside air film 0.68, outside air film 0.17.
- Half-inch drywall 0.45; half-inch plywood 0.63; OSB 0.51.
- Softwood studs about 1.25 per inch.
- Rigid foam per inch: EPS 4, XPS 5, polyiso 6.5.
- Vinyl siding 0.61; brick veneer 0.44.
Why the whole wall rates lower
Framing makes up about 25 percent of a wall with studs on sixteen-inch centers and 22 percent at twenty-four, by the California Energy Commission's figures. Heat takes both paths at once, so the calculator adds the conductance of each path by its share of the wall: that is the parallel-path method.
Rigid foam over the sheathing is the best fix, because it covers the studs as well as the cavities. Efficient framing that cuts the framing share toward 20 percent helps too.
R-value and U-factor
U-factor is the inverse of the total R-value: the lower the U, the less heat gets through. Windows are rated by U-factor, from about 0.88 for an old single-pane window to 0.28 for a good low-e double-pane one.
Common questions
How do I calculate the R-value of a wall?
Add the R-values of every layer, air films included, for the path through the insulation and again for the path through the studs. Then combine the two by their share of the wall; the calculator does all three.
What is the R-value of a 2x4 wall?
With fiberglass batts, OSB and vinyl siding, the path through the insulation comes out in the mid-teens, but the whole wall, studs included, lands lower. Try the first example above.
What is the R-value of drywall?
About 0.45 for half-inch drywall, by the California Energy Commission's table.
Does rigid foam increase R-value?
Yes, by about 5 per inch of XPS or 6.5 of polyiso, and because it also covers the studs, it raises the whole-wall value more than the same R-value inside the cavities.