How to use the R-value calculator
The R-value printed on a batt of insulation is not the R-value of the wall. Drywall, sheathing, cladding and the thin layers of still air on each face all add resistance, while studs and rafters let heat bypass the insulation. This calculator adds those parts up the way energy codes and building scientists do, so you can compare build-ups and see how close a design is to the code target.
- Pick the assembly, a wall or a roof or ceiling. That sets the inside air film and the code target.
- Add the layers from inside to outside. Choose a material, enter its thickness, and tick "in cavity" for insulation that sits between studs or rafters.
- Set the framing share. About 25% suits 2x4 or 2x6 walls at 16 inches on centre, 22% at 24 inches, and 10–15% for roof rafters or joists.
- Choose your IECC climate zone to compare the result with the residential U-factor limit.
R-value and U-value formulas
Layer R = thickness × R per inch (or a fixed R)
Cavity path R = air films + all layers
Framing path R = air films + other layers + cavity depth × R-1.25 per inch (wood)
Effective U = framing % ÷ R framing + (1 − framing %) ÷ R cavity
Effective R = 1 ÷ U · RSI = R × 0.1761 · U (W/m²K) = 1 ÷ RSI
This is the parallel path method from ASHRAE Fundamentals. It works well for wood framing. Steel studs conduct far more heat, so a steel-framed wall loses much more than this calculation shows and should be checked with the code's correction factors.
| Material | R per inch | RSI per 25 mm |
|---|---|---|
| Fibreglass batt | 3.2 | 0.56 |
| Mineral wool batt | 4.0 | 0.70 |
| Blown cellulose | 3.5 | 0.62 |
| Open-cell spray foam | 3.7 | 0.65 |
| Closed-cell spray foam | 6.5 | 1.14 |
| EPS / XPS / polyiso board | 4.0 / 5.0 / 6.0 | 0.70 / 0.88 / 1.06 |
| Softwood, plywood, OSB | 1.25 | 0.22 |
| Gypsum board | 0.9 | 0.16 |
| Brick / concrete | 0.2 / 0.08 | 0.035 / 0.014 |
Worked examples
A basic 2x4 wall
Inside air film 0.68, 1/2 inch drywall 0.45, 3.5 inches of fibreglass batt 11.2, 7/16 inch OSB 0.55, vinyl siding 0.61 and outside air film 0.17 add up to R-13.7 through the cavity. Through the studs, the 3.5 inches of wood give only R-4.4, so that path is R-6.8. With 25% framing the effective U is 0.25 ÷ 6.8 + 0.75 ÷ 13.7 = 0.091, or about R-11. The batt's label says R-11 to R-13, yet the whole wall barely matches it.
Adding an inch of polyiso outside
The same wall with 1 inch of polyiso over the OSB adds R-6 to both paths. The cavity path is R-19.7 and the stud path R-12.8, so the effective U drops to 0.058, about R-17.3. One inch of board added more than R-6 to the whole wall because it covers the studs too, while the batts lose a good part of their rating to the framing.
An attic in metric
A ceiling of 12.5 mm plasterboard with 300 mm of blown cellulose between and over 140 mm joists, entered as 140 mm in cavity and 160 mm continuous, comes to around RSI 7.1, or R-40.6, with 12% framing. That gives a U-value of about 0.14 W/m²K, or U-0.025 in imperial units. It meets the zone 3 limit of U-0.026 but falls just short of U-0.024 in zones 4 to 8, so about 25 mm more cellulose would get it there.
Who this calculator helps
- Architects and building designers comparing wall and roof build-ups before choosing a specification.
- Energy consultants and HERS raters making a quick check before a full model.
- Builders and insulation contractors explaining to clients why continuous insulation matters.
- Students learning how R-values, U-values and thermal bridging relate.
- Renovators and homeowners deciding how much insulation to add to an attic or wall.
Practical tips
- Use the R-value from the product data sheet when you know the product; generic values are averages.
- Glazing usually dominates heat loss; check how much of the facade is glass with the window-to-wall ratio calculator.
- Better insulation shrinks the cooling load; see the effect with the BTU calculator.
- Thicker walls change stud and rafter sizes; check spans with the joist and rafter span calculator.
- Keep the vapour control layer on the warm side in cold climates, and check the dew point for interior insulation.
Presenting an energy retrofit? Show the finished facade to the client with Exterior Redesign.
