Insulation R-Value Chart
The number that decides an insulation job is not R per inch on its own — it is R per inch multiplied by the depth you actually have. This chart pairs the two, so you can see immediately which targets are reachable in a 2×4 or 2×6 wall and which are not.
Generated from the same data the calculators use — a chart and a calculator on this site cannot disagree.
R-value per inch by material
| Material | R per inch | Fills cavities? | Notes |
|---|---|---|---|
| Fiberglass batt | R-3.2 | Yes | Typical R-3.1 to R-3.4 per inch |
| Blown fiberglass | R-2.5 | Yes | Typical R-2.2 to R-2.7 per inch, settles over time |
| Blown cellulose | R-3.5 | Yes | Typical R-3.2 to R-3.8 per inch |
| Mineral wool batt | R-4 | Yes | Typical R-3.8 to R-4.3 per inch |
| Open-cell spray foam | R-3.7 | Yes | Typical R-3.6 to R-3.8 per inch, air sealing included |
| Closed-cell spray foam | R-6.5 | Yes | Typical R-6.0 to R-7.0 per inch, also a vapour retarder |
| EPS rigid board | R-3.9 | Board only | Typical R-3.6 to R-4.2 per inch |
| XPS rigid board | R-5 | Board only | R-5 per inch, drops slightly over decades |
| Polyiso rigid board | R-6 | Board only | R-5.6 to R-6.5 per inch, loses R in cold weather |
Maximum R-value by cavity depth (cavity-fill materials)
| Cavity | Depth | Fiberglass batt | Blown fiberglass | Blown cellulose | Mineral wool batt | Open-cell spray foam | Closed-cell spray foam |
|---|---|---|---|---|---|---|---|
| 2×4 wall | 3.5" | R-11 | R-9 | R-12 | R-14 | R-13 | R-23 |
| 2×6 wall | 5.5" | R-18 | R-14 | R-19 | R-22 | R-20 | R-36 |
| 2×8 joist | 7.25" | R-23 | R-18 | R-25 | R-29 | R-27 | R-47 |
| 2×10 joist | 9.25" | R-30 | R-23 | R-32 | R-37 | R-34 | R-60 |
| 2×12 joist | 11.25" | R-36 | R-28 | R-39 | R-45 | R-42 | R-73 |
How to read this chart
A 2×6 wall cannot reach R-30 with fiberglass batt at standard density. Only closed-cell spray foam gets there in 5.5 inches, which is why high-R walls usually add continuous exterior insulation instead.
Manufactured batts are an exception to straight arithmetic: they are made to hit a rated R in a standard cavity by increasing density, so R-13 and R-15 exist for a 2×4 and R-21 for a 2×6. Use the label on the product.
Rigid board products — EPS, XPS and polyiso — are continuous exterior insulation, not cavity fill. Their advantage is covering the framing rather than sitting between it.
Whole-assembly R runs 10 to 20 percent below the cavity figure because wood framing conducts around the insulation. Studs are roughly R-1 per inch against R-3 or more for the fill, and framing occupies 15 to 25 percent of a typical wall.
Sources
- Published R-value per inch ranges by material
- US Department of Energy recommended insulation levels by climate zone
- Nominal lumber dimensions for cavity depths
R-value per inch varies by manufacturer, density and age. Use the figures printed on the product you are buying.
Frequently asked questions
What R-value do I need?
It depends on climate zone and assembly. The US Department of Energy publishes recommended levels by zone for attics, walls and floors, and local energy codes may require more.
Why is my wall’s real R lower than the insulation rating?
Thermal bridging through the studs. Framing conducts heat far better than insulation and makes up a fifth of a typical wall area, so the assembly performs below its cavity value.
Can I compress a thicker batt into a shallower cavity?
You can, but you lose R-value. An R-19 batt compressed into 3.5 inches delivers roughly R-13 to R-15 — better than an R-13 batt but well short of the label.
Why does polyiso lose R in cold weather?
Its blowing agent becomes less effective as temperature falls, so performance drops in exactly the conditions where insulation matters most. Cold-climate builders often derate it or choose XPS or mineral wool for exterior board.