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Insulation R-Value Chart
Quick answer: Fiberglass batt is about R-3.1 to R-3.4 per inch, cellulose R-3.2 to R-3.8, closed-cell foam R-6 to R-7. R-value adds by thickness, so a 3.5″ stud bay of standard batt gives roughly R-11 to R-13.
R-value per inch for every common insulation material, code minimums by climate zone, and the depth each material needs to reach R-30, R-49 or R-60.
Insulation R-value per inch
R-value measures resistance to heat flow. It is additive by thickness — two inches of a material rated R-3.5 per inch gives R-7 — and additive across layers, so a wall assembly’s total R is the sum of everything in it.
| Material | R per inch (low) | R per inch (high) | R in a 3.5″ bay | Notes |
|---|---|---|---|---|
| Fiberglass batt | 3.1 | 3.4 | 10.8 | Standard unfaced or kraft-faced batt |
| High-density fiberglass batt | 3.7 | 4.3 | 13.0 | R-15 in a 2x4 bay |
| Blown fiberglass (attic) | 2.2 | 2.9 | 7.7 | Settles; install to labelled depth |
| Blown cellulose | 3.2 | 3.8 | 11.2 | Dense-pack in walls runs higher |
| Mineral wool batt | 3.0 | 3.3 | 10.5 | Fire and sound performance |
| Open-cell spray foam | 3.5 | 3.6 | 12.2 | Vapour-permeable |
| Closed-cell spray foam | 6.0 | 7.0 | 21.0 | Also an air and vapour barrier |
| EPS rigid board | 3.6 | 4.2 | 12.6 | White beadboard |
| XPS rigid board | 4.7 | 5.0 | 16.4 | Pink/blue board; R drifts down with age |
| Polyiso rigid board | 5.6 | 6.5 | 19.6 | Derates in cold weather |
| Wood fiberboard | 2.5 | 2.8 | 8.8 | Sheathing |
The 3.5″ column is the depth of a 2x4 stud bay, using the low end of each range. Ranges reflect density and manufacturer variation — always use the figure on the product’s own label for a code submission.
Code minimums by climate zone
| Zone | Attic / ceiling | Wood-frame wall | Floor |
|---|---|---|---|
| 1 (hot — south Florida, Hawaii) | R-30 | R-13 | R-13 |
| 2 (Gulf Coast, south Texas) | R-49 | R-13 | R-13 |
| 3 (mid-South, coastal California) | R-49 | R-20 or R-13+5 | R-19 |
| 4 (mid-Atlantic, Pacific Northwest) | R-60 | R-20 or R-13+5 | R-19 |
| 5 (Midwest, New England) | R-60 | R-20 or R-13+5 | R-30 |
| 6 (northern Midwest, Maine) | R-60 | R-20+5 or R-13+10 | R-30 |
| 7–8 (Alaska, far north) | R-60 | R-20+5 or R-13+10 | R-38 |
Figures follow recent IECC residential provisions. A notation like R-13+5 means R-13 in the cavity plus R-5 of continuous exterior insulation. Your jurisdiction’s adopted code version governs.
Why continuous insulation appears in the code
Continuous exterior foam breaks that bridge because it covers the framing as well as the cavities. This is why codes increasingly specify R-13+5 rather than simply raising the cavity number: the extra five continuous does more than five extra in the bay would.
Depth needed to reach a target R
| Target | Fiberglass batt | Blown cellulose | Closed-cell foam | Polyiso board |
|---|---|---|---|---|
| R-13 | 3.5″ | 3.7″ | 2.2″ | 2.1″ |
| R-19 | 6.0″ | 5.4″ | 3.2″ | 3.1″ |
| R-30 | 9.5″ | 8.6″ | 5.0″ | 4.8″ |
| R-49 | 15.5″ | 14.0″ | 8.2″ | 7.8″ |
| R-60 | 19.0″ | 17.1″ | 10.0″ | 9.5″ |
Three things that quietly destroy R-value
- Compression. Stuffing an R-19 batt into a 3.5″ bay does not give R-19 — it gives roughly R-13, because R depends on the trapped air the fibres hold. Match batt to bay depth.
- Gaps. Insulation is a poor air barrier. A batt cut short at the top of a bay, or one squeezed around wiring instead of split behind it, loses far more than the missing area suggests.
- Moisture. Wet fiberglass loses most of its value and does not fully recover. Any insulation upgrade in a leaky assembly is money spent on the wrong problem.
Frequently asked questions
What is the R-value of fiberglass insulation per inch?
About R-3.1 to R-3.4 per inch for standard batt, so a 3.5 in stud bay gives roughly R-11 to R-13. High-density batt reaches R-3.7 to R-4.3 per inch, which is how R-15 fits in a 2x4 wall.
How thick does attic insulation need to be for R-49?
About 15.5 in of fiberglass batt, 14 in of blown cellulose, or 7.8 in of polyiso board. Blown fiberglass needs more depth than batt because it is less dense — install to the labelled depth, not by eye.
Is closed-cell spray foam worth the extra cost?
It depends what you need. At R-6 to R-7 per inch it is the only practical way to hit a high R in a shallow cavity, and it doubles as an air and vapour barrier. Where depth is not constrained, cellulose or batt reaches the same R for far less money.
What does R-13+5 mean?
R-13 in the stud cavity plus R-5 of continuous insulation on the exterior. The continuous layer covers the framing as well as the bays, which is what stops thermal bridging through the studs.