★ REFERENCE TABLE · CONSTRUCTION · INSULATION
R-value is resistance to heat flow: higher is warmer in winter and cooler in summer. From R-11 to R-49 you need 38 more R, which is 11.9 inches of fiberglass batts at R-3.2 per inch. Find your material below, read across to your target, and use the calculator for any starting point.
Insulation adds up by R-value, not by inches alone: an inch of closed-cell foam buys twice the R of an inch of fiberglass. Enter what you have, pick your target, choose the material, and the calculator converts the R gap into inches. The area is shown for context when you price the job; it does not change the thickness.
Thickness in inches = R-value ÷ R per inch. R-values add: existing R plus added R equals total R. The per-inch figures used here are the standard nominal values for each material: fiberglass batts R-3.2, blown cellulose R-3.5, mineral wool batts R-3.3, open-cell spray foam R-3.6, closed-cell spray foam R-6.5, and rigid XPS foam R-5.0. Every cell in the chart below is that division, rounded to one decimal place.
| Material | R/in | R-13 | R-19 | R-30 | R-38 | R-49 | R-60 |
|---|---|---|---|---|---|---|---|
| Fiberglass batts | 3.2 | 4.1 | 5.9 | 9.4 | 11.9 | 15.3 | 18.8 |
| Blown cellulose | 3.5 | 3.7 | 5.4 | 8.6 | 10.9 | 14.0 | 17.1 |
| Mineral wool batts | 3.3 | 3.9 | 5.8 | 9.1 | 11.5 | 14.8 | 18.2 |
| Open-cell spray foam | 3.6 | 3.6 | 5.3 | 8.3 | 10.6 | 13.6 | 16.7 |
| Closed-cell spray foam | 6.5 | 2.0 | 2.9 | 4.6 | 5.8 | 7.5 | 9.2 |
| Rigid XPS foam | 5.0 | 2.6 | 3.8 | 6.0 | 7.6 | 9.8 | 12.0 |
| Climate zone | Attic target | What it means in practice |
|---|---|---|
| Zones 1–2 (hot: South Florida, South Texas, Hawaii) | R30–R49 | Heat defence first; the lower end suits mild winters, the upper end suits year-round cooling loads. |
| Zone 3 (warm: Atlanta, Dallas, Los Angeles) | R30–R60 | Balanced heating and cooling; most existing attics here sit well below target and gain the most from topping up. |
| Zone 4 (mixed: Washington DC, St. Louis, Seattle) | R38–R60 | Real winters arrive, so the floor rises to R-38; aim mid-range unless the attic is easy to fill. |
| Zones 5–8 (cold to very cold: Chicago, Minneapolis, Alaska) | R49–R60 | Deep insulation pays fastest here; R-49 is the floor, not the aspiration, in the coldest zones. |
Walls and floors, briefly. Cavity walls typically target R-13 to R-21 depending on zone and stud depth, and floors over unconditioned space target roughly R-13 in warm zones rising to R-30 in cold ones. Walls are depth-limited by the studs, which is why high-R-per-inch materials matter there; attics have room to stack, so cheaper per-inch materials win. Always check your local energy code, which can exceed these general ranges.
Setup: existing R-11, target R-49, fiberglass batts at R-3.2 per inch.
Line 1 — added R. 49 − 11 = R-38.
Line 2 — thickness. 38 ÷ 3.2 = 11.875, rounded to 11.9 in.
Target R-60. From R-11 to R-60 you must add R-49, which is 49 ÷ 3.2 = 15.3 in of fiberglass batts.
Cellulose instead. The same R-38 gap in blown cellulose is 38 ÷ 3.5 = 10.9 in — a full inch thinner than fiberglass for the same insulation.
Starting from R-30. From R-30 to R-49 you must add R-19, which is 19 ÷ 3.2 = 5.9 in of fiberglass batts.
| Output | Formula | What it means |
|---|---|---|
| Added R | Target R − current R | R-38 at the defaults — the insulation gap you must close. |
| Thickness to add | Added R ÷ R per inch | 11.9 in of fiberglass batts at the defaults — how deep to fill. |
| Chart reading | Target R ÷ R per inch | Read any material across to any target: R-49 in closed-cell foam is only 7.5 in, against 15.3 in of fiberglass. |
The division is exact for the nominal per-inch values shown, but real products vary: batts compress, blown insulation settles, and gaps, framing, and air leaks cut effective R below the labelled total. Treat the chart as a planning figure and round thickness up, never down. Zone targets are DOE-style general ranges, not a substitute for your local building or energy code. Error policy: empty, non-numeric, or negative inputs, a zero area, and a target at or below the current R are reported as errors rather than computed.
Bright Side Kit. "Insulation R-Value Chart & Calculator." https://brightoolbox.com/insulation-r-value-chart/ Accessed October 24, 2026.Resistance to heat flow. Higher R means slower heat transfer: warmer in winter, cooler in summer. R-values of layers add together, which is why topping up works.
15.3 inches. R-49 divided by R-3.2 per inch gives 15.3125, rounded to 15.3 inches. In closed-cell foam the same R-49 is only 7.5 inches.
Because materials differ in R per inch. Closed-cell foam at R-6.5 per inch buys twice the insulation per inch of fiberglass at R-3.2, so it reaches any target in roughly half the depth.
It depends on your climate zone. Zones 1–2 target R30–49, zone 3 targets R30–60, zone 4 targets R38–60, and zones 5–8 target R49–60. Your local code can require more.
No. Thickness depends on the R gap and the material. Area only matters when you price the job or count bags and boards.
Yes, R-values add across layers. Existing fiberglass plus a new cellulose top-up counts as the sum of both layers' R, as long as each layer is installed at its full labelled thickness.
Because compression and gaps cut real performance. Rounding down leaves you short of the target before settling is even counted; rounding up is cheap insurance.
Up to the zone target, yes; far beyond it, returns shrink. Each added inch saves less than the one before it, and air sealing often beats extra inches once you are at target.