R-Value Chart for Insulation: per Inch, by Zone, by Thickness
Higher R-value means slower heat flow. Fiberglass batts give about R-3.1 to R-4.3 per inch, closed-cell spray foam about R-5.5 to R-7. The 2021 IECC asks for R-49 to R-60 in attics. R-60 takes about 23 inches of blown fiberglass or 17 inches of cellulose.
What R-value means
R-value measures how strongly a layer resists heat flow. US labels use ft²·°F·h/BTU: an R-10 layer lets 0.1 BTU per hour through each square foot for every degree of temperature difference. Three rules cover most decisions:
- Layers add up. R-13 batts behind ½″ drywall (R-0.45) and 7/16″ OSB (R-0.62) give about R-14 through the cavity.
- U-factor is the inverse. U = 1 ÷ R. Windows are rated in U-factor: a U-0.30 window is roughly R-3.3, which is why windows lose far more heat than walls.
- Returns shrink. Heat flow is proportional to 1/R. Going from R-11 to R-30 in an attic cuts its heat loss by 63%. Going from R-30 to R-60 cuts the rest in half, but that saves less than a third as much heat as the first upgrade.
Metric RSI values convert with R (US) = RSI × 5.678, so an RSI-5 product is about R-28.
R-value per inch by insulation type
Values are per inch at typical installed density. The "we use" column is the value the calculator above and the thickness table below are based on. Foam board ratings drift down as blowing agents age, which is why aged polyiso is listed below its fresh-board rating.
| Material | R per inch | We use | In 2x4 (3.5″) | In 2x6 (5.5″) |
|---|---|---|---|---|
| Fiberglass batt | R-3.1 – 4.3 | R-3.2 | R-11.2 | R-17.6 |
| Blown-in fiberglass (loose-fill) | R-2.5 – 3.7 | R-2.6 | R-9.1 | R-14.3 |
| Blown-in cellulose (loose-fill) | R-3.0 – 3.8 | R-3.5 | R-12.3 | R-19.3 |
| Mineral wool batt (rock wool) | R-3.0 – 4.3 | R-4 | R-14.0 | R-22.0 |
| Open-cell spray foam | R-3.5 – 3.8 | R-3.6 | R-12.6 | R-19.8 |
| Closed-cell spray foam | R-5.5 – 7.0 | R-6.5 | R-22.8 | R-35.8 |
| EPS foam board (expanded polystyrene) | R-3.9 – 4.2 | R-4 | R-14.0 | R-22.0 |
| XPS foam board (extruded polystyrene) | R-5.0 – 5.4 | R-5 | R-17.5 | R-27.5 |
| Polyiso foam board (foil-faced, aged) | R-5.5 – 6.8 | R-6 | R-21.0 | R-33.0 |
How thick is R-30, R-38, R-49 or R-60?
This is the table to use with a tape measure in the attic. It shows the installed inches needed for each common target. For blown-in insulation, the bag's coverage chart gives the settled thickness for your exact product; use that number when it differs from ours.
| Material | R-13 | R-19 | R-30 | R-38 | R-49 | R-60 |
|---|---|---|---|---|---|---|
| Fiberglass batt | 4.1″ | 5.9″ | 9.4″ | 11.9″ | 15.3″ | 18.8″ |
| Blown-in fiberglass (loose-fill) | 5.0″ | 7.3″ | 11.5″ | 14.6″ | 18.8″ | 23.1″ |
| Blown-in cellulose (loose-fill) | 3.7″ | 5.4″ | 8.6″ | 10.9″ | 14.0″ | 17.1″ |
| Mineral wool batt (rock wool) | 3.3″ | 4.8″ | 7.5″ | 9.5″ | 12.3″ | 15.0″ |
| Open-cell spray foam | 3.6″ | 5.3″ | 8.3″ | 10.6″ | 13.6″ | 16.7″ |
| Closed-cell spray foam | 2.0″ | 2.9″ | 4.6″ | 5.8″ | 7.5″ | 9.2″ |
| Polyiso foam board (foil-faced, aged) | 2.2″ | 3.2″ | 5.0″ | 6.3″ | 8.2″ | 10.0″ |
Code minimum R-values by climate zone (2021 IECC)
For new construction and additions, the International Energy Conservation Code sets minimum insulation levels by climate zone. "13+5ci" means R-13 in the stud cavity plus R-5 of continuous foam sheathing. "0+15" means R-15 of continuous insulation alone. Your state or city may have adopted an older or newer edition, so check with the local building department.
| Zone | Ceiling | Wood frame wall | Floor | Basement wall | Crawl space wall |
|---|---|---|---|---|---|
| 0–1 | R-30 | 13 or 0+10 | R-13 | 0 | 0 |
| 2 | R-49 | 13 or 0+10 | R-13 | 0 | 0 |
| 3 | R-49 | 20 or 13+5ci or 0+15 | R-19 | 5ci or 13 | 5ci or 13 |
| 4 except Marine | R-60 | 20+5ci or 13+10ci or 0+15 | R-19 | 10ci or 13 | 10ci or 13 |
| 5 and Marine 4 | R-60 | 20+5ci or 13+10ci or 0+15 | R-30 | 15ci or 19 or 13+5ci | 15ci or 19 or 13+5ci |
| 6 | R-60 | 20+5ci or 13+10ci or 0+20 | R-30 | 15ci or 19 or 13+5ci | 15ci or 19 or 13+5ci |
| 7–8 | R-60 | 20+5ci or 13+10ci or 0+20 | R-38 | 15ci or 19 or 13+5ci | 15ci or 19 or 13+5ci |
What R-value for the attic? Upgrading an existing house
For existing homes, ENERGY STAR gives retrofit targets based on the same code table. The middle column is for attics that already have 3–4 inches of old insulation: you top them up rather than start over. The last column converts the uninsulated target into inches of blown fiberglass and cellulose using our per-inch values.
| Zone | Attic, uninsulated | Attic with 3–4″ existing | Floor | Inches: fiberglass / cellulose |
|---|---|---|---|---|
| 1 | R-30 | R-25 | R-13 | 11.5″ / 8.6″ |
| 2 | R-49 | R-38 | R-13 | 18.8″ / 14.0″ |
| 3 | R-49 | R-38 | R-19 | 18.8″ / 14.0″ |
| 4A, 4B | R-60 | R-49 | R-19 | 23.1″ / 17.1″ |
| 4C, 5, 6 | R-60 | R-49 | R-30 | 23.1″ / 17.1″ |
| 7, 8 | R-60 | R-49 | R-38 | 23.1″ / 17.1″ |
When re-siding a wall, ENERGY STAR suggests adding R-5 insulated sheathing in zone 3 and R-5 to R-10 in zones 4–8. That is the "ci" part of the code table.
Nominal vs. real R-value of a wall
The number on the batt is the R-value of the cavity only. Studs, plates and headers are wood at about R-1.25 per inch, and in a typical wall they cover about 25% of the area. Heat takes the easy path through them. The calculator above and the table below use the parallel-path method from the ASHRAE Handbook of Fundamentals:
Both paths include outside air film (R-0.17), siding (R-0.62), 7/16″ OSB (R-0.62), ½″ drywall (R-0.45) and inside air film (R-0.68). Foam sheathing is added to both paths because it covers the studs too. That is why R-5 of foam raises a wall more than R-5 of extra cavity fill.
| Wall | Nominal | Real whole-wall R | U × 100 |
|---|---|---|---|
| 2x4, R-13 batt | R-13 | R-11.8 | 8.4 |
| 2x4, R-15 high-density batt | R-15 | R-12.7 | 7.9 |
| 2x6, R-19 batt | R-19 | R-16.3 | 6.1 |
| 2x6, R-21 batt | R-21 | R-17.1 | 5.8 |
| 2x4, R-13 + R-5 foam sheathing | R-18 | R-17.4 | 5.7 |
| 2x6, R-20 + R-5 foam sheathing | R-25 | R-22.4 | 4.5 |
| 2x4, R-13 + R-10 foam sheathing | R-23 | R-22.7 | 4.4 |
Why installed R-value falls short
- Compression. An R-19 batt is made 6¼″ thick. Squeezed into a 5½″ 2x6 cavity it gives about R-18, and in a 3½″ cavity much less. Buy the batt made for the cavity depth.
- Gaps. A batt that leaves gaps at the edges, or is cut loose around wires, loses a share of its rating out of proportion to the gap size, because air moves through the open space.
- Settling. Loose-fill cellulose settles over time. Bag charts list both installed and settled thickness, and the settled one is what counts.
- Air leaks. Fiberglass does not stop air. Seal attic bypasses (top plates, can lights, the attic hatch) before you add insulation. Otherwise warm air carries heat straight past the new layer.
- Moisture. Wet insulation conducts heat. Fix roof leaks and vent bath fans outdoors, not into the attic.
To see what better insulation does to the size of your furnace or AC, set the envelope level in the BTU calculator or the AC size calculator.
Sources
- International Code Council, 2021 IECC, Table R402.1.3 (codes.iccsafe.org).
- ENERGY STAR, Recommended Home Insulation R-Values (energystar.gov).
- R-value per inch: DOE Buildings Energy Data Book, ICC-ES ER-2833 (cellulose), Energy Saving Trust CE71 (foam boards), via the R-value (insulation) reference table. The DOE Energy Saver insulation pages were offline when we checked on November 14, 2026.
- ASHRAE Handbook of Fundamentals, ch. 27 (parallel-path method and surface film resistances).
Questions people ask
What's a good R-value for a house?
Use the code minimum for your climate zone: R-30 (zone 1) to R-60 (zones 4–8) in the attic, and R-13 to R-20 plus foam sheathing in walls. In most existing homes the attic is the cheapest place to add R-value.
How many inches is R-49 blown-in insulation?
About 19 inches of blown fiberglass at R-2.6 per inch, or about 14 inches of cellulose at R-3.5 per inch. Check the coverage chart on the bag: it lists the minimum settled thickness for R-49 for that exact product.
Is R-30 or R-38 better?
R-38 is better: about 21% less heat flow than R-30. In zones 2 and up, the code minimum for new attics is R-49 to R-60, so both are below today's target for an open attic.
Which is better, R-13 or R-19?
R-19 resists heat better, but it is made for 2x6 walls and floors. In a 2x4 wall it gets compressed and loses much of its advantage. Use R-13 or R-15 batts in 2x4 walls and R-19 or R-21 in 2x6 walls.
Should I remove old attic insulation before adding new?
Usually not. ENERGY STAR gives targets for attics with 3–4 inches already in place, and you add on top. Remove it only if it is wet, moldy, contaminated by rodents, or if it is vermiculite, which may contain asbestos and should be tested first.
What is the R-value of 1/2 inch drywall?
About R-0.45. Drywall adds very little insulation; its value is as an air barrier when the seams and edges are sealed.
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