Insulation R-Value Table
| Zone | Attic (uninsulated) | Attic (3-4 in) | Floor |
|---|---|---|---|
| 1 | R30 | R25 | R13 |
| 2 | R49 | R38 | R13 |
| 3 | R49 | R38 | R19 |
| 4A and 4B | R60 | R49 | R19 |
| 5, 6, and 4C | R60 | R49 | R30 |
| 7 and 8 | R60 | R49 | R38 |
How much insulation you need is not an opinion - it is a table. ENERGY STAR publishes the recommended retrofit R-values for existing wood-framed homes by IECC climate zone, based on the 2021 International Energy Conservation Code (Table R402.1.3): attics run from R-30 in Zone 1 up to R-60 across zones 4 through 8, floors from R-13 to R-38, and wall sheathing add-ons from R-5 to R-10 depending on zone. Pick your zone and what you are insulating above, and the tool reads the target straight off the table and translates any shortfall into inches of blown insulation.
The R-value math is unforgiving in both directions: doubling depth doubles resistance, so an attic sitting at R-19 in Zone 5 is less than a third of the way to the R-60 target no matter how thick it feels - and that gap is measured in winter dollars, since the same heat load keeps repeating every heating hour.
How to use
- Pick your climate zone (1-8) and where you are adding insulation - attic from scratch, attic topping up 3-4 existing inches, or floor over an unheated space.
- Enter your current R-value if known (the label on batts, or depth-inches times ~3.2 for blown fiberglass); the result shows the recommended R and the exact shortfall.
- Read the shortfall as inches: at roughly R-3.2 per inch of blown fiberglass, an R-22 gap is about 7 more inches across the ceiling.
Frequently asked questions
Which climate zone am I in?
The IECC map is county-level, so the honest answer is to check the county map on the code or ENERGY STAR sites. Rules of thumb that land close: Zone 1 is south Florida and Hawaii, Zone 2 the Gulf states, Zone 3 the mid-South, Zone 4 the mid-Atlantic and central states, Zone 5 the Chicago-to-Philadelphia band, Zone 6 the upper Midwest and northern New England, Zone 7 northern Minnesota and the Dakotas, Zone 8 Alaska's interior.
Why do zones 4 through 8 jump to R-60 attics?
Heating-dominated climates burn the insulation value every hour of a long winter, so the code's cost-optimal point moves up with heating degree days - the table gives zones 4A and 4B through 8 the same R-60 attic target while mild zones stop at R-30 to R-49. The 2021 IECC editions raised these targets from older R-38 advice, which is why many existing attics built to 1990s code are now measurably under-insulated.
I already have some insulation - do I top up or start over?
Top up, almost always. The table's middle column gives the target when your attic already has 3-4 inches, and blown or batt insulation adds to whatever resistance is already there - R-19 existing plus R-41 added equals R-60, no matter the mix. Only remove insulation if it is wet, moldy or vermin-infested; compressing existing fluffy insulation by walking on it is the real enemy.
Which insulation type gives the most R per inch?
Closed-cell spray foam leads at roughly R-6 to R-6.5 per inch, open-cell foam and blown cellulose sit near R-3.2 to R-3.8, and fiberglass batts or blown fiberglass run about R-2.2 to R-3.4 depending on density. Per-inch is not per-dollar: blown fiberglass or cellulose in an open attic costs a fraction of foam for the same total R, which is why the calculator above converts your gap using a typical blown-fiberglass inch.