R-value is the number that tells you how well insulation resists heat flow — higher means slower heat loss in winter and slower heat gain in summer — and the DOE recommendation changes by climate zone, location in the house, and whether the assembly is continuous or bridged by framing. An R-49 attic in Zone 5 is not the same job as an R-13 wall in Zone 3, and an R-23 batt stuffed into a 3.5-inch cavity does not magically become R-23. This guide owns the chart, the math, and the conditions that change the chart. For the broader material choice behind the number, see home insulation types; for what happens when the number is met but air still moves, see fiberglass vs spray foam; for the next step after you hit the target, the window U-factor guide translates the same physics to glass.
R-Value Explained So the Number Stops Being Abstract
R-value = resistance to conductive heat flow. U-factor = 1/R, used for windows and assemblies. R per inch is a material property (see table below); installed R is what the field assembly achieves after compression, gaps, and thermal bridging through studs. A 2×4 wood stud is about R-4.4, so a wall framed at 16 inches on center with R-13 batts is not R-13 whole-wall — it is closer to R-10 when the framing factor is included. That is why two walls with the same batt rating can differ by 30% on a blower-door test.
In US practice under the IRC and DOE, R-value targets are prescriptive minimums, not optimums. They assume the attic plane is air sealed, the wall is not wind-washed, and the vapor retarder matches the zone. Meet those three and the chart below is accurate. Skip them and adding R beyond the chart pays little.
| Material | R per inch | Typical installed R in common cavity |
|---|---|---|
| Fiberglass batt 3.5″ | 3.1 – 3.4 | R-13 in 2×4, R-15 high-density |
| Fiberglass batt 5.5″ | 3.1 – 3.4 | R-19 to R-21 in 2×6 |
| Blown cellulose | 3.2 – 3.8 | R-38 at ~10.5″, R-49 at ~13.5″ |
| Mineral wool batt 3.5″ | 3.3 – 4.2 | R-15 in 2×4, R-23 in 2×6 |
| Open-cell spray foam 5.5″ | 3.5 – 3.7 | R-20 in 2×6, air barrier included |
| Closed-cell spray foam 2″ + 3.5″ | 6.0 – 6.5 | R-12 at 2″, R-21 at 3.5″ |
| Polyiso rigid board | 5.5 – 6.5 | R-6.5 at 1″, R-13 at 2″ |
What R-Value You Need: DOE Chart by Climate Zone and Location
The DOE and IECC table is zone-based and location-specific. Use your IECC climate zone (check your county map — zones 1-2 are hot, 3-4 mixed, 5-6 cold, 7-8 very cold) and this prescriptive baseline:
| Location | Zones 1-3 | Zone 4 | Zones 5-6 | Zones 7-8 |
|---|---|---|---|---|
| Attic floor, vented | R-38 | R-49 | R-49 | R-60 |
| Attic roofline, conditioned | R-30 | R-38 | R-49 | R-49 |
| 2×4 wall cavity + continuous | R-13 | R-13 + R-5 | R-13 + R-10 | R-13 + R-15 |
| 2×6 wall cavity + continuous | R-20 | R-20 + R-5 | R-20 + R-5 | R-20 + R-10 |
| Floor over unconditioned space | R-13 | R-19 | R-30 | R-30 |
| Basement wall, interior | R-0 to R-5 | R-10 | R-15 | R-15 to R-19 |
| Slab edge | R-0 | R-10, 2 ft depth | R-10, 4 ft | R-15, 4 ft |
Two practical notes US inspectors enforce: a vented attic at R-49 is about 13 to 14 inches of blown cellulose or 14 to 15 inches of blown fiberglass — measure depth with a ruler at three points, don’t trust bag count. And a 2×4 wall at R-13 alone never meets code in zones 4-8 without continuous exterior insulation; the table’s R-13 + R-5 means batts plus rigid board outside the sheathing, not thicker batts inside.
Installed vs Labeled R-Value: Why Field Performance Drops
Four field conditions change the number you actually live with. Compression — an R-19 batt (6.25 inches) stuffed into a 3.5-inch bay performs near R-13, not R-19, because the air pockets that do the insulating are crushed. Gaps and voids — a 2% gap area (a 1-inch gap along one stud) cuts effective R by about 25% in that bay per Oak Ridge tests. Wind wash — air blowing through soffit vents across the top of the batt at the eave strips heat laterally; fix with baffles that keep the vent path above the insulation, not through it. Moisture — wet fiberglass or cellulose at 10% moisture content loses 30 to 40% of its resistance and grows mold; never cover a damp batt — dry the assembly first.
Thermal bridging is the fifth and largest effect in walls. Wood framing at 20 to 25% of the wall area creates a parallel heat path at R-4.4. The wall average is not the cavity R, it is (cavity R × cavity share) + (stud R × framing share). That is why continuous rigid foam matters: R-5 outside the studs lifts a nominal R-13 wall from about R-10 whole-wall to about R-15 effective — a larger real gain than jumping from R-13 to R-15 batts inside.
How Much R-Value Is Enough: Diminishing Returns and the Sealing-First Rule
The payoff curve flattens. Going from R-0 to R-13 in a wall cuts heat flow dramatically. Going from R-13 to R-23 cuts the remaining flow much less, and from R-23 to R-30 even less, while cost per R per square foot rises. That is why energy models in most US homes show air sealing to 2.5 to 3.0 ACH50, duct sealing when ducts are in the attic, and attic top-up to the zone target beat super-insulating walls beyond code unless walls are already open.
For budgeting: attic blow to R-49 in a 1,000 sq ft attic is about $1,200 to $1,800 installed and often saves 10 to 15% on heating and cooling; moving that same attic to R-60 adds about $300 to $500 and saves another 1 to 2%. Wall continuous foam R-5 on a 1,500 sq ft wall area is about $2,500 to $4,000 installed and saves both energy and comfort (fewer cold corners, less risk of interior condensation). Model the house with a blower-door number, not with R alone — two houses at R-49 can differ by 20% on the utility bill if one leaks twice the air of the other.
Measuring and Proving You Hit the Target
Before adding insulation, measure what exists. Probe three spots per attic bay with a ruler to verify depth and convert to R (cellulose ~ R-3.5/inch, fiberglass ~ R-2.5/inch blown). In walls, an infrared scan on a cold day shows missing fills as cold stripes around outlets and plates — that is faster than opening drywall. After the job, require the contractor to provide depth markers (ruler cards) in the attic, an invoice stating installed R by location, and, ideally, a blower-door or infrared spot check. Code inspectors check depth; comfort and bills check continuity.
If you are also deciding between spray foam and batts for how the R will be achieved, the comparison below maps R per inch to installed reality and cost. And if you are planning the job in sequence with other envelope work, the insulation cost page will help you price per square foot before you call for bids.
Frequently Asked Questions About R-Value
Is a higher R-value always better? Up to the zone target, yes. Beyond it, air sealing and continuity deliver more comfort per dollar than extra R in the same cavity.
Can I mix insulation types in one attic? Yes — air seal and keep the original dry layer, then blow a different material on top. Do not block soffit ventilation and keep baffles in place.
What is the R-value of a 2×6 wall with R-20 batts? About R-14 whole-wall without continuous foam, about R-19 with R-5 exterior foam. The stud bridging is the difference.
How many inches of cellulose is R-49? About 13 to 14 inches settled, depending on density and manufacturer chart — check the bag coverage chart for your density.
Do I need a vapor barrier with high R-value? Depends on zone and assembly. Closed-cell spray foam at 2 inches is already a Class II retarder; cellulose with latex paint is a Class III permeable retarder — neither needs polyethylene inside the drywall in most mixed climates.
