Overview
R-value measures thermal resistance: how well a material resists conductive heat flow. The higher the number, the more it slows heat moving through it. That is the whole concept. A material rated R-3 per inch resists heat half as well as one rated R-6 per inch, so you need twice the depth to hit the same target. Heat always moves from warm to cold, so in winter R-value slows heat leaving the house and in summer it slows heat coming in.
The number that matters in the field is additive across an assembly. Stack a layer of R-13 cavity insulation behind R-6 of continuous rigid foam and the assembly hits roughly R-19 before you account for the framing and sheathing. That additive math is how you build up to a code target with mixed materials.
The code that governs the minimums is the International Energy Conservation Code (IECC), and most jurisdictions adopt it. The IECC does not set one R-value for the whole house. It sets a minimum by climate zone (colder zones demand more) and by location in the building (attic, wall, floor, and crawlspace each carry their own number). Spec to the zone and the location, not to a single round figure you heard once.
R-Value per Inch by Material
Every material carries a published R-value per inch. That figure, times the installed depth, is your insulating value. Here is where the common materials land:
- Fiberglass batt: ~2.9-3.8 per inch
- Blown cellulose: ~3.2-3.8 per inch
- Mineral wool (rock wool): ~3.0-3.3 per inch
- Open-cell spray foam: ~3.5-3.7 per inch
- EPS rigid board: ~3.6-4.2 per inch
- XPS rigid board: ~5 per inch
- Polyiso rigid board: ~5.6-6.5 per inch
- Closed-cell spray foam: ~6-7 per inch
Closed-cell foam and polyiso pack the most R into the least thickness because their cells trap a low-conductivity gas instead of air. That density is why you reach for them when depth is tight, like a rim joist, a cathedral ceiling, or rigid foam on the exterior where every inch of wall thickness costs you.
Two cautions on the rigid foams. Polyiso loses R per inch as temperature drops below about 25 degrees, so its winter performance in a cold roof runs lower than the label. XPS settles toward roughly R-4.5 per inch over decades as its blowing agent off-gasses. Fiberglass, mineral wool, cellulose, and EPS hold their rated R indefinitely as long as they stay dry and uncompressed. The Department of Energy insulation reference covers the material tradeoffs in depth.
Required R-Value by Climate Zone
The IECC splits the country into eight climate zones. Zone 1 is the hot south (south Florida, Hawaii). Zone 8 is the coldest (interior Alaska). As you move toward colder zones, the required R climbs at every location in the building.
For ceilings and attics, the jump is sharp. Zones 1 through 3 require around R-30. Zones 4 and 5 require R-38. Zones 6 through 8 require R-49 under the 2021 IECC, and the 2024 update pushes the coldest zones to R-60. The attic carries the biggest number because heat rises and that plane sees the largest temperature swing.
Walls move less but still climb. Wood-frame walls run a minimum of R-13 in the warmer zones (1 through 4) and step up to R-20, or R-13 cavity plus R-5 continuous, in the colder zones. Continuous exterior insulation enters the equation in zone 4 and above because it breaks the thermal bridge through the studs.
Floors run R-13 in the warm zones and climb to R-30 or R-38 over unconditioned space up north. These are minimums. Pull the IECC table for your exact zone and location, and check whether your jurisdiction adopted the 2021 or 2024 edition, because the numbers shifted.
Where R-Value Matters Most
Not every plane in the building earns the same R. Spend where the heat moves, and the code targets reflect that priority.
The attic gets the highest number, full stop. Heat rises, and the ceiling plane sees the largest temperature difference between conditioned space and the outside, so it leaks the most. That is why the attic target runs R-49 to R-60 while walls sit at R-13 to R-20. Get the attic right before you obsess over anything else.
Walls come next. They carry less R than the attic because the temperature delta is smaller and the wall cavity limits depth, but they cover the largest surface area in the house, so they still matter.
Then come the floors over unconditioned space, the crawlspace, and the rim joist. The rim joist is the spot most crews skimp on, and it is a notorious leak: a thin band of framing wrapping the whole perimeter with almost nothing behind it.
Here is the part the R-value number alone will not tell you. Air sealing multiplies the value of every R you install. Insulation slows conductive heat flow, but air leaks carry heat straight through it by convection. An attic at R-49 with unsealed top plates and can lights performs like a fraction of its rating. Seal the leaks, then the R does its job.
Depth, Compression, and Effective R
Hitting a target R is arithmetic. Divide the target by the material's R per inch to get the depth you need. For R-49 in an attic with blown cellulose at roughly R-3.5 per inch, that is about 14 inches of settled depth. For the same R-49 in closed-cell foam at R-6.5, you need under 8 inches. The material sets the depth, and the depth sets whether it even fits the cavity.
Compression kills R-value, and it catches crews constantly. An R-19 fiberglass batt is rated at its full loft, around 6.25 inches. Stuff that same batt into a 5.5-inch 2x6 cavity and you compress it to roughly R-18. Cram an R-30 batt into a 2x8 floor cavity and you lose even more. The rule: the batt has to fill its rated depth without crushing.
The rated R also assumes a Grade I installation, the highest of three quality grades. Grade I means the insulation fills the cavity completely, with no gaps, voids, compression, or air paths around it. Grade III, with visible gaps and missed corners, can cut effective R by a quarter or more.
One more invisible loss: thermal bridging. Wood studs run about R-1 per inch, far below the insulation in the cavity. With studs every 16 inches, that framing drags the whole-wall R well below the cavity rating. Continuous exterior insulation is the fix because it covers the studs.
Common Mistakes
The same handful of mistakes show up on inspection after inspection.
Under-insulating below code. Someone hits R-30 in the attic because that is what they have always done, in a zone that now requires R-49. Pull the table for your zone before you order material, not after.
Compressing batts. Jamming an R-30 batt into a cavity sized for R-19 does not give you R-30. You lose loft, you lose R, and you paid for value you threw away. Match the batt to the cavity.
Gaps and voids. This is the Grade III install: batts cut short, stuffed around wiring instead of split, corners missed, plumbing left uncovered. Every gap is a hole in the thermal envelope, and a wall with scattered voids underperforms its rated R badly.
Ignoring thermal bridging. Cavity insulation alone leaves the studs as uninsulated highways for heat. In colder zones, continuous exterior insulation is not optional, it is in the code for a reason.
Skipping air sealing. The most common and most expensive miss. Insulation without air sealing is a screen door on a submarine. Seal top plates, penetrations, and the rim joist first.
Confusing R-value with cost. A high R per inch does not mean the best value for the job. That is a separate calculation, covered in our insulation prices guide.
FAQ
Is a higher R-value always better? Higher R always means more thermal resistance, so for a given thickness, higher is better at slowing heat. But more R than code requires hits diminishing returns, and at some point the cost outruns the energy saved. Spec to the code minimum for your zone, then go higher only where the payback or comfort justifies it.
Does R-value tell me the cost? No. R-value is purely thermal resistance. Two materials at the same R can cost very different amounts, and a high R per inch like closed-cell foam usually costs more per board foot. Cost is a separate decision, covered in our insulation prices guide.
Why does my insulated wall still feel cold? Usually air leaks or thermal bridging, not the R-value itself. Air moving through gaps carries heat past the insulation, and the wood studs conduct heat around the cavity fill. Air-seal the penetrations and add continuous exterior insulation to break the bridge.
What R-value do I actually need? It depends on your IECC climate zone and the location in the building. Attics run R-30 in the south up to R-49 or R-60 in the north, walls run R-13 to R-20-plus, and floors climb with the zone. Pull the IECC table for your exact zone.