R-Value Calculator

Build a wall, ceiling or floor from layers and this R value calculator returns the nominal R, the whole-assembly R after framing, and the IECC minimum for your zone.

By CalcBuilt Editorial TeamUpdated September 9, 2026Formulas checked against manufacturer specs, see how we calculate.

Quick answer: To find a real R-value, add the layers along the cavity and along the framing, then area-weight the two paths in U-factor. A 2x4 wall at 16 in on center with an R-13 batt, OSB sheathing and vinyl siding is R-15.8 at the cavity. Whole wall it is R-12, a U-factor of 0.084.

Your measurements

One wall: area is wall length times height, minus each door and window.

AssemblySets the framing depth, the framing area fraction and the two air films. Walls use a 0.68 inside film and a 0.17 outside film, a ceiling uses 0.61 on both faces and a floor 0.92.Cavity insulationPer-inch values from manufacturer data. An R-13 batt is 3.5 in of the 3.8 per inch grade, and an R-15 high density batt runs about 4.3 per inch.Insulation thicknessinIn a wall this is the stud depth, 3.5 in or 5.5 in. In an attic it is the blown depth, and anything deeper than the 9.25 in joist buries the framing, which is why attics bridge far less than walls.SheathingStructural panel behind the siding. Sheathing adds under R-1, so it never rescues an assembly, but the code tables still count it.Continuous exterior insulationFoam or mineral board outside the framing. This is the only layer that covers the studs, so it raises the cavity path and the framing path by its full R-value.Continuous insulation thicknessinFoam over 1 in usually means furring strips, longer fasteners and window bucks, so price the trim work before choosing 2 in.CladdingChoose None for a ceiling or floor. Brick veneer earns most of its R-1.44 from the 1 in air space behind it, not from the brick.IECC climate zoneThe zone is assigned by county in IECC Figure R301.1. Your building department can confirm it, and several states amend the table.

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Results

Whole-assembly R-value: R-12; Framing covers 25% of the area, so the assembly U-factor is 0.084. This is the figure that belongs in a heat loss calculation
Nominal R-value at the cavity: R-15.8; 3.5 in of insulation at R-3.8 per inch, plus 1/2 in gypsum board, 7/16 in OSB, vinyl siding and the two air films, worth R-2.53 together
Through the framing: R-6.9; A 2x4 stud is R-4.4 at R-1.25 per inch. That path is the thermal bridge
Insulation only: R-13.3; All of it inside the framing. Continuous insulation is what the code table means by the number after a plus sign
2021 IECC zone 5: R-30 or U-0.045; Warning: R-13.3 of insulation is under the R-30, R-20 plus R-5 continuous, or R-13 plus R-10 continuous path and U-0.084 is over the U-0.045 limit, so this assembly fails both 2021 IECC paths

R-Value formula

Cavity path R = air films + finish + insulation thickness x R per inch + sheathing + continuous insulation + cladding
Framing path R = the same layers, with framing depth x 1.25 in place of the insulation
Assembly U = framing fraction / framing path R + (1 - framing fraction) / cavity path R
Whole-assembly R = 1 / Assembly U

Worked example

A 2x6 wall with studs 24 in on center, in climate zone 6. It has R-23 mineral wool in the 5.5 in cavity, 1 in of polyiso outside the OSB, and vinyl siding. The cavity insulation is 5.5 x 4.2 = R-23.1 and the polyiso is R-6.0. The layers every path shares add up to 0.68 + 0.45 + 0.62 + 6.0 + 0.61 + 0.17 = R-8.53.

The cavity path is 8.53 + 23.1 = R-31.63. The framing path swaps the wool for a 5.5 in stud at R-6.88, so it is R-15.41. Framing is 22 percent of that wall, so U = 0.22 / 15.41 + 0.78 / 31.63 = 0.0389, which inverts to R-25.7 whole wall. Zone 6 asks for R-30 of insulation on the R-value path, so this wall misses that. It still complies, because 0.0389 sits inside the U-0.045 limit in Table R402.1.2.

Quick reference

Nominal and whole-wall R-value, all with 1/2 in gypsum, OSB, vinyl siding and air films worth R-2.53
Wall assembly	Insulation	Nominal R	Whole-wall R	U-factor
2x4 at 16 in, R-13 batt	R-13.3	R-15.8	R-12.0	0.084
2x4 at 16 in, R-15 batt	R-15.1	R-17.6	R-12.7	0.079
2x4 at 16 in, 3.5 in closed cell foam	R-22.8	R-25.3	R-15.2	0.066
2x4 at 16 in, R-13 plus 1 in polyiso	R-19.3	R-21.8	R-18.6	0.054
2x6 at 16 in, R-21 batt	R-20.9	R-23.4	R-17.1	0.059
2x6 at 24 in, R-21 batt	R-20.9	R-23.4	R-17.6	0.057
2x6 at 24 in, R-23 mineral wool	R-23.1	R-25.6	R-18.6	0.054
2x6 at 16 in, R-21 plus 2 in XPS	R-30.9	R-33.4	R-28.3	0.035

Nominal R-value against whole-assembly R-value

Adding layer R-values gives the nominal, center of cavity number, and it is only true in the middle of a stud bay. Studs, plates, headers and corners run from inside to outside with almost no insulation in them, and wood is only R-1.25 per inch. A 2x4 stud is R-4.4 where the batt beside it is R-13.

To combine the two paths, work in U-factors, because heat flow adds and R-value does not. Take the framing fraction divided by the framing path R, add the remaining fraction divided by the cavity path R, and invert the answer. At 25 percent framing, a nominal R-15.8 wall lands near R-12. That gap is thermal bridging, and it is why a code table asks for R-30 in zone 5 rather than R-20.

Continuous insulation is the layer that fixes bridging

Insulation outside the studs counts in both paths, so it raises the framing path by its full value. On a 2x4 wall, 1 in of polyiso takes the whole-wall figure from about R-12 to about R-18.6, which is more than doubling the cavity insulation achieves. That is why the IECC writes wall requirements as R-20 plus R-5 continuous rather than one larger cavity number.

Foam outside changes the detailing. Siding fasteners have to reach 1-1/4 in into framing through the foam, and windows need bucks or extension jambs. The water resistive barrier and flashing order also has to be settled before the foam goes up. IRC Table R702.7.1 sets the continuous R-value that keeps sheathing warm enough to skip an interior vapor retarder in zones 4 and up. Thickness is a moisture decision as much as an energy one.

What this calculator leaves out

The result covers one clear field of an assembly. It does not model headers, corners, band joists, wall intersections or the framing around openings. Those push a real framing fraction past 27 percent on a house with a lot of glass. It ignores installation quality, and a batt cut short or stuffed behind wiring performs below its label. It also leaves out air leakage, which the IECC tests separately at 3 ACH50 in zones 3 to 8, and windows and doors, which are rated by U-factor instead. The foam values here are aged R-values at 75 F; polyiso in particular loses some R-value in deep cold.

Code limits that apply

IECC Table R402.1.3checked
Wood frame wall R-30 in zones 4 to 8, R-20 in zone 3, R-13 in zones 1 and 2
If you miss it: Plan review rejects the wall section, and the framing has to be furred out or foamed before insulation
IECC Table R402.1.3
Ceiling R-60 in zones 4 to 8, R-49 in zones 2 and 3, R-30 in zone 1
If you miss it: An attic blown to the older R-38 depth fails the insulation inspection and has to be topped up
IECC Table R402.1.2checked
Assembly U-factor path: frame wall U-0.045 in zones 4 to 8, ceiling U-0.024
If you miss it: Missing the R-value path and the U-factor path both leaves the job with no energy compliance certificate
IECC R402.1.3
Insulation squeezed into a cavity thinner than its label must still deliver the table R-value
If you miss it: An R-19 batt jammed into a 2x6 bay counts as about R-18 and comes up short at inspection
IECC R402.2.1
R-49 may stand in for R-60 where it runs over the wall top plate at its full height
If you miss it: Insulation crushed at the eave loses R-value exactly where the ceiling is coldest
IRC Table R702.7.1
Minimum continuous insulation R-value that lets a wall skip the interior vapor retarder
If you miss it: Foam too thin for the zone puts the dew point in the sheathing and it stays wet through winter

Provision	Limit	If you miss it
IECC Table R402.1.3checked	Wood frame wall R-30 in zones 4 to 8, R-20 in zone 3, R-13 in zones 1 and 2	Plan review rejects the wall section, and the framing has to be furred out or foamed before insulation
IECC Table R402.1.3	Ceiling R-60 in zones 4 to 8, R-49 in zones 2 and 3, R-30 in zone 1	An attic blown to the older R-38 depth fails the insulation inspection and has to be topped up
IECC Table R402.1.2checked	Assembly U-factor path: frame wall U-0.045 in zones 4 to 8, ceiling U-0.024	Missing the R-value path and the U-factor path both leaves the job with no energy compliance certificate
IECC R402.1.3	Insulation squeezed into a cavity thinner than its label must still deliver the table R-value	An R-19 batt jammed into a 2x6 bay counts as about R-18 and comes up short at inspection
IECC R402.2.1	R-49 may stand in for R-60 where it runs over the wall top plate at its full height	Insulation crushed at the eave loses R-value exactly where the ceiling is coldest
IRC Table R702.7.1	Minimum continuous insulation R-value that lets a wall skip the interior vapor retarder	Foam too thin for the zone puts the dew point in the sheathing and it stays wet through winter

Rows marked checked produce a warning in the results when an entry crosses the limit. The others are on the plans or on site and this calculator does not test them. Local amendments can be stricter; the adopted edition and the inspector govern.

Mistakes that cost money

Quoting the batt label as the wall R-value. An R-13 wall is about R-12 after framing, and less again at corners and headers.
Counting continuous foam only in the cavity path. Foam outside the studs raises both paths, which is why R-5 of it beats R-5 of batt.
Stuffing a 6.25 in R-19 batt into a 5.5 in bay. It performs at about R-18, and the R-21 batt made for 2x6 costs little more.
Foam thicker than 1 in with standard fasteners. Siding nails still have to reach 1-1/4 in into the stud through the foam and furring.
Foam too thin for the climate. IRC Table R702.7.1 sets the continuous R that keeps the sheathing above the dew point in zones 4 and up.
Ignoring headers, corners and the band joist. Real framing fractions run 27 percent or more on a house with many openings.

Key facts

Wood framing covers about 25 percent of a 2x4 wall at 16 in on center, and about 22 percent of a 2x6 wall at 24 in on center.
Softwood framing runs about R-1.25 per inch, so a 2x4 stud is R-4.4, a 2x6 stud is R-6.9 and a 2x10 joist is R-11.6.
A 2x4 wall with an R-13 batt, OSB sheathing and vinyl siding is R-15.8 at the cavity but about R-12 once the studs are counted.
The 2021 IECC Table R402.1.3 calls for R-30 wood frame walls in climate zones 4 through 8, R-20 in zone 3 and R-13 in zones 1 and 2.
The 2021 IECC ceiling minimum is R-60 in climate zones 4 through 8, R-49 in zones 2 and 3, and R-30 in zone 1.

Frequently asked questions

What is the R-value of a 2x4 wall?

With an R-13 batt, OSB sheathing, vinyl siding and 1/2 in drywall, the cavity path is R-15.8 and the whole wall is about R-12. Studs at 16 in on center cover roughly a quarter of the area and are only R-4.4.

How do I calculate the R-value of a wall with several layers?

Add the R-values of every layer along the cavity, then add them again along a stud with wood at R-1.25 per inch in place of the insulation. Combine them with U = 0.25 / framing R + 0.75 / cavity R and invert the result.

Does framing really cut R-value that much?

Yes. At 25 percent framing, a nominal R-15.8 wall performs at about R-12, a loss near 24 percent. Attic ceilings lose far less, because blown insulation deeper than the joists buries the framing.

Is continuous foam better than a thicker batt?

Per R-value, yes, because it covers the studs. On a 2x4 wall, 1 in of polyiso outside takes the whole wall from about R-12 to R-18.6. Filling the same bay with closed cell foam instead only reaches about R-15.2.

Sources and references

Chapter 4 [RE] Residential Energy Efficiency, Table R402.1.2 assembly U-factors and Table R402.1.3 R-values, 2021 IECC (ICC)
Recommended Home Insulation R-Values by climate zone (ENERGY STAR)
FOAMULAR NGX extruded polystyrene insulation product data (Owens Corning)
COMFORTBATT stone wool batt insulation product data (Rockwool)

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