Wall R-Value Calculator

Whole-wall effective R-value with thermal bridging, continuous insulation and a code check.

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Whole-wall effective R

R-11.8 RSI 2.08

Labels add to R-13.0; framing costs 24% of the wall's R. U-factor 0.084.

Your wallZone 5 code: R-22.2 effective (R-20 + 5 ci, R-13 + 10 ci or R-0 + 15 ci)

Fails zone 5. Add 2 in of Polyiso to pass.

CladdingSheathingCavityDrywallOutsideInsideWood stud shown as the bridging path
Outside air filmR-0.2
Vinyl sidingR-0.6
OSB 7/16 inR-0.6
Fiberglass R-13R-13.0
Drywall 1/2 inR-0.5
Inside air filmR-0.7

280 sq ft of this wall loses about $55 a year in gas heat in zone 5.

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How many batts or bags? Insulation calculator. Payback on the upgrade: energy savings calculator. Hanging drywall after: drywall calculator.

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  • R-11.8 before
  • R-23.0 nominal
  • R-22.6 effective

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Project plan · built from your numbers

Insulate 280 sq ft of wall to effective R-22.6

Re-insulate 280 sq ft of 2x4 wall at 16" on center with R-13. Framing is 25% of the wall and costs 11% of the R-value. Today the wall is R-11.8 effective and misses the zone 5 code; 2.0 in of Polyiso brings it to R-22.6 and passes, saving about $26 a year in heating.

Materials
$1,173
Time
1 weekend
Level
Easy
7 steps 7 tools 6 materials
  1. 1

    Open and inspect the wall

    With drywall off, check for moisture stains, rot, or pests in the stud bays. Turn off any circuits in the wall before working near boxes.

    • Kill power to circuits in the wall
    • Look for wet or stained sheathing and fix leaks first
    • Pull old insulation and bag it
  2. 2

    Air seal the cavities

    Air leaks undo insulation. Seal every penetration and the joints where sheathing meets the plates before any batt goes in.

    • Caulk sheathing to top and bottom plates
    • Foam around wire and pipe holes
    • Seal window and door rough openings with low-expansion foam
  3. 3

    Cut and fit R-13 batts

    Cut batts 1/2" long so they press snugly into each of the 27 bays. Insulation only performs at its rating when it fills the cavity without gaps or compression.

    • Friction-fit full-height pieces
    • Fill short bays above and below windows
    • No gaps at the plates
01 · Your numbers
Before
R-11.8
Nominal
R-23.0
Effective
R-22.6
Code
Passes zone 5
02 · The plan

7 steps · 7 tools · 6 materials

4 more steps, the full tool list and a shopping list you can check off as you go.

03 · Finished

Your plan is 60% built. Save it.

3/5

Insulate 280 sq ft of wall to effective R-22.6

Finished in 1 weekend · $1,173 DIY

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Borrow these 7 tools from neighborsFull how-to guide: Insulate the attic

The parallel-path formula, worked

  1. List the layers. Air films, siding, foam, sheathing, cavity insulation, drywall. Add up everything except the cavity and studs.
  2. Build two paths. Cavity path = the other layers + the cavity insulation. Stud path = the other layers + the stud (R-1.25 per inch of wood).
  3. Weight by framing fraction. U = framing % / stud path + (1 - framing %) / cavity path. Use 25% at 16 in o.c., 22% at 24 in.
  4. Invert. Effective R = 1 / U. Compare U with the IECC maximum for your climate zone.

Worked example, 2x4 R-13 at 16 in with vinyl, OSB and drywall: the other layers add to R-2.5. Cavity path = R-15.5; stud path = R-2.5 + 3.5 x 1.25 = R-6.9. U = 0.25 / 6.90 + 0.75 / 15.53 = 0.084, so effective R = R-11.8. Without siding, sheathing and films the same wall is about R-10 to R-11.

What R-value insulation for walls? IECC 2021 by climate zone

Wood-frame walls, IECC 2021 Tables R402.1.2 and R402.1.3; ci = continuous insulation
ZonePrescriptiveMax U-factorEffective R equivalent
1R-13 or R-0 + 10 ci0.084R-11.9
2R-13 or R-0 + 10 ci0.084R-11.9
3R-20, R-13 + 5 ci or R-0 + 15 ci0.060R-16.7
4R-20 + 5 ci, R-13 + 10 ci or R-0 + 15 ci0.045R-22.2
5R-20 + 5 ci, R-13 + 10 ci or R-0 + 15 ci0.045R-22.2
6R-20 + 5 ci, R-13 + 10 ci or R-0 + 15 ci0.045R-22.2
7R-20 + 5 ci, R-13 + 10 ci or R-0 + 15 ci0.045R-22.2
8R-20 + 5 ci, R-13 + 10 ci or R-0 + 15 ci0.045R-22.2

R-value per inch of common materials

MaterialR per inchNotes
Fiberglass batt3.1 to 4.3R-13 / R-15 in 2x4, R-21 in 2x6
Dense-pack cellulose3.7Fills around wires and pipes
Open-cell spray foam3.7Air seals; not a vapor retarder
Closed-cell spray foam6.5Air and vapor barrier
EPS foam board4Cheapest rigid foam
XPS foam board5Ages to about R-4.5
Polyiso foam board6Derate to about R-5 below 40 F
Mineral wool board4.2Vapor open, fire resistant
Softwood stud1.25The bridge
Steel stud0.003Far worse bridge; use the correction factor

Steel stud correction factors

Effective cavity R as a share of the batt label, IECC / ASHRAE 90.1 method
Studs16 in o.c.24 in o.c.R-13 or R-21 becomes
2x4 (3.5 in)0.460.55R-13 to R-6.0 or R-7.2
2x6 (5.5 in)0.360.44R-21 to R-7.6 or R-9.2

Size the batts or bags with the insulation calculator, see payback with the energy savings calculator, and read the insulation R-value cheat sheet for attics and floors. Closing the wall up after? The drywall cheat sheet covers hanging and taping.

Wall R-value calculator FAQ

What R-value do I need for exterior walls?

IECC 2021: R-13 in zones 1 and 2; R-20 or R-13 + 5 continuous in zone 3; R-20 + 5 ci, R-13 + 10 ci or R-0 + 15 ci in zones 4 to 8. Check your local code, since many states adopt an older edition.

Is R-13 enough for 2x4 walls?

It meets code in zones 1 and 2. A 2x4 R-13 wall performs at about R-11.8 once the studs are counted, so colder zones need R-15 batts plus exterior foam.

What is the effective R of 2x4 vs 2x6 walls?

With vinyl, OSB and drywall at 16 in o.c.: 2x4 with R-13 is about R-11.8, 2x6 with R-21 about R-17.1. Going to 24 in spacing adds a little more.

Does drywall add R-value?

A little: 1/2 in drywall is R-0.45. The inside air film adds R-0.68, more than the drywall itself.

What is thermal bridging?

Heat taking the easy path through framing. Wood studs are about R-1.25 per inch against R-3.7 for the insulation beside them, so a quarter of the wall leaks faster. Steel is far worse.

How can I reduce thermal bridging?

Add continuous exterior foam or mineral wool, use 24 in advanced framing, insulated sheathing like ZIP-R, or a double-stud wall. Even 1 in of foam over the studs helps a lot.

What's the framing factor for typical walls?

About 25% at 16 in o.c. counting studs, plates, headers and cripples, 22% at 24 in, and 15 to 20% with advanced framing.

Does continuous exterior insulation help?

Yes. It adds its full R-value to the whole wall, covers the studs and keeps the sheathing warm, which cuts condensation risk. R-13 + 5 ci beats R-19 in the cavity alone.

How does stud spacing affect R-value?

24 in spacing has less wood per square foot, adding roughly 3 to 5% to effective R over 16 in.

Are steel studs worse for thermal bridging?

Much worse. A 2x4 steel wall with R-13 performs at about R-8.5 with sheathing and drywall. Steel framing needs continuous exterior insulation.

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