Jake Morrison, Licensed General Contractor · Last reviewed June 25, 2026

R-Value Calculator

Stack the insulation layers in your wall, attic, or floor and get the total R-value and U-factor instantly. Add each material by thickness, or drop in a known R-value, and the calculator sums them — then checks the assembly against the recommended level for your climate zone.

R-value calculator: stack insulation layers to get total R-value and U-factor

Layers add together
Add a layer to total it up

Total R-value

R-—

Add your first layer

U-factor (1 / R)

—

Lower is better

Stack the materials in your wall, attic, or floor — each layer's R-value adds up. Pick a state or zone above to check the total against the recommended level for your climate.

Method: Total R = sum of every layer's R-value (thickness × R-per-inch, or a known R). U-factor = 1 ÷ total R.

Estimates only. Layered R-values add for a center-of-cavity number; the whole-wall figure is lower because heat shortcuts through the framing (thermal bridging). R-per-inch varies by product, density, and install quality, and recommended levels vary by county. Air sealing matters as much as R-value. Check your local building code and the product spec sheet before buying.

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How to use this R-value calculator

  1. Add your layers. Hit "Add material layer" for each insulation, sheathing, or finish layer in the assembly and pick the material plus its thickness in inches. The calculator multiplies thickness by that material's R-per-inch and shows the layer's R-value on the right. Layers stack top to bottom — the order doesn't change the total, so just get them all in.
  2. Or drop in a known R-value. If a product is labeled directly — "R-19 batt," "R-13 cavity" — use "Add a known R-value" instead of computing it from thickness. Mix the two freely: a typed R-19 batt plus 1.5 inches of continuous polyiso is a common cold-climate wall.
  3. Add air films for a true U-factor. Check the air-films box to include the interior and exterior surface resistances (about R-0.85 combined). That gives you the air-to-air U-factor energy codes and window/wall comparisons actually use.
  4. Read the total. The result card shows the summed R-value and the U-factor (1 ÷ R) the instant any input changes — no Calculate button. Lower U-factor means less heat flows through.
  5. Compare against code. Pick your state (or set the climate zone directly) and what you're building. The calculator color-codes your total against the ENERGY STAR recommended band — below, meets, or exceeds — and tells you how much R you're short if you're under.
  6. Save or share it. The PDF button downloads a branded, dated layer-by-layer breakdown. The share button copies a link that re-builds your exact assembly — hand it to a contractor, an inspector, or save it for the supply run.

Why this R-value calculator is different

Most "R-value calculators" online are really just one box — thickness × R-per-inch for a single material — or a static chart you read sideways. A real wall, attic, or floor is a stack of layers, and the number that matters is the sum. This calculator is built for the assembly, not one slab of foam:

  • It adds up a real multi-layer assembly. Cavity insulation, continuous exterior foam, sheathing, drywall — type each layer once and watch the total climb. Single-material calculators can't tell you that R-13 batt plus 1.5 inches of polyiso is R-22, which is the whole point of a continuous-insulation wall.
  • It gives you the U-factor, not just R. Energy codes, window specs, and heat-loss math all speak in U-factor (U = 1 ÷ R). Most R-value calculators stop at R and leave you to flip it yourself. This one shows both, live.
  • Material mode and known-R mode in one stack. Compute a layer from thickness when you know the material, or type a labeled R-value when the product gives it to you directly. Mixing the two is exactly how real assemblies get specced.
  • Optional air films for a true assembly U-factor. The interior and exterior surface resistances are a real part of the air-to-air number. One checkbox adds them — most calculators ignore them entirely or bury them.
  • Climate-zone code check built in. The total isn't just a number — pick your state and the calculator tells you whether the assembly meets the recommended level for your zone, and how far short it is if not.
  • Embeddable and free. If you run a home-energy blog, a building-science resource, or a weatherization page, the iframe at the bottom hosts the same layered calculator with attribution.

How it works (the math behind the numbers)

R-value measures resistance to heat flow — higher is better. In a layered assembly, the resistances add in series, so the total is simply the sum of every layer. The U-factor is the reciprocal of that total. 

layer R     = thickness (inches) × R-per-inch (material)
              OR a known R-value typed directly
air films   = interior (≈0.68) + exterior (≈0.17)   // optional
Total R     = sum of all layer R-values (+ air films)
U-factor    = 1 ÷ Total R
Status      = Total R ≥ recommended max  →  exceeds
              Total R ≥ recommended min  →  meets
              else                       →  below

The R-per-inch design values come from the U.S. Department of Energy, the recommended totals by climate zone come from ENERGY STAR's wood-frame tables, and the interior/exterior surface resistances are the ASHRAE still-air (interior) and 15 mph (exterior) film values. A fixture file with ten input → output pairs runs as a build-time gate so the numbers can never drift from what the page displays. If you'd rather read the per-inch and by-zone numbers as static tables, the companion R-value insulation chart lays them out and lets you check a single material against your climate.

R-value per inch, by material

These are the design values the calculator uses for material-mode layers. Closed-cell foam and polyiso pack the most R into the least depth; blown fiberglass the least. Structural and finish layers (sheathing, drywall) are included because they're real, if small, contributors to a whole-assembly U-factor.

  • Closed-cell spray foam — R-6.5/in. Highest of the common insulations, plus an air and vapor seal.
  • Polyiso rigid board — R-6.0/in. Foil-faced; derates toward R-5 in deep cold, so a conservative design value is used.
  • XPS rigid board — R-5.0/in. The pink/blue board; moisture-resistant, common on foundations.
  • EPS rigid board — R-3.9/in. White bead board; lowest-cost rigid foam.
  • Open-cell spray foam — R-3.6/in. Softer, cheaper, still air-seals.
  • Blown cellulose — R-3.5/in. Best R-per-inch of the loose fills.
  • Mineral wool batt — R-3.3/in. Fire- and water-resistant, good sound barrier.
  • Fiberglass batt — R-3.2/in. The cheapest, most DIY-friendly cavity fill.
  • Blown fiberglass — R-2.5/in. Light loose fill that won't load a ceiling.
  • Plywood/OSB sheathing — R-1.25/in and drywall — R-0.9/in. Minor, but real in the U-factor.

Three real-world assemblies

A 2x6 cold-climate wall with continuous foam

Building a Zone 6 exterior wall: 5.5 inches of fiberglass batt in the 2x6 cavity (5.5 × 3.2 = R-17.6) plus 1.5 inches of continuous polyiso on the outside (1.5 × 6.0 = R-9.0). Stack them and you get R-26.6, comfortably past the R-21–25 a Zone 6 wall wants. The continuous foam is doing double duty here — it adds R and breaks the thermal bridge through the studs that the cavity number alone can't capture. Check the box for air films and the air-to-air U-factor lands around 0.04.

An attic top-up that crosses code

A Zone 5 attic with 10 inches of existing blown cellulose (R-35) is well short of the R-60 target. Add a second layer of 12 inches of blown fiberglass over the top (R-30) and the calculator sums to R-65 — over the line. This is exactly the layered situation single-material calculators choke on: the answer is the sum of the old layer and the new one, and you want to see both. For reading what's already up there before you top up, the R-value chart handles the single-layer "is mine enough?" check.

A finished basement wall

Finishing a Zone 4 basement: 2 inches of XPS against the foundation (R-10) plus an interior 2x4 wall with R-13 batt typed in directly as a known R-value. The stack totals R-23, past the R-13–15 a Zone 4 basement wall needs. A conditioned, well-insulated basement holds humidity, which is where a basement-rated dehumidifier earns its keep once the walls are sealed.

What affects how accurate this is

The arithmetic is exact; the real-world assembly is where variation creeps in:

  • Thermal bridging is the big one. Adding layer R-values gives you the center-of-cavity number. Wood studs are only about R-1.2 per inch, so heat shortcuts through the framing and the whole-wall R is lower — often 10–20% lower for a 2x4 wall on 16-inch centers. Continuous exterior insulation is what closes that gap, and it's why this calculator handles a separate continuous-foam layer.
  • Compression kills R-value. An R-19 batt only delivers R-19 at full loft. Stuff it into a shallower cavity and you lose a chunk. Enter the actual installed thickness, not the bag label.
  • Polyiso derates in the cold. Foil-faced polyiso is R-6/in at room temperature but drops toward R-5/in in deep cold. The calculator uses a conservative design value; for a Zone 6–7 roof or wall, treat the polyiso number as optimistic.
  • Air sealing comes first. R-value resists conductive heat flow, but a leaky assembly lets air bypass the insulation entirely. R-60 over a leaky ceiling underperforms R-38 over a sealed one. Seal, then insulate.
  • Air films are orientation-dependent. The 0.85 combined film value is for a vertical wall with moving outside air. A horizontal attic floor or a still-air interior shifts those numbers slightly. For a wall U-factor it's close; for precise heat-loss modeling, use direction-specific film values.

Insulation is also half of an indoor-air-quality equation: a tighter envelope means less fresh-air exchange. If you're sealing things up, it's worth reading Jake's notes on home air purifiers before you button up a tight house.

Frequently asked questions

How do you calculate the total R-value of a wall?

Add the R-value of every layer. R-values in series add directly, so a wall with R-13 cavity batt, R-9 of continuous foam, and an R-0.5 layer of drywall is R-22.5 total. The order of the layers doesn't change the sum. This calculator does that automatically — add each layer by material and thickness (or as a known R-value) and it keeps the running total. For the true air-to-air number, add the interior and exterior air films (about R-0.85 combined).

How do I convert R-value to U-factor?

U-factor is the reciprocal of R-value: U = 1 ÷ R. An R-20 assembly has a U-factor of 0.05; an R-50 attic is U-0.02. Lower U-factor means less heat flows through, so for U-factor lower is better while for R-value higher is better — they're two ways of saying the same thing. The calculator shows both at once, recomputing the U-factor every time you add or change a layer.

Do R-values add up when you layer insulation?

Yes — for layers stacked in series (one behind the next), R-values add directly. Two inches of XPS (R-10) over an R-13 batt is R-23. The exception is parallel paths, like insulation between studs versus the studs themselves — those don't simply add, which is the thermal-bridging effect that makes a whole-wall R lower than the center-of-cavity sum. This calculator gives you the center-of-cavity total; subtract roughly 10–20% for a framed wall to estimate the whole-wall figure.

What R-value do I need for my wall, attic, or floor?

It depends on your climate zone and the assembly. ENERGY STAR recommends roughly R-13 to R-21 for walls, R-25 to R-30 for floors, and R-30 to R-60 for attics, with cold zones at the high end. Pick your state in the calculator and it shows the recommended band for whichever assembly you select, then color-codes your total against it. The companion R-value chart has the full by-zone recommendation table if you want to read it directly.

Can I embed this R-value calculator on my site?

Yes — copy the iframe snippet at the bottom of this page. The embedded version is a streamlined variant built for home-energy blogs, weatherization and energy-assistance nonprofits, building-science resource pages, real-estate and home-inspection sites, and DIY forums. Required attribution is built into the snippet. Free, no signup, no analytics attached to the embed.

Whether you're building up a wall assembly or topping up an attic, here's the kit Jake reaches for on every insulation job — and the efficiency upgrades that pay off once the envelope is tight:

  • Best Hand Tool Sets — a sharp utility knife, a good tape, and a straightedge are what cut batts and foam board clean and measure cavity depth accurately.
  • Best LED Shop Lights — attics and crawlspaces are pitch dark. A bright, portable LED makes it safe to see joists, wiring, and gaps while you work overhead.
  • Best Shop Vacs — for clearing old, settled blown-in and construction debris before the new layer goes down.
  • Best Smart Thermostats — insulation holds the heat; a learning thermostat schedules it. Together they're the single biggest lever on a heating and cooling bill.
  • Best Dehumidifiers — a tighter, well-insulated home traps moisture. A dehumidifier keeps a sealed basement or conditioned crawlspace dry and mold-free.
  • Best Space Heaters — for the one stubborn room that stays cold while you finish insulating, a good spot heater bridges the gap.

Sources & methodology

The R-per-inch and recommended-by-zone values are reviewed annually against the DOE and ENERGY STAR sources and the prevailing IECC code cycle. About Jake · Last reviewed June 25, 2026.

Embed this calculator on your site

Free for home-energy and weatherization blogs, energy-assistance and housing nonprofits, building-science resource pages, home-inspection and real-estate sites, and DIY forums. Required attribution is included in the snippet. No fee, no account, no analytics attached to the embed. 

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  R-value calculator by
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  &middot; Reviewed by Jake Morrison, Licensed GC
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