Insulation R-Value Calculator

Build up a real wall, attic, floor, basement wall, or crawlspace wall from its actual material layers and see its total R-value — the same layer-by-layer approach used by builders and energy auditors. Add air film resistance for a more complete total, turn on structural framing to see an effective whole-assembly R-value that accounts for thermal bridging through studs, and compare your result against the IECC code minimum for your climate zone. If you're upgrading existing insulation, enter your current R-value to see exactly how much more you need.

Choose your climate zone and assembly type, add your material layers, then calculate. Formula: R (per layer) = Thickness × R-per-inch, summed across all layers + air films if included.

Climate Zone & Assembly

Your climate zone sets the IECC code-minimum R-value this calculator compares your result against.

IECC Climate Zone

Assembly Type

Material Layers

Not sure what a field means? Tap the ⓘ next to its label. Add every layer in your assembly — insulation, sheathing, drywall — for the most accurate total.

Units

R-value-per-inch figures are standard published values (ASHRAE / U.S. DOE) for each material type and can vary by manufacturer, density, and installation quality. Select Custom to enter your own known value.

Air Films, Framing & Current R-Value

All three are optional — turn on what applies to your situation.

Include Air Films

Include Structural Framing

Current R-Value — optional, imperial R

Your Results

Total R-Value

—

°F·ft²·hr/BTU

Effective R-Value

—

with framing

U-Value

—

1 ÷ R

Code Minimum

—

for your zone

Compliance

—

vs. code minimum

Additional R Needed

—

to reach code minimum

Formula: —

Layer Contribution to Total R-Value

Each bar segment shows how much one material layer contributes to your total R-value.

Material	Thickness	R-Value
Total (before air films / framing)		—

Results are for reference and estimation only. Actual thermal performance varies with installation quality, moisture, compression, and material variability. See full disclaimer below.

How to Use the Insulation R-Value Calculator

Select Your Climate Zone and Assembly Type

Choose your IECC climate zone (1 through 8) and the assembly you're insulating — wall, attic/ceiling, floor, basement wall, or crawlspace wall.

Add Your Insulation Layers

Add one or more material layers, choosing a preset material and entering its thickness, or select Custom to enter your own R-value per inch.

Choose Optional Settings

Toggle on air film resistance for a more complete total, enable structural framing to see an effective whole-assembly R-value, and optionally enter your current R-value to see how much more insulation is needed.

Review Your Results

Click Calculate to see your total R-value, effective R-value if framing is included, U-value, and whether your assembly meets the code minimum for your climate zone. Use Print to capture the results as they appear on screen.

Related Tools

Want to know what your energy costs, not just your R-value? Try the Appliance Energy Cost Calculator.

Frequently Asked Questions

What is R-value?

R-value stands for thermal resistance value — a measure of how well a material resists the flow of heat. The higher the R-value, the better a material insulates. It's expressed in °F·ft²·hr/BTU (or m²·K/W, called RSI, in metric units) per inch of thickness, and R-values are additive: two layers of R-13 insulation stacked together give a combined R-26.

How is R-value calculated for a multi-layer wall or ceiling?

Each layer's R-value is its thickness multiplied by its material's R-value per inch. The total R-value of the assembly is the sum of every layer's R-value, plus the still-air film resistance on the interior and exterior surfaces if you include it. This calculator adds each layer as you enter it, so you can build up a real-world wall, attic, or floor assembly one material at a time.

What does "Effective R-Value" mean, and why is it lower than my insulation's R-value?

Wood and steel framing conduct heat much faster than insulation, so a wall isn't uniformly insulated — some of it is stud, not cavity. The Effective R-Value applies the ASHRAE parallel-path method to blend the framing's R-value with your insulation's R-value, weighted by how much of the wall each one covers. This typically reduces the whole-assembly R-value by 20 to 30 percent compared to the insulation alone, which is why it's always lower.

Why does my climate zone matter for R-value requirements?

The International Energy Conservation Code (IECC) divides the U.S. into 8 climate zones based on how cold or hot the region typically gets, and sets a minimum required R-value for each zone and building component. Colder zones require higher minimums to prevent heat loss. Selecting your zone lets this calculator show you the code-minimum target to compare your assembly against.

How accurate is this calculator compared to a professional energy audit?

This calculator uses standard published R-value-per-inch figures and the ASHRAE parallel-path method, which are the same general approach used by building professionals, but it applies simplified assumptions — a single air-film figure per assembly type, a single framing fraction per stud spacing, and no allowance for compression, gaps, penetrations, or moisture. For code compliance verification or permit applications, consult a licensed energy rater, engineer, or your local building department.

Why doesn't this calculator estimate material cost or energy savings?

Cost per square foot and heating-fuel savings depend on constantly changing material prices and utility rates, which is a different kind of estimate than thermal resistance. Keeping this tool focused on R-value means the formula only has to be maintained in one place. For the cost side of your energy use, see the Appliance Energy Cost Calculator.