Starlight Tools

Insulation R-Value Calculator

Calculate the cumulative R-value (thermal resistance) of your wall, ceiling, or floor assembly by stacking materials. 100% Client-side.

Assembly Calculator

Assembly preset

Units and target

ZIP lookup uses an approximate local prefix table. Pick the zone manually when in doubt.

Editable layers

Thermal Resistance

Total R-Value

R-0.0

Choose a preset or edit layers to calculate automatically.

US R-value R-0.0
Metric RSI RSI 0.00
U-factor 0.000 Btu/h-ft²-F
U-value 0.00 W/m²-K
Layer Thickness R / inch Layer R Share

Understanding Insulation R-Values

This calculator supports two common workflows. Stacked layer mode adds continuous layers in series, which is useful for attic depth, rigid foam, drywall, sheathing, and simple material comparisons. Whole-wall mode estimates effective R-value by accounting for framing paths through studs, joists, rafters, or other thermal bridges.

Whole-wall formula

Whole-wall mode calculates two parallel paths: one through the insulated cavity and one through the framing. It converts each path to U-value, area-weights those U-values by framing percentage, and converts the result back to R-value. Continuous layers such as drywall and exterior foam are added to both paths before the U-value average.

Climate target comparison

The target panel compares your result with common retrofit guidance by assembly type and climate zone. It is a planning check, not a code determination. Local code, product labels, fire requirements, vapor control, and building-specific moisture conditions can change the final assembly.

Worked Examples

2x4 wall with R-13 batt

Inputs: 1/2 inch drywall, R-13 cavity insulation, 7/16 inch OSB, wood framing at 25% area.

Expected result: about whole-wall R-10.0 before siding and exterior foam.

Add continuous exterior foam

Inputs: same 2x4 wall, plus 1 inch XPS as a continuous exterior layer.

Expected result: about whole-wall R-15.7 because the foam covers the framing bridge.

Attic blown cellulose

Inputs: 10 inches loose-fill cellulose at about R-3.5 per inch.

Expected result: about R-35, or roughly U-0.029 Btu/h-ft²-F.

Convert R-value to U-value

Inputs: any assembly with total R-20.

Expected result: U-factor 0.050 Btu/h-ft²-F and U-value about 0.28 W/m²-K.

Material Database Notes

Insulation materials

The selector includes fiberglass batts by typical density, blown fiberglass, loose-fill and dense-pack cellulose, mineral wool batt and board, open-cell and closed-cell spray foam, EPS, XPS, polyiso, and wood fiber board.

Assembly layers

Common non-insulation layers include gypsum board, OSB, plywood, softwood, brick, concrete block, air films, air spaces, siding, asphalt shingles, and roof deck materials.

Custom data

Choose custom material to enter R per inch, a known total R-value, or lambda in W/m-K with thickness. Lambda is converted to RSI, then to US R-value.

Why ranges vary

R-values vary by product, density, age, moisture, installation quality, compression, temperature, facers, and test method. Use manufacturer labels for purchased products.

Data and Limitations

Last reviewed: June 30, 2026.

Values in this calculator are typical estimates for planning. Product labels, manufacturer technical data, local code, and project-specific design should control final decisions. Compressed fibrous insulation, gaps, moisture, air leakage, and poor fit can reduce real performance. Radiant barriers are not treated as standalone R-value layers unless the full tested assembly is known.

Insulation R-Value FAQ

How do I calculate total R-value?

For continuous layers, multiply R per inch by thickness for each layer, then add the layer R-values.

What R-value do I need for an attic or wall?

It depends on assembly type and climate zone. Use the comparison panel as a planning target and verify with local code.

Do R-values add together?

Yes for layers in series. Use whole-wall mode when framing creates parallel heat-flow paths.

How do studs affect R-value?

Wood, steel, or masonry framing bypasses cavity insulation. Area-weighted U-value estimates that thermal bridge.

What is R-value vs U-value?

R-value resists heat flow. U-value transfers heat. U equals 1 divided by R.

How do US R and RSI compare?

RSI equals US R divided by 5.678. US R equals RSI multiplied by 5.678.

Does compressed insulation lose R-value?

Usually yes. Compression reduces total thickness and the air structure that makes fibrous insulation work.

Does air sealing matter?

Yes. Air leakage can carry heat around insulation, so sealing gaps is part of real assembly performance.

Do radiant barriers have R-value?

Radiant barriers are normally rated by emissivity and assembly conditions, not a simple standalone R-value.

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