Practical insulation
Insulation thickness calculator from target R-value
Use this insulation thickness calculator when you know the target R-value and need to estimate how many inches or millimeters of material are required. Enter the target R-value, optionally enter existing insulation R-value, choose a material preset, or type a custom R per inch from a product label. The calculator first finds the additional R-value needed, then divides that number by R per inch. Typical values vary by product density, facing, aging method, temperature, and installation quality, so the result is a planning estimate rather than a purchase specification.
Educational estimates only. Always check local building code and product documentation before specifying insulation.
Quick answer
Insulation thickness calculator from target R-value
Thickness = (target R-value - existing R-value) / R-value per inch.
- Formula
thickness_in = max(0, target_R_US - existing_R_US) / R_per_in- Example
- Example: target R-38 with existing R-19 leaves R-19 to add. At R-3.8 per inch, the estimate is 5.0 in or about 127 mm.
Working calculator
Enter the core dimensions and check the result directly on this page.
Typical R-value per inch
Use this table as a planning starting point. Real values vary by product, density, temperature, aging method, and installation quality.
| Material | Typical R/in | Typical US k | Note |
|---|---|---|---|
| Fiberglass batt | 3.2 | 0.313 | common cavity batt planning value |
| Mineral wool | 4.2 | 0.238 | often denser than fiberglass batt |
| Loose-fill cellulose | 3.5 | 0.286 | depends on settling and blown density |
| EPS foam board | 4.0 | 0.250 | varies by board grade |
| XPS foam board | 5.0 | 0.200 | check long-term declared value |
| Polyiso board | 5.6 | 0.179 | can vary with temperature |
Worked examples
Example 1
Inputs
target_R_US = 38existing_R_US = 19R_per_in = 3.8
Formula
thickness_in = max(0, target_R_US - existing_R_US) / R_per_in
Steps
38 - 19 = 1919 / 3.8 = 5.00 in
Result
thickness = 5.00 in
Related calculator
Insulation thicknessExample 2
Inputs
target_R_US = 49existing_R_US = 11R_per_in = 3.5
Formula
thickness_in = max(0, target_R_US - existing_R_US) / R_per_in
Steps
49 - 11 = 3838 / 3.5 = 10.86 in
Result
thickness = 10.86 in
Related calculator
Attic R-valueInsulation thickness calculator from target R-value
Example: target R-38 with existing R-19 leaves R-19 to add. At R-3.8 per inch, the estimate is 5.0 in or about 127 mm.
This calculator gives educational estimates only. Always check local building code and product documentation.
Practical note
How to interpret the calculator result
Insulation thickness calculator from target R-value is intended for quick option checks and technical discussion before detailed execution. The result depends on the selected units, declared material values, and chosen surface resistances, so each change in layer or thickness should be treated as a separate variant.
The calculator does not automatically verify every local rule, thermal bridge, moisture condition, structural connection, or installation tolerance. If the result is close to a requirement, treat it as a reason for deeper verification rather than a final decision.
For better comparisons, test several realistic thicknesses, check current product data sheets, and review the complete assembly. A calculated value is most useful when the assumptions are clear: material, thickness, layer order, units, and data source.
For insulation or U-value tools, layer order and correct units are especially important. For concrete, electrical, plumbing, or heating tools, the result should be read as a quick quantity or plausibility check before standards and execution conditions are reviewed.
Save the result with the date, material name, and assumptions. If the product, diameter, cable section, or thickness changes later, do not compare the numbers alone without checking which inputs changed.
For calculator pages, clear separation between inputs and result is essential. If a value looks surprising, check units and default fields first, then review the project assumptions.
Insulation Thickness Calculator
Insulation Thickness Calculator
Estimate required insulation thickness from target R-value, existing R-value, and material R per inch.
Assembly thickness
Assembly thickness
Thickness is estimated from target R-value, existing R-value, and material R per inch. The result is shown in inches and millimeters.
U-value
U-value
R-values can be added for layers in the same heat-flow path, but framing, gaps, compression, air leakage, and moisture details can reduce real assembly performance.
CategoryLambdaLayerMaterialThickness
How it works
Material values vary
Material presets are typical planning values. Product labels and datasheets should override the preset when they are available.
Limitations
Cavity depth, compression, air sealing, moisture control, and thermal bridges can change the installed result.
Insulation Thickness Calculator
Thickness = (target R-value - existing R-value) / R-value per inch.
Use this insulation thickness calculator when you know the target R-value and need to estimate how many inches or millimeters of material are required. Enter the target R-value, optionally enter existing insulation R-value, choose a material preset, or type a custom R per inch from a product label. The calculator first finds the additional R-value needed, then divides that number by R per inch. Typical values vary by product density, facing, aging method, temperature, and installation quality, so the result is a planning estimate rather than a purchase specification. Example: target R-38 with existing R-19 leaves R-19 to add. At R-3.8 per inch, the estimate is 5.0 in or about 127 mm.
Add layerRestore default setupRemove
Assembly nameThickness unitInternal Rsi (m2K/W)External Rse (m2K/W)
Calculation assumptions
The calculators use visible formulas and explicit unit conversions. Treat the result as a preliminary check, not a complete building design.
- SI and US units are converted separately; R, RSI, U-value, and U-factor are not mixed without the unit factor.
- Enter positive values and compare the result with the selected product datasheet.
- Local codes, thermal bridges, fasteners, and installation quality can change the requirement.
- Last formula review: 2026-04-27.
See how formulas, unit conversions, rounding, and limitations are handled. Methodology details.
Using US units?
Open the R-value, U-factor, and insulation thickness in inches tools.
Materials and comparisons
Check lambda values, insulation comparisons, and reference pages before choosing layers.
Practical insulation FAQ
They can be added for layers in the same heat-flow path, but framing and parallel paths need whole-assembly checks.
No. They are planning values. Product labels, density, temperature, and installation method can change R per inch.
No. These calculators are educational estimates and local code or product documentation should decide final requirements.
Many US R-value products are sold in inches, while international projects often need metric dimensions.
Yes. It is designed for layered assemblies such as external walls, flat roofs, pitched roofs, floors, and slabs. For unusual assemblies, add every relevant layer and treat the result as a planning check before formal verification.
Yes. It is intended for fast concept-stage calculations, insulation comparison, and envelope optimisation before detailed design. It is best used to narrow choices, not to replace a code check or project-specific thermal bridge assessment.
Yes. You can switch between millimeters, centimeters, and inches, and the calculator keeps the values consistent. For fewer mistakes, choose one unit system at the start of a project and review converted thicknesses before export.
Use Insulation thickness calculator from target R-value as a first-pass reference. Before specifying anything, compare the result with the slab thickness, actual project dimensions, product data sheet, and local requirements.