Practical insulation
Roof R-value calculator
Roof R-value is not always the same problem as attic R-value. A roof calculator is useful for sloped roofs, rafter bays, above-deck insulation, cathedral ceilings, and retrofit layers where the thermal path follows the roof plane. Enter existing R-value, added thickness, target R-value, and material. The result is nominal and should be checked against ventilation, moisture control, structural depth, and thermal bridging through rafters.
Educational estimates only. Always check local building code and product documentation before specifying insulation.
Quick answer
Roof R-value calculator
Roof R_total = R_existing + thickness × R per inch.
- Formula
R_total = R_existing + thickness_in * R_per_in- Example
- Example: existing R-10 plus 5.5 in at R-3.8 per inch adds R-20.9 and reaches about R-30.9.
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
R_existing = 19thickness_in = 6R_per_in = 3.7
Formula
R_total = R_existing + thickness_in * R_per_in
Steps
6 * 3.7 = 22.219 + 22.2 = 41.2
Result
R_total = R-41.2
Related calculator
Added insulation R-valueExample 2
Inputs
R_existing = 10thickness_in = 4R_per_in = 5.0
Formula
R_total = R_existing + thickness_in * R_per_in
Steps
4 * 5.0 = 20.010 + 20.0 = 30.0
Result
R_total = R-30.0
Related calculator
Roof R-valueRoof R-value calculator
Example: existing R-10 plus 5.5 in at R-3.8 per inch adds R-20.9 and reaches about R-30.9.
This calculator gives educational estimates only. Always check local building code and product documentation.
Practical note
How to interpret the calculator result
Roof R-value calculator 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.
Roof R-value Calculator
Roof R-value Calculator
Estimate roof or rafter insulation R-value from existing R-value, added thickness, and material.
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
Sloped roof layers must leave required ventilation or use an assembly designed for unvented conditions.
Limitations
Use the attic calculator when insulation lies flat on the ceiling plane rather than along the roof plane.
Roof R-value Calculator
Roof R_total = R_existing + thickness × R per inch.
Roof R-value is not always the same problem as attic R-value. A roof calculator is useful for sloped roofs, rafter bays, above-deck insulation, cathedral ceilings, and retrofit layers where the thermal path follows the roof plane. Enter existing R-value, added thickness, target R-value, and material. The result is nominal and should be checked against ventilation, moisture control, structural depth, and thermal bridging through rafters. Example: existing R-10 plus 5.5 in at R-3.8 per inch adds R-20.9 and reaches about R-30.9.
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.
US R-value mini-hub
Move between R-value calculators, material tables, insulation comparisons, and assembly calculators without scanning the whole navigation.
R-value calculators
Material R-values
Insulation comparisons
Assembly calculators
Reference tables
Relevant materials for this assembly
Check typical R-value ranges and material limits before choosing the layer build-up.
Using European units?
Open the U-value, lambda, and metric insulation thickness 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 Roof R-value calculator as a first-pass reference. Before specifying anything, compare the result with the manufacturer data, actual project dimensions, product data sheet, and local requirements.