R-value and U-factor
k-value vs R-value: what each unit means
Lower k-value is better for a material, while higher R-value is better for the completed layer. This page is built for searchers who need a practical answer first and a transparent formula immediately after it. Use it for early estimating, product comparison, and checking whether a result belongs in metric or imperial notation.
The formulas use explicit SI and US conversions, so λ, k-value, R-value, U-value, and U-factor are not mixed silently.
k-value vs R-value: what each unit means
Two products can share a similar k-value, but the thicker product can still deliver a much higher R-value.
separate material conductivity per inch from installed thermal resistance. For compliance work, confirm product data, framing fractions, air films, and local code requirements before using the number in a final report.
Working calculator
Enter the core dimensions and check the result directly on this page.
Quick answer
What is the R-value of 100 mm insulation?
A 100 mm layer is about R-16.2 in US units when lambda is 0.035 W/mK. In SI units, the same layer is about 2.86 m²K/W.
| Lambda | R SI | R US |
|---|---|---|
| 0.030 | 3.33 | 18.93 |
| 0.035 | 2.86 | 16.22 |
| 0.040 | 2.50 | 14.20 |
| 0.045 | 2.22 | 12.62 |
The table covers a single layer without surface resistances. Use the calculator for a complete wall, roof, or floor assembly.
Practical note
How to interpret the calculator result
k-value vs R-value: what each unit means 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.
k-value vs R-value
k-value vs R-value
Lower k-value is better for a material, while higher R-value is better for the completed layer. It keeps the calculation small enough for a quick check while still linking to deeper U-value, lambda, and material pages for complete assembly work.
Assembly thickness
Assembly thickness
Thickness can be handled in millimetres, inches, or feet depending on the task, but the conversion path is always explicit.
U-value
U-value
U-value and U-factor are shown only after the relevant R-value has been calculated, which prevents a common unit-system error.
CategoryLambdaLayerMaterialThickness
How it works
Unit discipline
R-value belongs to the US convention and rises as performance improves. U-value and U-factor move in the opposite direction, so the calculator labels each output before showing the number. separate material conductivity per inch from installed thermal resistance.
Practical use
Use these pages to translate product sheets, compare assemblies, and decide whether a detailed layer-by-layer calculation is needed. Lower k-value is better for a material, while higher R-value is better for the completed layer.
k-value vs R-value
Lower k-value is better for a material, while higher R-value is better for the completed layer. Two products can share a similar k-value, but the thicker product can still deliver a much higher R-value.
This page is built for searchers who need a practical answer first and a transparent formula immediately after it. Use it for early estimating, product comparison, and checking whether a result belongs in metric or imperial notation. For compliance work, confirm product data, framing fractions, air films, and local code requirements before using the number in a final report.
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Assembly nameThickness unitInternal Rsi (m2K/W)External Rse (m2K/W)
R-value and U-factor FAQ
R-value is thermal resistance in US notation. A higher R-value means the layer resists heat flow better, but it must be tied to the actual installed thickness.
U-factor is the US heat transfer rate through an assembly. It is the inverse of total R-value in US units, so a lower U-factor means better thermal performance.
For US units, divide 1 by U-factor to get R-value. For SI U-value, first convert to US U-factor by dividing by 5.678263337, then invert it.
Calculate R-SI as thickness in metres divided by lambda in W/mK. Multiply the SI resistance by 5.678263337 to get US R-value.
US product labels and energy codes commonly express insulation performance as R-value because it is easy to add layer resistances and compare product thicknesses.
Yes. Metric mode keeps λ, U-value, W/mK, and m²K/W visible. Imperial mode switches the interface to R-value, U-factor, k-value, inches, feet, and Btu units.
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.