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Thermal Resistance (R Value) Calculator

Work out thermal resistance (r value) instantly with clear inputs, formula shown and shareable results.

About Thermal Resistance (R Value) Calculator

R-value is thickness divided by conductivity, so it measures how strongly a layer resists heat flow per square metre. 100 mm of mineral wool at k = 0.038 gives R = 2.63 m2 K/W and a U-value of 0.38 W/(m2 K). Layers in series add their R-values, which is how a full construction is assessed.

Formulas

R-value

R = thickness / conductivity

U-value

U = 1 / R(total)

Worked example

With the default values below, Thermal Resistance (R Value) Calculator returns R-Value of 2.6316. Change any input above and every figure here is recalculated in your browser.

Inputs used

Layer thicknessminimum 0 mm
100 mm
Thermal conductivityminimum 0 W/(m K)
0.038 W/(m K)
Areaminimum 0 m2
20 m2

Results

R-Value
2.6316 m2 K/W
Corresponding U-Value
0.3800 W/(m2 K)
Absolute Resistance Of The Element
0.13158 K/W
Heat Loss Per Kelvin Of Difference
7.6000 W/K

Step by step

  1. 1Values used
  2. 2R-value
  3. 3U-value
  4. 4R-Value= 2.6316 m2 K/W
  5. 5Corresponding U-Value= 0.3800 W/(m2 K)
  6. 6Absolute Resistance Of The Element= 0.13158 K/W
  7. 7Heat Loss Per Kelvin Of Difference= 7.6000 W/K

Assumptions and limits

  • The result is derived only from the 3 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
  • Figures are rounded for display to 5 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

Do R-values add?

Yes, for layers in series - including the internal and external air films, which contribute about 0.13 and 0.04 m2 K/W.

Why do imperial R-values look bigger?

Because the imperial unit is about 5.68 times smaller. An R-13 batt in imperial is only R-2.3 in SI.

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