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Insulation R-Value Calculator

Add up the thermal resistance (R-value) of each layer in a wall, roof or floor build-up and get the overall R-value and U-value. Enter product R-values directly, and/or a material as thickness and conductivity — the tool sums them in series and can add the standard inside/outside surface resistances.

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How to use this calculator

Enter Values

Enter your values and press Calculate to see results.

Before you rely on this: First-pass guide only. Verify safety-critical or regulated work against the relevant standards, your project requirements and a qualified professional.

How to use this calculator

  1. Enter the R-value (m²·K/W) of each insulation or material layer in the Layer 1–5 boxes — these add together.
  2. Optionally enter one extra layer as a thickness (mm) plus its conductivity λ (W/m·K); the tool converts it to R = (t/1000)/λ.
  3. Put 1 in the surface-resistances box to add the standard still-air films Rsi 0.13 + Rso 0.04, then read the total R and U-value.

How it works

Layers in series add their resistances: total R = R1 + R2 + … A material given by thickness and conductivity has R = (thickness in metres) / λ = (t_mm/1000) / λ. The U-value (thermal transmittance) is the reciprocal of total resistance, U = 1/R, in W/m²·K — lower U (higher R) means better insulation. Optional surface resistances Rsi = 0.13 (inside) and Rso = 0.04 (outside) m²·K/W represent the still-air films on each face of a typical wall.

Worked example

Worked example. A wall has an R2.5 insulation batt and an R1.5 lining, plus a 90 mm layer of λ = 0.04 W/m·K insulation. The extra layer adds (90/1000)/0.04 = 2.25 m²·K/W, so total R = 2.5 + 1.5 + 2.25 = 6.25 m²·K/W and U = 1/6.25 = 0.16 W/m²·K.

Common mistakes

  • Mixing up R-value and U-value — R adds up across layers, U is 1/R (they move in opposite directions).
  • Entering conductivity λ without a thickness (or vice-versa) — the thickness/λ pair needs BOTH values.
  • Forgetting that this simple series model ignores thermal bridging through studs and framing, which lowers the real-world R-value.

Frequently asked questions

What is the difference between R-value and U-value?

R-value is thermal resistance (how well a layer resists heat flow) and adds up across layers in series. U-value is thermal transmittance, the reciprocal of the total resistance (U = 1/R). A higher R-value and a lower U-value both mean better insulation.

Should I include the surface resistances?

For a total (air-to-air) U-value of a wall or roof, yes — tick the option to add Rsi 0.13 and Rso 0.04 m²·K/W. If you only want the resistance of the material build-up itself (e.g. to compare products), leave them out. The exact surface values vary with orientation and heat-flow direction.

Does this account for studs and thermal bridging?

No. This is a simple one-dimensional series calculation for a single path through the build-up. Timber or steel framing bridges the insulation and reduces the effective R-value, so a compliance calculation uses a framing-corrected method to AS 4859 / ISO 6946 / the NCC.

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