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Insulation Calculator

Thermal resistance (m²K/W): -
Calculation explained

How this result was calculated

The formula, your substituted values, and the evaluated result are shown below.

This is a planning estimate. Confirm dimensions, allowances, material specifications, applicable codes, and supplier requirements before purchasing or construction.
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How the Insulation Calculator works

Calculate insulation thermal resistance.

Before using the result

This is a planning estimate. Confirm dimensions, allowances, material specifications, applicable codes, and supplier requirements before purchasing or construction.

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About this calculator

Calculate insulation thermal resistance.

Calculator guide

How the insulation calculator works

Insulation is rated by its thermal resistance, the R-value: the higher the number, the better it resists heat flow. For a uniform material layer, the R-value is its thickness divided by its thermal conductivity:

R (m²K/W) = thickness (m) ÷ thermal conductivity λ (W/m·K)

Thermal conductivity (lambda, λ) is a property of the material. Lower is better. Enter the thickness in metres, so 90 mm is 0.09.

Worked example

  1. A 90 mm glasswool wall batt with λ = 0.035 W/m·K
  2. R = 0.09 ÷ 0.035 = 2.57 m²K/W, in line with the R2.5 wall batts sold for 90 mm stud walls
  3. A 200 mm ceiling batt with λ = 0.040: 0.2 ÷ 0.040 = R5.0

Typical conductivity values

MaterialApprox. λ (W/m·K)
Glasswool / rockwool batts0.032 – 0.044
Polyester batts0.040 – 0.045
Expanded polystyrene (EPS)0.033 – 0.038
Polyisocyanurate (PIR) board0.022 – 0.026
Timber (for comparison)0.12 – 0.16

Material R-value vs total R-value

This calculator gives the R-value of the insulation layer itself. Building codes, including Australia's National Construction Code, often set a minimum total R-value for the roof, wall or floor, which also counts plasterboard, air gaps, cladding and surface air films. Studs and joists also bridge the insulation and lower its real-world performance. For compliance, use the product's certified R-value and your energy assessor's calculation.

Getting the most from insulation

What R-value do I need for ceiling insulation?

It depends on your climate zone and the code in force. In Australia, ceiling insulation of about R3.5 to R6.0 is common. Check the NCC requirements for your zone.

Is a higher R-value always better?

Higher R-values reduce heat flow, but the savings flatten out. Doubling from R2 to R4 saves far more energy than going from R6 to R8.

Does insulation thickness matter more than material?

Both do. R-value is thickness divided by conductivity, so a thinner board with low conductivity can match a much thicker batt.

Written by Sam Alex, civil engineer. About the author