Insulation material comparison: lambda, R and target thickness
Compare two products using declared thermal conductivity and the same installed thickness, or calculate the thickness each needs for a target layer R-value. Labels A and B represent your own products. Input values are illustrative, not universal material ratings.
Calculate with your inputs
Compare like-for-like layers
RSI = d / λ; RUS = RSI × 5.678263; dtarget[mm] = Rtarget × λ × 1000
Use thickness in metres and lambda in W/(m·K). Higher layer R means greater thermal resistance. At the same thickness a lower lambda gives greater R. For the same target R, it needs a thinner layer. This comparison excludes surface resistances and other assembly layers.
Example: conductivity 0.040 versus 0.035
At 100 mm, A at lambda 0.040 has RSI 2.500. B at lambda 0.035 has RSI 2.857, about 14.29% greater resistance at that thickness. For target RSI 3.0, A needs 120 mm and B 105 mm. Check available thicknesses, declared product values and installation tolerances before selecting either product.
Thermal resistance is one selection criterion
Match the product to its location and system. Compare moisture behaviour, fire classification, compressive strength, installation method, acoustic needs and manufacturer approvals. Cost per square metre should refer to comparable installed resistance and include labour and ancillary materials. A thermal comparison alone cannot establish the best product.
From layers to whole-wall U
The existing wall and other layers remain part of the assembly. Use the linked layered thermal calculator with consistent surface conditions to compare final U. Assess bridges and junctions separately. Record the product declaration, installed thickness, values and date with each variant. When changing products or thicknesses, recalculate rather than comparing old numbers without their assumptions.
Worked examples
| Inputs and method | Result |
|---|---|
| A: 100 mm / λ 0.040 | RSI = 2.500 |
| B: 100 mm / λ 0.035 | RSI = 2.857 |
| Target RSI 3.0 | A: 120 mm; B: 105 mm |
Result scope
The form compares homogeneous material layers using your conductivity values. It does not give the complete assembly U, rank fire or moisture safety, recommend a universal material or establish code compliance.