Insulation: thickness, lambda and R-value explained

Insulation planning has two separate questions: how much material to buy and what thermal resistance the selected layer provides. The Insulation calculator keeps both visible.

For a homogeneous layer, R = thickness in metres ÷ lambda. With 100 mm and λ = 0.035 W/m·K, R ≈ 2.86 m²·K/W. At 140 mm with the same material, R becomes 4.00. At the same thickness, a lower lambda gives a higher R.

Quantity and performance are independent

A 20 m² surface with 5% cutting allowance becomes 21 m². If one pack covers 5.4 m², four packs are required. Buying an extra pack because of rounding does not increase the R-value of the wall; it only changes the amount of material on site.

Use the product’s declared lambda or certified R-value rather than a generic value.

A layer is not the whole wall

The simple e/λ formula describes a homogeneous layer. Studs, fixings, thermal bridges, air layers and other materials affect the real performance of an assembly. The calculator is therefore useful for comparing the insulation layer, not for certifying the complete building element.

Thickness available, moisture control, fire behaviour, acoustics and installation method may also determine which system is appropriate.

For a lined partition use the Drywall calculator for boards and framing. Record the actual pack coverage and product data before ordering.