UK Insulation Lambda Values Reference

Lambda (λ) is the thermal conductivity of a material, measured in watts per metre kelvin (W/mK). It indicates how easily heat passes through 1 metre thickness of the material for each degree of temperature difference. A LOWER lambda value means BETTER insulating performance. For example, PIR board at 0.022 W/mK is a much better insulator than mineral wool at 0.035 W/mK — you need less thickness to achieve the same U-value.

The thermal resistance (R-value) of insulation = thickness (m) / lambda (W/mK). To achieve a target U-value, you need the total R-value of all wall/roof layers to equal approximately 1/U-value. For example, to get a wall U-value of 0.18 W/m²K, you need a total R-value of about 5.56. If using PIR (lambda 0.022), the insulation thickness needed is roughly 5.56 x 0.022 = 0.122 m (122 mm), after accounting for other wall layers.

PIR (polyisocyanurate) has a lambda of 0.022 W/mK compared to mineral wool at 0.032-0.040 W/mK. This means PIR achieves the same thermal resistance in roughly 60-65% of the thickness. In situations where space is limited (cavity walls, roof rafters, floor build-ups), PIR saves valuable space despite the higher cost per m². Phenolic foam (0.018-0.020 W/mK) is thinner still but more expensive.

Natural insulation materials like sheep wool (lambda ~0.038 W/mK), wood fibre (0.038-0.042), hemp (0.040), and cellulose (0.035-0.040) have low embodied energy and are often made from recycled or renewable sources. Mineral wool is also relatively low-impact. PIR and phenolic foams have higher embodied energy but achieve excellent performance in thin profiles. The best choice depends on balancing thermal performance, thickness constraints, budget and environmental priorities.