Question 1

What does lambda value mean?

Accepted Answer

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.

Question 2

How do I calculate insulation thickness from lambda?

Accepted Answer

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.

Question 3

Why is PIR more expensive but thinner than mineral wool?

Accepted Answer

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.

Question 4

What insulation has the best environmental credentials?

Accepted Answer

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.

Material	Lambda (W/mK)	Form	Common Applications
Phenolic foam	0.018–0.021	Rigid board	Walls, roofs where thin profile critical
PIR (polyisocyanurate)	0.022–0.023	Rigid board (foil-faced)	Walls, roofs, floors — Celotex, Kingspan
PUR (polyurethane)	0.023–0.028	Rigid board / spray foam	Cavity fill (spray), flat roofs
XPS (extruded polystyrene)	0.029–0.036	Rigid board	Below ground, inverted roofs, high moisture
EPS (expanded polystyrene)	0.030–0.038	Rigid board / beads	External wall insulation, cavity beads
Mineral wool (glass)	0.032–0.044	Rolls, batts, slabs	Lofts, timber frame, partitions
Mineral wool (stone/rock)	0.034–0.040	Slabs, batts	Cavity batts, external walls, fire barriers
Cellulose (recycled paper)	0.035–0.040	Loose fill / blown	Lofts, timber frame cavities
Sheep's wool	0.035–0.040	Rolls, batts	Timber frame, lofts, eco builds
Hemp fibre	0.038–0.042	Batts, rolls	Timber frame, lofts, eco builds
Wood fibre	0.038–0.043	Flexible batts / rigid boards	Timber frame, external sheathing, roofs
Cork	0.040–0.050	Rigid boards, granules	Floors, walls, eco / heritage buildings

Material	U = 0.30 W/m²K (mm)	U = 0.18 W/m²K (mm)	U = 0.15 W/m²K (mm)
PIR (0.022)	~60	~110	~135
Mineral wool (0.035)	~95	~175	~215
Wood fibre (0.040)	~110	~200	~245

UK Insulation Lambda Values Reference

Insulation Lambda Values (Thermal Conductivity)

Thickness Needed for Common U-Value Targets

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