Thermal conductivity, written lambda (λ) and expressed in W/(m.K), runs from roughly 0.022 for a PIR board to 0.046 for wood fibre. It is what dictates the thickness to install, through R = e/λ. The part the data sheet leaves out: an energy assessment only credits the product's lambda when the insulant's thermal resistance is known and evidenced. Where only the thickness is recorded, the calculation falls back on a default lambda of 0.04 W/(m.K), whatever was actually installed.
Lambda (λ) in practice: what conductivity really measures
A simple definition of λ: heat passing through the insulation (W/m·K)
Lambda (λ) is a material's thermal conductivity. It expresses the heat power that passes through 1 m of insulation when the temperature gap is 1 K. The lower λ is, the less heat "gets through." The unit is W/m·K.
What a low λ means on a job site: performance and comfort
On the ground, a low λ means that at equal thickness you get better insulation. The result: a lower U-value for the element, warmer walls, less of a "cold wall" effect, and more stable winter comfort. In summer, the incoming heat flow is also slowed down. Still, keep thermal bridges and airtightness in mind.
Don't confuse λ, thermal resistance R, and thickness
Lambda alone isn't enough to compare two solutions. For sizing, you mainly look at thermal resistance. The rule is simple. R = e/λ (with e in meters). So an insulator that's slightly less efficient in λ can reach the same R if you increase the thickness, provided the installation is careful and continuous.
Reading a technical data sheet without mistakes: λD, declared λ, and measurement conditions
λD (declared lambda): the value to use when comparing insulation materials
The λD (declared lambda) is the thermal conductivity shown on the Declaration of Performance. It's the value to use to calculate the thermal resistance R = e/λ and to compare two products at equal thickness. The lower the lambda, the more the insulation slows heat loss.
The conditions that make λ vary: moisture, temperature, aging
A λ value measured in a lab doesn't exist in isolation. Moisture increases conductivity, especially for water-sensitive insulation materials. Temperature also changes the result. Some products see their performance evolve with aging, foams for example. On a data sheet, identify the "dry" state and the reference temperature, often 10°C.
CE marking, standards, and useful documents to secure your choices in 2026
In 2026, secure your choices with UKCA or CE marking and the DoP, which gives the λD and the performance classes. Check the product standard (BS EN 13162 to BS EN 13171 series depending on the family) and a BBA Agrément certificate to make the declared values more reliable. For calculations, the BS EN ISO 10456 standard helps translate product data into sizing assumptions.
From lambda to thermal resistance: calculating insulation thickness for your target
A field formula: R = thickness / λ (with quick examples)
On site, you go from lambda (λ) to thickness in one line. R (m².K/W) = thickness (m) / λ (W/m.K). Example: mineral wool with λ = 0.035 and a target R = 7 gives 0.245 m, or about 24 cm. For a wall targeting R = 3.7 with the same lambda, count on 13 cm. The calculation gives the order of magnitude, the installation makes the performance.
Choosing the thickness by wall type: attics, roof slopes, walls, floors
Keep practical benchmarks based on the targeted grant and the space available. In unheated attics, you often aim for an R around 7 to 10, or 24 to 35 cm depending on the lambda. In roof slopes, 6 to 8 is common, with special attention to the rafters. In walls, 3.7 to 5 depending on internal or external insulation. In ground floors, 3 to 4 to cut the cold-floor effect.
Avoiding common mistakes: thermal bridges, settling, installation
Three classic pitfalls keep coming up. Thermal bridges at junctions, hatches, and bearing points. Settling, especially with loose-fill, if the density or usable height falls short. Installation: open joints, poorly managed vapour barrier, compressed insulation. Clean continuity is sometimes worth more than 2 extra cm. To go further, also see the linear thermal bridge and its impacts.
Comparing insulation families with the right criterion: when λ isn't enough
Mineral, bio-based, synthetic: order-of-magnitude λ values and suitable uses
Lambda (λ) gives the rate of heat loss, but not everything. The thickness column below matches an R of 7 m².K/W, the usual loft target, worked out from e = R × λ.
| Insulation family | Typical λ, W/(m.K) | Thickness for R = 7 |
|---|---|---|
| PIR / PUR | 0.022 to 0.028 | 15 to 20 cm |
| Graphite EPS | 0.030 to 0.032 | 21 to 23 cm |
| Glass wool | 0.030 to 0.040 | 21 to 28 cm |
| Rock wool | 0.034 to 0.041 | 24 to 29 cm |
| White EPS | 0.035 to 0.038 | 25 to 27 cm |
| Cellulose | 0.038 to 0.042 | 27 to 30 cm |
| Wood fibre | 0.036 to 0.046 | 25 to 32 cm |
| Default value, resistance not evidenced | 0.040 | 28 cm |
That last row is not a product, it is what the calculation assumes in your place. Installing 20 cm of 0.032 wool without recording it means the assessment counts R = 5.0 instead of R = 6.25.
Other decisive criteria: fire reaction, water vapour, acoustics, durability
Look at fire reaction (Euroclasses), especially in attics and technical rooms. On the water vapour side, permeance and the vapour-barrier/vapour-control pairing make the difference between a healthy wall and one that condenses. Also think about acoustics (mineral wool often performs well) and durability, mechanical strength, moisture sensitivity, settling, rodents, and technical approvals.
Summer comfort and thermal lag: how to round out your choice beyond λ
For summer comfort, add density, heat capacity, and thermal lag. On a roof, a denser, thicker insulator delays the heat peak, like a lamp being dimmed rather than switched off. Check the allowable weight, the installation (continuity, airtightness), and hygrothermal compatibility. This way you choose a coherent whole, not just the best λ.
Choosing the right insulation on site: a simple method compliant with 2026 grants
Adapting the choice to the building and the client: moisture, ventilation, space constraints
On the technical survey, record the moisture state of the substrate, whether ventilation is present and what it actually extracts, and the clear height genuinely available. Those three readings settle the insulant and vapour-control pairing well before lambda does. Where height is tight, that is where a low lambda earns its keep: it buys centimetres, and it is the only thing that justifies the price gap against a cheaper standard wool.
Consistency with the certification and funding rules: proof and points of attention
To remain eligible, the product and installation must justify the targeted U-value. Keep clear product proof (BBA Agrément certificate or declaration of performance): on an external wall insulation job, Argile carries the thickness and thermal resistance of the chosen insulant from the catalogue through to the quote. On site, watch out for weak points, continuity, and ventilation. A good insulator on paper doesn't forgive a "gappy" installation.
Selection checklist: comparing, pricing, justifying on the quote and handover report
- Calculate the thickness. e = R x lambda.
- Compare in €/m² installed and in real R gain (compression, settling).
- Quote: insulator, thickness, evidenced R, surface area, brand, and reference.
- Handover report: photos, treated zones, discrepancies, and rework.
