Thermal conductivity λ (lambda): understanding and choosing insulation

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: 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 CE marking and the DoP, which gives the λD and the performance classes. Check the product standard (EN 13162 to EN 13171 series depending on the family) and, in France, an ACERMI certificate to make the declared values more reliable. For calculations, the 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 vapor 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. As an order of magnitude, mineral insulation (glass wool, rock wool) is often around 0.032 to 0.040 W/m.K. Synthetic materials range from EPS around 0.030 to 0.038, and PIR can go down to 0.022 to 0.028 when thickness is constrained. Bio-based materials (cellulose, wood fiber) are more around 0.038 to 0.045, useful when you're also targeting summer comfort.

Other decisive criteria: fire reaction, water vapor, acoustics, durability

Look at fire reaction (Euroclasses), especially in attics and technical rooms. On the water vapor side, permeance and the vapor-barrier/vapor-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 site, start by reading the building. Signs of moisture, cold walls, absent ventilation or a tired mechanical ventilation system. These signals point toward a more or less breathable insulator, and toward the right vapor-barrier management. Then look at the available thickness. When it's limited, aim for a better thickness ratio, without forgetting acoustics and fire reaction.

Consistency with RGE, MaPrimeRénov', and CEE: proof and points of attention

To remain eligible, the product and installation must justify the targeted thermal resistance. Keep clear product proof (ACERMI certification or declaration of performance). 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, R, surface area, brand, and reference.
• Handover report: photos, treated zones, discrepancies, and rework.