Understanding insulation lambda without stopping at a simple definition
Definition in one sentence: what is insulation lambda, in practical terms?
Insulation lambda (λ) is thermal conductivity. It expresses the heat flow through 1 m of insulation for a 1 K temperature difference. Its unit is W/m.K. The lower the λ, the better the insulation resists heat loss. On site, you mainly use it to convert to a usable value: R = e/λ.
Thermal conductivity of insulation: how to read values on a product datasheet (λD, λB, ACERMI)
On a Declaration of Performance, look for λD. This is the declared value, measured according to the product standard, generally at 10 °C. If the product is ACERMI-certified, rely on the certified value and the R associated with the thickness. When a λB value is shown, treat it as a calculation value set by the certification scheme. Do not mix columns. Choose a single reference value for the quote and the project file.
What lambda does not tell you: settlement, moisture, thermal bridges and installation
Lambda does not account for blown-in settlement, moisture uptake, thermal bridges or condensation risk. Record the installed thickness, continuity, compression around services, and interfaces with framing and joinery. A theoretically perfect R-value on paper can drop if the installation creates stray air gaps or leakage paths.
Moving from lambda to R: the formula that really matters on site
Lambda calculation: applying R = e / λ with the correct units (m, W/m·K)
Insulation lambda (λ) describes conductivity. On site, what matters for the quote and inspection is thermal resistance R. Work in metres for thickness e, and in W/m·K for λ. Example: 120 mm becomes 0.12 m. Then R = 0.12 / 0.040 = 3.00 m²·K/W.
Common thicknesses: worked examples (80, 100, 120, 140, 160, 200 mm) depending on lambda
| Thickness (mm) | e (m) | R if λ = 0.032 (m²·K/W) | R if λ = 0.040 (m²·K/W) | R if λ = 0.045 (m²·K/W) |
|---|---|---|---|---|
| 80 | 0.08 | 2.50 | 2.00 | 1.78 |
| 100 | 0.10 | 3.13 | 2.50 | 2.22 |
| 120 | 0.12 | 3.75 | 3.00 | 2.67 |
| 140 | 0.14 | 4.38 | 3.50 | 3.11 |
| 160 | 0.16 | 5.00 | 4.00 | 3.56 |
| 200 | 0.20 | 6.25 | 5.00 | 4.44 |
Going the other way: finding thickness e = R × λ to reach a target performance
When you start from a target R, you work back to thickness. Simple formula: e = R × λ. This is useful to find the thickness and weigh a target performance against the available thickness allowance (sloping ceilings, linings, supports).
| Target R (m²·K/W) | e if λ = 0.032 (mm) | e if λ = 0.040 (mm) | e if λ = 0.045 (mm) |
|---|---|---|---|
| 3.0 | 96 | 120 | 135 |
| 4.0 | 128 | 160 | 180 |
| 5.0 | 160 | 200 | 225 |
2026 insulation lambda table by family: compare quickly before pricing
2026 order-of-magnitude values. For pricing, always start from the product’s declared λ (λD), ideally ACERMI-certified. The “insulation lambda” then lets you quickly convert to thickness via R = e/λ.
| Family | Common product | Typical λ (W/m.K) |
|---|---|---|
| Mineral | Glass wool | 0.032 to 0.040 |
| Mineral | Rock wool | 0.035 to 0.041 |
| Synthetic | EPS | 0.030 to 0.038 |
| Synthetic | XPS | 0.029 to 0.036 |
| Synthetic | PUR/PIR | 0.022 to 0.028 |
| Bio-based | Cellulose wadding | 0.038 to 0.042 |
| Bio-based | Wood fibre | 0.036 to 0.050 |
| Bio-based | Hemp | 0.039 to 0.045 |
Mineral insulation: glass wool, rock wool (lambda ranges)
These are steady values. On a quote, note the λD, the installed thickness and the target thermal resistance. Check continuity and compression, otherwise the paper R-value will not hold up.
Synthetic insulation: EPS, XPS, PUR/PIR (where you save thickness)
When space is tight, PUR/PIR often saves a few centimetres for an equivalent R-value. Watch thermal bridges at junctions, and make sure accessories are compatible with the substrate.
Bio-based insulation: cellulose wadding, wood fibre, hemp (thermal resistance watchpoints)
The lambda is often higher. To achieve the target R, you need more thickness and tighter installation control. Check density, settlement, site moisture and vapour barrier according to the system.
Real thickness to allow for: a simple method by application and site constraints
Walls, lofts, sloping ceilings, floors: aiming for the right R without losing space
| Building element | Minimum R (m².K/W) | Thickness for λ 0.035 (mm) | Thickness for λ 0.040 (mm) |
|---|---|---|---|
| Walls | 3.7 | 130 | 150 |
| Loft spaces | 7.0 | 245 | 280 |
| Sloping ceilings | 6.0 | 210 | 240 |
| Ground floors | 3.0 | 105 | 120 |
With a fixed target R, move from the declared insulation lambda to thickness, e = R × λ, then convert to mm. After that, you decide based on available space, layout and layer continuity.
Tolerances and installed thickness: linings, framing, settlement, bonding/fixing
| Cause | Typical impact (mm) | To record on the quote |
|---|---|---|
| BA13 lining | 13 | Finished total thickness |
| Framing, battens, service cavity | 45 to 70 | Set-asides, supports |
| Blown installation, settlement | +5 to +10% | Initial installed thickness |
| Bonding, fixing plugs, local compression | -5 to -15 | Flatness, supports, fixings |
Keep a site margin when tolerances are tight, especially on sloping ceilings and soffits. A few millimetres lost can drop you below the target R.
Site checks: quick references to confirm the installed thickness matches the calculation
On site, check
- Product reference and declared lambda on the label.
- Measured thickness before closing, at several points.
- No crushing at supports, hangers or fixings.
- Time-stamped photos and measurements recorded in the file.
Practical R-value and thickness examples: what the snippet cannot tell you
Wall example: choosing between two insulations (same R, different thicknesses) and impact on reveals
| Insulation | Lambda λ (W/m.K) | Target R (m².K/W) | Thickness e (mm) |
|---|---|---|---|
| Mineral wool | 0.036 | 4.0 | 144 |
| PIR | 0.022 | 4.0 | 88 |
At constant R, thickness follows e = R × λ. Use the product’s declared insulation lambda. The thickness difference shows up immediately in renovation work with deeper window reveals, jambs that need adjustment and sometimes a redesigned insulation return.
Loft example: gaining R with limited available thickness (and when to switch insulation families)
| Constraint | Lambda λ (W/m.K) | Available thickness (mm) | Resulting R (m².K/W) |
|---|---|---|---|
| Standard blown insulation | 0.040 | 240 | 6.0 |
| More conductive product | 0.032 | 240 | 7.5 |
When rafter depth is limited, you gain R by lowering λ, without compression. Only switch families if the system remains coherent: fire reaction, vapour barrier, density and wind resistance.
Calculation memo to keep: quick “R ↔ thickness” table ready to use in 2026
| Lambda λ (W/m.K) | R (m².K/W) | Thickness e (mm) |
|---|---|---|
| 0.032 | 4 | 128 |
| 0.032 | 6 | 192 |
| 0.040 | 6 | 240 |
Keep this memo with your take-offs. You can move from R to layout planning in a few seconds, then fine-tune using the declared resistances and installation tolerances.
