Understanding the R-value: definition, unit and what is actually enforceable
In insulation work, the R-value is used to describe the declared performance of a material at a given thickness. It is a product data point that should be properly recorded in your job documentation.
Simple definition of the R-value: thermal resistance of the insulation, not of the whole wall
The R-value is the thermal resistance of a layer of insulation. The basic formula is R = e/λ, with e in m and λ in W/m·K. Do not confuse this R with the R-value of the complete wall assembly. The wall also includes supports, linings, air gaps and surface resistances. Classic trap. A thickness entered in cm instead of m creates an error by a factor of 100.
Unit and reading: m²·K/W, why a higher R-value insulates better
The unit is m²·K/W. The higher the R-value, the more heat flow is resisted for the same surface and temperature difference. Resistances add up by layer. For a single homogeneous layer, U = 1/R. For a wall assembly, work from the sum of the useful R-values and a global U-value.
Enforceable value: ACERMI certificate or product DoP for the installed product
During inspections, the enforceable value is the one on the ACERMI certificate or the DoP for the installed product, not a sales brochure figure. Record the exact reference, thickness, declared λ, ACERMI number or DoP identifier. And for grants, check the minimum R required by the current version of the CEE sheets or MaPrimeRénov’ rules at the time of commitment.
R-value formula: R = thickness / λ (lambda) and common mistakes to avoid
The formula to remember: thickness in metres (m) divided by lambda λ (W/m·K)
The R-value is calculated simply. R = e/λ, with e in metres and λ in W/m·K. The result is expressed in m²·K/W. Example: 120 mm of insulation with λ = 0.032 gives e = 0.12 m and R = 3.75. Note the unit as soon as you measure, then lock it into your spreadsheets.
The classic trap: calculating in centimetres (x100 error) and how to spot it
If you enter 12 instead of 0.12, you multiply R by 100. The clue is immediate. A common insulation product does not come out at R = 150. Do a rough sanity check. 0.10 m with λ = 0.040 gives R of around 2.5. On quotes and calculation sheets, enforce a unit-consistency check.
Keep the distinction clear: insulation R-value vs thermal resistance of the complete wall
The R above applies to the insulation only. For the complete wall, add the resistance of each layer, plus the internal and external surface resistances according to the method in force. That total R is what is used for technical compliance. In your documents, clearly write “insulation R” and “wall R”.
Layer-by-layer addition: calculating an overall R-value when you stack several insulation layers
Total R = R1 + R2 + …: layer-by-layer method (with linings kept separate)
When layers are stacked on the same wall, you add their thermal resistances. For each layer, calculate R = e/λ with e in m. Keep linings (plasterboard, OSB, etc.) separate if you are declaring only the insulation on the grant line.
| Layer | Thickness (m) | Lambda (W/m.K) | R (m².K/W) |
|---|---|---|---|
| Mineral wool | 0.100 | 0.035 | 2.86 |
| PU board | 0.045 | 0.032 | 1.41 |
| Total | 4.27 |
Common renovation case: internal lining + additional loft insulation, how to add them without mistakes
You do not add a wall lining and additional loft insulation together. These are two different assemblies, so they require two separate R-values. Only add together the layers that are continuous along the same thermal path, and check the minimum R required by the current operation sheet.
On-site check: verify that the declared R matches the thickness actually installed
On site, record the installed thickness (mm), the product reference and the λ value from the ACERMI certificate or performance declaration. Recalculate the expected R-value and compare it with the quote and inspection reports. Classic trap: using cm instead of m, or confusing the insulation R with the complete wall R.
Moving from R-value to U-value performance: link with U = 1 / R and impact on the wall
Quick conversion: U (W/m²·K) = 1 / R and how to read the order of magnitude
The U-value is the inverse of R. The lower the U, the better the wall resists heat losses. Quick order-of-magnitude guide.
| Total R (m²·K/W) | U (W/m²·K) | Site reading |
|---|---|---|
| 2 | 0.50 | Good insulation |
| 4 | 0.25 | Good level |
| 6 | 0.17 | Very high performance |
For a wall/roof/floor: add all other resistances (supports, air gaps, finishes) before concluding
The R to invert is the total R. Add the real layers: insulation, support, linings, renders, air gaps, plus internal and external surface resistances. The classic mistake is to conclude from the insulation R alone, when the complete wall does not match.
What clients and design offices ask for: when to speak in R and when to speak in U
In quotes, the R-value is used to describe the installed product. Enforceable value. ACERMI certificate or performance declaration. In a design office, U is used more often to integrate the wall into a calculation, for example in a full thermal assessment. Thermal bridges are then handled with their dedicated coefficients.
2026 table: insulation thickness needed to reach typical R-values according to lambda
How to use the table: choose your lambda, target the required R, read the minimum thickness in cm
During the technical visit, start from the certified lambda of the product (ACERMI or DoP). Then choose your target thermal resistance and read the corresponding thickness. The table gives a minimum thickness in cm, rounded up. Keep the insulation ordered, the thickness actually installed and the R-value shown on the quote consistent.
Thickness table (cm) = R × λ × 100: examples for λ 0.032 / 0.035 / 0.038 / 0.040 / 0.045
Units: R in m².K/W. λ in W/m.K. Formula: e (cm) = R × λ × 100. Common trap: entering cm as if they were m, which is a factor-100 error.
| Target R (m².K/W) | λ 0.032 (cm) | λ 0.035 (cm) | λ 0.038 (cm) | λ 0.040 (cm) | λ 0.045 (cm) |
|---|---|---|---|---|---|
| 3.7 | 12 | 13 | 15 | 15 | 17 |
| 4.0 | 13 | 14 | 16 | 16 | 18 |
| 6.0 | 20 | 21 | 23 | 24 | 27 |
| 7.0 | 23 | 25 | 27 | 28 | 32 |
Grants 2026: check the minimum R by wall on the current sheets and keep proof (ACERMI/DoP)
For CEE and 2026 schemes, never rely on a “usual” threshold. Check the minimum R by wall on the current standardised sheet on the commitment date. During inspection, the enforceable value is the one on the ACERMI certificate or the DoP for the installed product. Record: product reference, thickness, treated area, retained R-value and supporting documents in the file.
