The thermal resistance formula fits on one line: R = e / λ, the thickness e in metres divided by the thermal conductivity λ in W/m.K, which gives R in m².K/W. That figure governs access to energy saving certificates. To calculate the thermal resistance of a whole element, you add up the R of its layers and the two surface resistances defined by ISO 6946, Rsi on the inside and Rse on the outside: 0.13 and 0.04 for horizontal heat flow. What the French scheme sheets call the additional resistance is that of the insulation you install and nothing else, without the existing layers and without the surface resistances. The sheets in force require R ≥ 7 in a cold loft and R ≥ 6 on rafters (BAR-EN-101), R ≥ 3.7 on walls (BAR-EN-102) and R ≥ 3 on ground floors (BAR-EN-103). Two clauses in those same sheets get read less often: the resistance of insulation already in place does not count towards the total, and seven clear days must separate acceptance of the quote from the start of installation. All three sheets are repealed on 1 May 2027.
Thermal resistance R: what exactly does it mean on an insulation job?
R, lambda and thickness: the concepts to master without mistakes
On site, thermal resistance R indicates how well a wall or surface slows the passage of heat. The higher the R, the more the insulation "blocks the cold". It's calculated with a simple rule: R = thickness (in m) divided by lambda λ (in W/m·K). So a high-performance material installed too thin quickly loses its advantage. And a more "average" material can do the job if the thickness follows.
R and actual performance: the impact of thermal bridges and installation defects
The R on the product data sheet doesn't tell the whole story. Thermal bridges at junctions, compressed insulation, poorly joined cuts, air leaks: all of this causes overall performance to drop. You can have a high theoretical R and a disappointing result in practice. On site, the continuity of the insulation and careful attention to special points often make the difference.
R by wall type: walls, lofts, floors, sloped ceilings... the most common cases
You reason wall by wall, because the constraints aren't the same everywhere. If needed, you can use a thermal resistance calculator to quickly check R based on thickness and lambda.
- Loft insulation. Continuous installation, uniform thickness.
- Sloped ceilings (rafters). Managing space, moisture, ventilation.
- Walls. Treatment of window reveals, slab edges, fixings.
- Ground floors. Watch out for utility runs and access hatches.
Thermal resistance calculation: formula, units and classic pitfalls
The thermal resistance formula: R = e / λ, with the right units
The thermal resistance of a uniform layer is its thickness divided by its thermal conductivity. All the risk of error lies in the units, which data sheets mix freely.
| Quantity | Symbol | Unit used in the calculation | Where to read it |
|---|---|---|---|
| Thermal resistance | R | m².K/W | data sheet, ACERMI certificate, roll label |
| Thickness | e | m | quote, label (often in mm) |
| Thermal conductivity | λ | W/m.K | data sheet (sometimes in mW/m.K) |
The calculation step by step, for 120 mm of insulation at λ 0.032:
- Convert the thickness to metres: 120 mm = 0.12 m.
- Check that λ is in W/m.K: 32 mW/m.K = 0.032 W/m.K.
- Divide: R = 0.12 / 0.032 = 3.75 m².K/W.
The formula turns round to give the thickness you need to install: e = R × λ. For an R of 3.7 with the same insulation, e = 3.7 × 0.032 = 0.118 m, i.e. 120 mm as a commercial thickness. The lower λ is, the better the insulation performs at equal thickness: our article on thermal conductivity λ gives the orders of magnitude by family of materials. On a certified product, the R declared by the manufacturer is what counts, not your own recalculation.
Adding resistances: stacking layers and the case of multilayer insulation
When you stack layers, you add up the R values. Total R = R1 + R2 + ... + Rsi + Rse if you include the surface resistances. For multilayer insulation, don't settle for a "equivalent to... wool" comparison. Rely on the declared R value from standardised tests, and check the installation conditions. For a wall, going from this total R to the U-value is set out in our method for calculating the U-value of walls.
Rsi and Rse are not a matter of choice: they depend on the direction of heat flow through the element, and ISO 6946 sets them.
| Direction of heat flow | Element concerned | Rsi (m².K/W) | Rse (m².K/W) | Rsi + Rse |
|---|---|---|---|---|
| Upwards | ceiling, sloping ceiling, flat roof | 0.10 | 0.04 | 0.14 |
| Horizontal | external wall, gable, wall onto an unheated space | 0.13 | 0.04 | 0.17 |
| Downwards | floor over a crawl space, over a cellar, over an open passage | 0.17 | 0.04 | 0.21 |
Two habits follow from that table. Where the element gives onto an unheated space rather than outside air, the cold side takes the internal value, 0.13 instead of 0.04, because no wind sweeps the surface. And on a ground floor, Rsi + Rse is worth 0.21 m².K/W, close to 7% of the 3 m².K/W threshold: enough to make or break a U-value calculation, never enough to rescue an insufficient additional resistance, since that one is assessed on the installed insulation alone.
Worked example for a multilayer element: concrete block wall insulated from the inside
Take a facade wall in 20 cm hollow concrete blocks, rendered outside, lined on the inside with 120 mm of insulation at λ 0.032 and a 13 mm plasterboard. The resistances of the common materials are tabulated values from the French Th-U rules, to be replaced by those of the actual product when you have them.
| Layer, from inside to outside | Calculation | R (m².K/W) |
|---|---|---|
| Rsi, horizontal flow | fixed value | 0.13 |
| Plasterboard 13 mm | 0.013 / 0.25 | 0.05 |
| Insulation 120 mm, λ 0.032 | 0.12 / 0.032 | 3.75 |
| Hollow concrete block 20 cm | tabulated value | 0.23 |
| Cement render 15 mm | 0.015 / 1.15 | 0.01 |
| Rse | fixed value | 0.04 |
| Total R of the element | sum | 4.21 |
Two readings not to confuse. For the CEE file, only the 3.75 of the installed insulation counts, and it clears the 3.7 wall threshold. For the thermal performance of the home, it is the 4.21 of the whole element that matters, since that is what gives the U-value.
Common mistakes: crushed thickness, moisture, insulation continuity and settling
The classic pitfalls come from the job site. Compressed insulation loses usable thickness, and therefore R. Moisture degrades performance, especially if airtightness and vapour management are poorly handled. Finally, a break in continuity, a thermal bridge, or settling over time cause actual performance to drop.
From R to the U-value: U = 1 / total R
The thermal transmittance U, in W/m².K, is the inverse of the element's total resistance: U = 1 / total R. Where R measures what holds heat back, U measures what gets through, per square metre and per degree of difference between inside and outside. U is what goes into heat loss calculations, the DPE and the Ubat coefficient of the envelope.
On the example wall, with 100 m² of facade and a 20 °C difference between inside and outside:
| Concrete block wall | Total R (m².K/W) | U (W/m².K) | Flow through 100 m² for a 20 °C difference |
|---|---|---|---|
| Uninsulated, plaster inside | 0.46 | 2.17 | about 4,350 W |
| With 120 mm at λ 0.032 | 4.21 | 0.24 | about 475 W |
The flow is U × area × temperature difference. The insulated wall lets nine times less heat through, leaving aside thermal bridges, which are counted separately in W/m.K along the junctions. The inverse relationship also explains why the last centimetres pay back less: going from R 4 to R 8 halves U but only removes 0.125 W/m².K, whereas going from R 0.5 to R 1 removed a whole one. That is the whole point of reasoning about the optimal insulation thickness.
Choosing your insulation based on the target R: how to compare materials without getting fooled
Comparing mineral wool, cellulose, wood fibre, PUR/PIR: a quick read of the data sheets
To compare quickly, start from thermal resistance (R) and lambda. At equal R, the thickness changes depending on the material. Also check fire reaction, mechanical strength (especially on roofs), and usage classification. Mineral wool: good cost ratio. Cellulose: high-performing and worthwhile for blown-in applications. Wood fibre: denser, often useful for summer comfort. PUR/PIR: very insulating at low thickness, but more sensitive to installation quality.
The same R doesn't always mean the same result: thermal mass, phase shift and summer comfort
Two insulation materials with the same R don't always give the same feel. Density, thermal capacity and moisture management affect thermal mass and phase shift. In lofts, a denser solution can limit overheating. Conversely, a very lightweight insulation material can let heat spikes through more quickly. Look at the specific heat data and density, not just the R.
Matching R to the wall type and the building: older buildings, partial renovation, ITE vs ITI
You don't aim for the same R in walls, sloped ceilings or floors. In older buildings, the right choice avoids moisture pitfalls. In a partial renovation, hunt down thermal bridges and check the continuity of the vapour barrier. External wall insulation (ITE) protects the masonry better and limits thermal bridges, but changes the facades. Internal wall insulation (ITI) is simpler, but requires flawless installation and well-treated special points. To go further on choosing between these two approaches, see ITI vs ITE.
Thermal resistance and 2026 requirements: what the aid schemes and trade rules demand
What R to target in 2026: thresholds by element and matching thicknesses
In renovation, thermal resistance (R) is the threshold that opens access to the grant. The values below are those of the standardised operation sheets in force since 1 January 2025, with the minimum thickness they imply, worked out from e = R × λ for two common insulants.
| Element insulated | Minimum R of the insulation installed | Thickness at λ 0.040 | Thickness at λ 0.032 | Scheme sheet |
|---|---|---|---|---|
| Cold loft | ≥ 7 m².K/W | 280 mm | 224 mm | BAR-EN-101 |
| Roof rafters | ≥ 6 m².K/W | 240 mm | 192 mm | BAR-EN-101 |
| Wall, facade or gable | ≥ 3.7 m².K/W | 148 mm | 119 mm | BAR-EN-102 |
| Ground floor over a basement, crawl space or open passage | ≥ 3 m².K/W | 120 mm | 96 mm | BAR-EN-103 |
These thicknesses are calculated minimums: round up to the next commercial thickness, and order blown insulation with an allowance for settling on top. The conditions of each sheet are detailed in our article on the CEE sheets BAR-EN-101 to BAR-EN-107.
Outside the grant schemes, two other texts come up in clients' questions. The RE2020 only covers new construction and sets no R per element: it judges the whole building on its needs (Bbio) and its consumption. In a renovation without a grant, the RT existant element by element (arrêté of 3 May 2007 as amended) applies, with minimums assessed on the whole element and, most of the time, below those of the CEE sheets. Aiming for the CEE threshold therefore covers both cases.
Three details decide the fate of a file. The R is that of the insulation installed: whatever was already there is not added to it, even where it is sound and kept in place. It is assessed to NF EN 12664, 12667 or 12939 for non-reflective insulants, and to NF EN ISO 22097 for reflective ones, which closes the door on the equivalences claimed by thin multifoils. Finally, all three sheets are repealed on 1 May 2027, so operations have to be committed before that date.
MaPrimeRénov' and CEE files: how to document R with solid evidence
To avoid back-and-forth, rely on a manufacturer's data sheet or an ACERMI certificate. On the quote and invoice, include the thickness, lambda, surface area, R and the exact product reference. On the CEE side, the sworn statement must match the job site, with no grey areas: Argile is connected to the scheme providers, so the grant is figured into the pricing and the file leaves with the right paperwork.
Two calendar obligations belong in the sales process, because losing them loses the grant. The technical survey of the building happens before the quote is drawn up, and it is that survey which settles whether existing insulation is kept, made good or stripped out. Then a minimum of seven clear days runs between acceptance of the quote and the laying of the insulant. A job signed on Monday cannot start before the Tuesday of the following week.
RGE and inspection: traceability, reports, labelling and evidence in case of verification
During an inspection, you're mainly asked to prove what was actually installed. Keep the labelling (dated photos of the rolls or panels), delivery notes, the handover report and before/during/after photos. The date of the prior technical survey has been part of the required evidence since the 2025 version of the sheets: record it in the file rather than trusting memory.
Practical thermal resistance calculation cases: three worked figures to reuse as they stand
Loft insulation: calculating R with blown wool and checking the final thickness
Simple reminder: thermal resistance is calculated with R = e/λ. Example: blown wool, λ 0.040. Target R 8. You need e = 8 x 0.040 = 0.32 m. On site, announce a slightly higher blown thickness to account for the final thickness after settling, then check with a ruler.
Walls: calculating R for internal lining and watch out for insulation breaks/returns
Internal lining. Insulation λ 0.032 at 120 mm. R = 0.12/0.032 = 3.75, just above the 3.7 threshold. That R is only reached if the insulation is continuous. Take care with insulation returns at window reveals and slab edges, and keep an eye on thermal bridges around supports.
Ground floors: calculating R and managing special points (hatches, edges, utility runs)
Underside of the floor. Panel λ 0.035 at 100 mm. R = 0.10/0.035 = 2.86. That is below the threshold of 3 in sheet BAR-EN-103: you need e = 3 × 0.035 = 0.105 m, in practice a 110 or 120 mm panel. For the result to be real, treat the special points. Insulated hatch, continuity at the edges, and utility runs passed through without holes left "in a draught".
To run these calculations on a multilayer element without doing the divisions by hand, Argile's thermal resistance calculator adds up the layers, applies Rsi and Rse for the element type and gives the matching U-value.
