Thermal resistance R is expressed in m².K/W, is worked out from R = e/λ and governs access to energy saving certificates. The total resistance of an element is the sum of its layers plus 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 All three sheets are repealed on 1 May 2027.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.
Calculating R correctly in practice: formulas, units and classic pitfalls
The basic formula: R = e / λ, with the right units (m and W/m·K)
The thermal resistance of a layer is calculated simply. Take the thickness in m and divide it by the conductivity λ in W/m·K. Example: 120 mm of insulation, i.e. 0.12 m, with λ = 0.032 gives R = 0.12 / 0.032 = 3.75 m²·K/W. Watch out with data sheets: they sometimes give λ in mW/m·K or the thickness in mm.
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.
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.
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.
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
2026 benchmarks: minimum R expected by insulation work type
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.
| Element insulated | Minimum R of the insulation installed | Scheme sheet |
|---|---|---|
| Cold loft | ≥ 7 m².K/W | BAR-EN-101 |
| Roof rafters | ≥ 6 m².K/W | BAR-EN-101 |
| Wall, facade or gable | ≥ 3.7 m².K/W | BAR-EN-102 |
| Ground floor over a basement, crawl space or open passage | ≥ 3 m².K/W | BAR-EN-103 |
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. This R is theoretical 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 = 2.85. 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".
