The rafter slope is the sloping face of the roof seen from inside, the one that becomes the pitched ceiling of a converted loft. To insulate it, the thermal resistance to reach is 6 m²·K/W, against 7 in a cold loft: the French CEE measure BAR-EN-101, version vA64-6 applying to work committed since 1 January 2025, sets both thresholds and states that existing insulation is not counted. Turned into depth by e = R × λ, that is 18 cm at 0.030 W/(m·K) and 25.2 cm at 0.042, where a common rafter offers 80 mm. The whole fitting method follows from that gap: one layer between the rafters, a second cross-battened layer under them on hangers, and a continuous warm-side vapour barrier with an Sd above 18 m in the sense of NF DTU 45.10. The rest is what goes on the invoice so the audit passes.
Rafter slope: definition, and how it differs from a cold loft
The sloping part under the covering, in the plane of the rafters
The rafter slope is the underside of a roof pitch: the inclined plane carried by the rafters, between the wall plate at the eaves and the ridge. In a converted loft it serves as the ceiling, and so it is what separates the room from the outside. Insulating on the rafter slope means making the insulation follow the pitch instead of laying it on the floor. The full vocabulary of the roof structure, from purlin to plasterboard (in French) reads off a roof-space section.
Cold loft or loft converted under the slope: two different envelopes
The difference does not lie in the roof, it lies in what is heated. A cold loft stays cold: the insulation goes on the floor, over the ceiling of the top storey, and it is the cheapest measure in the building, covered in our article on cold-loft insulation. A loft converted under the slope joins the heated volume: the insulation follows the pitch, the gables and the dormer cheeks, and it goes in as boards or rolls rather than blown loose-fill.
| Criterion | Cold loft | Loft converted under the slope |
|---|---|---|
| Plane insulated | Loft floor | Roof pitches, between and under rafters |
| R required by BAR-EN-101 | 7 m²·K/W | 6 m²·K/W |
| Usual product | Blown wool or cellulose | Rolls and boards, two layers |
| Vapour barrier | Depends on the substrate | Continuous on the warm side, Sd > 18 m |
| Effect on the volume | None | Headroom and floor area lost |
The R required on rafters, and why it is not the cold-loft figure
Two thresholds in one measure, and the line where files fail
BAR-EN-101 covers the cold loft and the rafter slope in a single document, with an identical payment and two different requirements. That asymmetry is what causes rejections, because it turns on the nature of the substrate rather than on the product installed.
| Measure | R required on the insulation fitted | Payment in zone H1 | Zone H2 | Zone H3 |
|---|---|---|---|---|
| Cold loft | 7 m²·K/W | 1,700 kWh cumac/m² | 1,400 | 920 |
| Rafter slope | 6 m²·K/W | 1,700 kWh cumac/m² | 1,400 | 920 |
The payment is rigorously the same on both sides, only the requirement changes. The conventional lifetime is 30 years, and the measure is withdrawn for work committed from 1 May 2027. The full schedule and its calculation by area are set out on our BAR-EN-101 sheet.
Existing insulation does not count towards the R
The measure is explicit: the thermal resistance of any existing insulation is not taken into account. On rafters already carrying 100 mm that are being topped up, the R retained is that of the added layer alone, and it has to reach 6 on its own. That is the most ordinary reason a file is refused, and it is settled on the survey by measuring what is in place before pricing. The measure in fact requires a decision at that point: either the existing insulation can stay as it is, or it is made good or stripped out during the works, and that call is made before the quote, not in front of the van.
From R to thickness: e = R × λ, insulant by insulant
Thickness is not negotiated, it is calculated. At constant R only lambda moves the line, and thermal conductivity remains the parameter that governs everything else.
| Declared lambda | Total thickness for R = 6.0 | Balance under an 80 mm rafter |
|---|---|---|
| 0.030 W/(m·K) | 18.0 cm | 10.0 cm |
| 0.032 | 19.2 cm | 11.2 cm |
| 0.035 | 21.0 cm | 13.0 cm |
| 0.038 | 22.8 cm | 14.8 cm |
| 0.040 | 24.0 cm | 16.0 cm |
| 0.042 | 25.2 cm | 17.2 cm |
The right-hand column is the one that decides the job: it says how many centimetres come off the habitable volume, and it is the figure to put in front of the client before anyone talks about finishes.
Insulating a rafter slope from inside: the method that holds 6 m²·K/W
Survey the substrate before pricing
Four measurements are enough and none of them can be guessed from the ground. The real depth of the rafters, taken at several points on the slope, because a traditional roof structure is not calibrated. The spacing, which gives the cut width and the number of rolls. The presence and type of sub-roof membrane, breathable or not, which decides the air gap. And the condition of the timber, moisture and infestation included, because timber treatment happens before the insulation and never after.
The cross-battened second layer, and how the depth is split
The first layer goes between the rafters, at the exact depth of the timber, without compression and without a gap at the verge. The second goes in cross-battened underneath, on metal hangers and furring, and covers the rafters: that is the layer that removes the structural cold bridge, which accounts for a far from negligible share of the slope area at 400 mm centres. Putting the whole depth between the rafters is impossible once the target is R = 6, and forcing insulation in under compression degrades the declared lambda, and so the real R. Continuity of the layer is the rule that takes precedence here over ease of fitting. The hangers, furring and boards of the underside are then priced on the slope area, which plasterer quoting software takes from the site survey rather than from a sketch copied out at the office.
The ventilated air gap under the covering, when the membrane does not breathe
With no sub-roof membrane, or with a non-breathable one, the insulation must not touch the roof deck: a ventilated air gap is kept, open at the eaves and at the ridge. This is where a cold roof gets turned into a warm roof without anyone noticing, and the choice between the two build-ups is settled before the insulation is ordered. Blocking the eaves ventilation with the first layer is the move that manufactures pathologies two winters later.
The vapour barrier: Sd, continuity and a service void
Sd on the warm side, and what the CEE measure expects of it
NF DTU 45.10 works to a vapour barrier with an Sd above 18 m fitted on the warm side, meaning the inside, under the whole of the insulation. BAR-EN-101 sets no value but imposes the principle: a vapour barrier or any equivalent device is fitted where the insulation has to be protected from moisture transfer, and it appears in the list of associated works the evidence of completion must name. A smart variable-Sd membrane earns its place under a breather membrane, where the build-up has to be able to dry inwards; that choice rests on a hygrothermal calculation or the manufacturer's opinion, never on the fitter's habit.
The service void, the only way to run services
The membrane goes on in strips with overlaps, tapes and sealants from the same system, returned to the gables, the purlins, the roof-window frames and the flues. Underneath it, a 40 to 60 mm service void between hangers and furring takes the conduits and the boxes. That void is what lets the electrician work without piercing the membrane, and its absence is why so many correctly insulated rafter slopes still leak air. The boxes that remain are dealt with as airtight boxes, as on any other element of the envelope.
The penetrations that produce the claim
Three families concentrate the failures. Recessed downlights, which need boxing or a protective screen and which are named among the works the measure expects. Flues, whose clearance distance comes from the flue manufacturer's instructions and not from the standard alone. Roof windows, where the membrane has to be returned to the frame with the manufacturer's parts rather than folded over and stapled. Each of these is photographed before closing up, because none of them can be checked once the board is on.
Insulating rafter slopes from outside: sarking when the depth is not there
Continuous insulation above the rafters
When the habitable volume cannot give up 18 to 25 cm, the insulation goes above the structure. Sarking gives a continuous layer of rigid boards, with no structural cold bridge and no loss of headroom under the slope, but it forces the covering off. The applicable reference and the real added depth to allow for are set out in our article on sarking.
What the added depth costs at verges and ridge
Fitting 140 mm of PIR above the rafters lifts the roof plane by the same amount, and everything touching that plane follows: verges, eaves, stacks, roof-window flashings, vent terminals. Those reworks are priced line by line, and they often weigh more than the insulation itself. That is also what makes sarking hard to defend outside a re-roof that has already been decided: on a sound covering, the arithmetic does not close.
Trading habitable volume against stripping the roof
The decision rule comes down to two questions. Is the covering at the end of its life, in which case stripping is on the programme anyway and sarking becomes the obvious option. And does the headroom under the slope after internal insulation stay acceptable, which is checked with a tape on site and not on the drawing. Between the two, the hybrid exists, a reduced layer between the rafters plus thin sarking, but it calls for a check on where the dew point sits in the build-up.
The headroom lost under the slope, to price before the insulation
How far the 1.80 m line moves
In a converted loft, every centimetre fitted under the rafters is paid for in floor area. French habitable area and floor area alike only count the parts more than 1.80 m high, and measurement disputes come precisely from lofts. A thickness e taken at right angles to the slope moves that line inwards by e divided by the sine of the pitch. With λ 0.035 and an 80 mm rafter, 13 cm of insulation remain under the rafters, plus a 5 cm service void and 13 mm of board, about 19 cm in all.
| Roof pitch | Shift of the 1.80 m line (19 cm under rafters) | Area lost, two pitches over 8 m of length |
|---|---|---|
| 30° | 38 cm | 6.1 m² |
| 40° | 30 cm | 4.7 m² |
| 45° | 27 cm | 4.3 m² |
| 55° | 23 cm | 3.7 m² |
The shallower the pitch, the heavier the loss: it is on roofs at 30 or 35° that sarking or a low-lambda insulant makes the best case. The calculation is done with the dimensions measured on site, not those on the drawing, and the full trade-off between thickness and floor area is set out in our article on converting a loft.
Price of rafter insulation per m², and grants in 2026
Orders of magnitude, not a price list
The ranges below are orders of magnitude commonly seen on the French market, excluding VAT, supply and fitting, before grants. They move with access, height, the condition of the roof structure and the number of roof windows, and they do not replace a price built on a survey.
| Solution | Order of magnitude excl. VAT | What moves the price |
|---|---|---|
| From inside, mineral wool, two layers + membrane, board excluded | €40 to €70/m² | Lambda, access, number of penetrations |
| From inside, plasterboard and finish included | €70 to €110/m² | Dormer cheeks, dormers, hatches |
| From inside, wood fibre | + €15 to €30/m² | Greater depth for the same R |
| From outside, sarking | €150 to €250/m² | Re-roofing, verges and stacks on top |
From inside, the gap rarely comes from the insulation: it comes from the treatment of roof windows, flues and downlights, which the quote should price as separate lines.
What remains in grants, and what has gone
The CEE payment under BAR-EN-101 remains the lever for the measure on its own, at the lump sum in the table above, multiplied by the area insulated, as long as the work is committed before 1 May 2027. MaPrimeRénov' has changed in nature: since decree no. 2026-822 of 25 August 2026, roof and loft insulation has left the single-measure pathway for applications filed from 1 September 2026, and it is the filing date that decides the regime (in French). Rafter slopes are now funded by MaPrimeRénov' only within the supported pathway, as part of a whole-house renovation. The 5.5% VAT rate and the zero-rate eco-loan (éco-PTZ) remain available on the energy part, to be kept clearly apart from the fit-out lines on the quote.
The file: what gets recorded, and in what order
The technical survey before the quote, and the seven clear days
The sequence is imposed and it is checked. The technical survey of the building takes place before the quote is drawn up; the contractor confirms there that the insulation system suits the building and rules on any existing insulation. Then at least seven clear days separate acceptance of the quote from the start of fitting. A survey dated after the quote, or fitting started the day after signature, is enough to bring the whole operation down. The rules of that sequence and the other traps in the file are set out in our article on securing certification and CEE paperwork.
The mandatory wording on the evidence of completion
The invoice carries the fitting of loft or roof insulation, the make and reference of the insulant, its thickness and the area installed, the thermal resistance of the insulation fitted, the associated works carried out and the date of the survey. Where the thermal resistance is absent, the thickness becomes mandatory for references sold in several depths. The contractor's certification has to fall under the relevant categories of the qualification decree. That wording is prepared at pricing stage, from a catalogue where thermal resistance and thickness are already attached to the reference, rather than re-keyed onto the invoice.
Photographs and self-checks before closing up
Before boarding, a short series and always the same one: the membrane fitted and its overlaps, the return to the gables and at the eaves, the treatment of roof windows, the boxing around downlights and flues, the hatch upstand. A smoke test, or a simple blower door where the job warrants it, completes the file. Plasterwork, hatches and making good are priced as second-fix works in their own right: that is the part a cheaper quote absorbs into a lump sum, and the part that ends up in dispute. That leaves summer comfort, which depth alone does not solve, and whose levers in a converted loft are dealt with at the same time as the winter insulation.
