Most people start with the windows. Yet it is the least cost-effective job of all, and that is a joiner who fits windows telling you so. Here is the order that makes every euro count, the ADEME figures to understand it, and the particular case of our stone houses in the Pyrenees.
When I coordinate a full renovation, insulation is always the first thing people want to discuss, and often in the wrong order. To put the ideas straight, one table is enough: where does the heat actually escape?
| Area | Share of losses | The right reflex |
|---|---|---|
| Roof and loft | 25 to 30% | Deal with it first |
| Walls | 20 to 25% | Second job |
| Air leaks and air renewal | 20 to 25% | Ventilation to review at every stage |
| Windows | 10 to 15% | Last, unless single glazing |
| Ground floors | 7 to 10% | If the cellar or crawl space is accessible |
| Thermal bridges | 5 to 10% | Dealt with along with the walls |
Hot air rises: the roof is where a house empties itself of its heat. And it is also where insulation costs the least. A non-converted loft can be blown with wool insulation for €20 to €35 per m², a morning's work on site, and the job pays for itself in 3 to 5 years on the heating bill. No other energy renovation job does better. Walls come next: allow €40 to €90 per m² from the inside, €120 to €270 from the outside. Windows come last in pure cost-effectiveness, at around 10 to 15% off the heating bill for several thousand euros. They win on another front, comfort: no more cold surface, noise or draughts. I explain when they are genuinely worth it in when to replace your windows.
The rule I repeat on every visit: heating comes last. A boiler or a heat pump is sized to the needs of the house. If you replace it before insulating, you pay for a machine that is too powerful for a house that wastes heat. I saw the case in the Campan valley: a new boiler installed a year before the loft and walls were insulated, the result being an oversized unit that runs in fits and starts and wears out faster. The full order of a project, trade by trade, is in what order to do your renovation work in.
That is ADEME's instruction, word for word, and it is the one people forget most. Insulating means plugging air leaks. But those leaks were also how the house breathed. If the ventilation is not reviewed, the moisture produced by daily life, a family gives off several litres of water vapour a day, has no way out: condensation, mould, stale air. A costs between €350 and €1,050 fitted: on an insulation budget, it is the cheapest line and the most important one.
The gold-underlined words are explained on hover. All definitions are in the joinery glossary.
In Gerde, in Bagnères, in the valleys, many houses have stone walls 50 or 60 cm thick. These walls live with moisture: it comes in, it goes out, and the balance has held for a century. The worst thing you can do to them is wrap them in airtight materials, polystyrene and plastic vapour barriers first among them. The moisture ends up trapped in the wall: hidden condensation behind the lining, saltpetre, embedded floor timbers that rot, stone that freezes and crumbles. I have stripped out enough 1980s wall linings to tell you this is not theory.
On these walls, you insulate with materials that let water vapour through: wood wool or wood fibre, hemp-lime, cork, with a suitable vapour-control membrane rather than an airtight film. It costs a little more per m², and it is what keeps the house healthy. That respect for old buildings is the common thread of my article renovating an old house: where to start.
The landscape has moved a lot. MaPrimeRénov', France's main renovation grant, reopened in February 2026 after a suspension, but wall insulation has been dropped from the job-by-job route: it is now only funded within large-scale renovations, with a compulsory audit and support adviser. Lofts and pitched roofs are still funded job by job, windows only when replacing single glazing. The CEE incentives, rebates funded by the energy suppliers, continue at around €10 to €13 per m² on lofts. And VAT at 5.5%, the reduced French rate for energy-saving work, applies to this work in homes over two years old, provided the materials fitted reach the required performance levels. In the Hautes-Pyrénées, the department's Rénov'Occitanie desk offers free, neutral advice: make use of it.
A point of transparency: I am not RGE-certified (RGE is the French eco-certification an installer needs for most grants). The 5.5% VAT does not depend on that label: it rests on the performance of the materials fitted, not on the tradesman's certification. MaPrimeRénov', the CEE incentives and the éco-PTZ, France's interest-free eco-loan, do require an RGE tradesman: for those jobs I tell you from the quote onwards, and when I coordinate a renovation, I bring in RGE firms on the lots where the grants weigh heavily. My added value lies elsewhere: the right order, the building respected, and a single point of contact. That is exactly the point of my pre-works advice visit.
Almost always with the non-converted loft: it is the cheapest job (€20 to €35 per m² blown), the one that stops the most heat loss, and it pays for itself in 3 to 5 years. With the rest of the budget, you review the ventilation. A watertight roof, an insulated loft, healthy air: it is the best first step there is.
Spreading it out is entirely possible, provided you follow the right order and keep an overall vision: each stage must prepare the next, not get in its way. It is when you insulate piece by piece without a plan that condensation problems and thermal bridges appear. An audit or an advice visit at the start costs little and avoids these traps.
Thermally, it is the best: it removes thermal bridges and does not eat into the rooms. But it costs 2 to 3 times more than insulating from the inside, and on a beautiful stone facade it is aesthetically unthinkable. On the old houses around here, we often favour the inside, with materials that let the wall breathe.
I come and see your house, I spot the real priorities, and I give you a plan of attack in the right order, with the right materials. Then you decide, item by item. I speak English, so feel free to call or write in English.
+33 6 03 57 45 47Reply within 48 hours, or in writing: contact@agencementklein.fr
