In an older house, wall insulation is one of the most effective ways to cut heat losses. But the choice between external wall insulation (ITE — isolation thermique par l'extérieur) and internal wall insulation (ITI — isolation thermique par l'intérieur) is not only a question of price: it decides the actual performance, the square metres you keep, the permits you will need, and — since 2026 — the subsidy route you are entitled to.
This decision has become structural for one precise reason. Since 1 January 2026, wall insulation — internal as well as external — has been excluded from the single-action route of MaPrimeRénov' (ministerial order of 12 June 2026): it can now only be funded as part of a full-scale renovation, with an energy audit, an approved supervisor and a gain of at least two classes on the DPE (Diagnostic de Performance Énergétique — the French Energy Performance Certificate). Getting the ITE/ITI trade-off wrong now risks an extra cost of several thousand euros and a rating that stalls. Here is how to decide.
ITE often costs close to double ITI — as a guide, around €100 to €250 per m² of insulated wall for ITE against €50 to €90 per m² for ITI, according to ADEME ranges; but ITE handles thermal bridges better and does not eat into your living space.
What this article covers
The technical difference between ITE and ITI, their respective impact on performance and thermal bridges, the cost ranges published by ADEME and APUR, the loss of living space specific to ITI, the planning and homeowners'-association constraints that may force one or the other, the 2026 subsidy framework (MaPrimeRénov', CEE, eco-PTZ, 5.5% VAT), an illustrative worked example, the common mistakes and a decision grid based on your situation.
ITE and ITI: two ways to insulate walls, two opposite logics
Wall insulation aims to reduce losses through the vertical walls — a work item that, according to ADEME, accounts for around 20% to 25% of the heat losses of an uninsulated detached house. Two techniques exist, and they are not equal on every criterion.
External wall insulation (ITE) wraps the building in an insulating coat fitted to the outer face of the walls, finished with render or cladding. Internal wall insulation (ITI) places the insulant on the room side, behind a counter-partition (usually a plasterboard lining). The first acts as a continuous coat; the second, as an internal lining, room by room.
Handling thermal bridges: the decisive advantage of ITE
A thermal bridge is a break in the insulation through which heat escapes — typically the junctions between walls and floors, balconies and window surrounds. This is where ITE and ITI diverge most clearly.
ITE forms a continuous envelope that wraps the façades and neutralises most of the structural thermal bridges (intermediate floors, partition walls). ITI, fitted inside, is interrupted at every floor and every partition wall: these junctions remain weak points, harder to treat, where thermal bridges persist and where the risk of condensation and mould is higher if workmanship is poor. At a comparable insulant thickness, ITE therefore delivers higher overall envelope performance.
Profile: detached house from the 1970s, uninsulated block walls, façades with no heritage constraint — the case where ITE expresses its full potential, with no loss of space and complete treatment of thermal bridges.
Thermal mass and summer comfort: a point for ITE
By placing the insulant outside, ITE keeps the mass of the walls (the block, the stone, the brick) on the heated side. This mass acts as a thermal flywheel: it dampens temperature swings and improves summer comfort, an issue set to grow in importance in the DPE — summer comfort is due to be strengthened in the certificate by 2028 under the French national climate-change adaptation plan. ITI, which insulates on the inside, deprives the home of this mass: the room heats up and cools down faster.
When ITI remains the right choice
ITI keeps real strengths. It is cheaper, generally requires no heavy planning permission, can be carried out room by room, and does not change the building's external appearance — a decisive point when the façade is protected or the homeowners' association refuses to alter it. On a dressed-stone façade, on timber framing, or in a protected heritage area, ITE is often simply prohibited: ITI then becomes the only way to insulate the walls.
Cost, space, constraints: the ITE vs ITI comparison
The choice between ITE and ITI turns on four variables: cost per square metre, living space lost, permits required and performance expected. The ranges below are orders of magnitude published by ADEME and APUR (Atelier parisien d'urbanisme — the Paris urban-planning agency); the actual price depends on the insulant, the substrate, the finish (render or cladding for ITE) and the region. Treat them as benchmarks, not as a quote.
| Criterion | ITE (external) | ITI (internal) |
|---|---|---|
| Indicative cost per m² of wall | ~€100 to €250/m² (render or cladding), per ADEME | ~€50 to €90/m², per ADEME |
| Cost gap | ITE often costs close to double ITI, at comparable insulant (ADEME/APUR order of magnitude) | |
| Thermal bridges | Largely treated (continuous envelope) | Persist at floors and partition walls |
| Living space | Preserved | Reduced (thickness of the lining) |
| Thermal mass / summer comfort | Preserved (mass on heated side) | Reduced |
| External appearance | Altered (façade reworked) | Unchanged |
| Delivery | Whole-building site, scaffolding | Possible room by room |
The loss of living space: the hidden cost of ITI
ITI is paid for in square metres. The insulating counter-partition has thickness — of the order of 10 to 16 cm on each wall facing outwards, insulant and board included. On a flat or a modestly sized house, this thickness translates into a noticeable reduction in living space, and therefore in the property's value. As a guide, for a 70 m² home where several outer walls are lined, the loss can amount to a few square metres — a trade-off to weigh against the price per square metre in your area, which can wipe out part of the saving made on the cost of the works. ITE, fitted outside, does not affect the interior space.
Planning and homeowners' associations: who decides?
ITE alters the building's external appearance: it therefore falls under planning rules. A prior works declaration is generally required, and the local urban plan (PLU — Plan local d'urbanisme) may regulate the appearance of façades, or even prohibit ITE in certain areas. Near a listed monument or in a remarkable heritage site, the opinion of the architecte des Bâtiments de France (the heritage architect) may be required and may lead to a refusal. The law has, however, made ITE easier by allowing, under conditions, an exceedance of building-envelope rules for external insulation works (French planning code).
In a homeowners' association (copropriété), external walls are common parts: ITE must be voted at a general meeting. The legislator has lowered the majority required for energy-saving works to make these decisions easier, but collective approval remains an unavoidable prerequisite. If the association refuses ITE, the individual owner has, for their walls, only ITI as an individual option.
2026 subsidies: ITE and ITI reserved for full-scale renovation
This is the major change that makes the choice urgent. Since 1 January 2026, wall insulation — ITI as well as ITE — has been excluded from the single-action route of MaPrimeRénov', the French national renovation grant distributed by the French National Housing Agency (ANAH). The change was set by the ministerial order of 12 June 2026 (amending the order of 17 November 2020): these work items can now only be funded by MaPrimeRénov' as part of a full-scale renovation.
In practice, funding your wall insulation through MaPrimeRénov' in 2026 means meeting three cumulative conditions: a prior energy audit, a contract with a Mon Accompagnateur Rénov' (MAR, the approved supervisor who follows the project), and a works bundle targeting a gain of at least two DPE classes. Added to this, since the application window reopened on 23 February 2026, is a mandatory preliminary appointment with a France Rénov' adviser (the public home-renovation service) before filing any full-scale application. A quote offering you external insulation "funded by MaPrimeRénov'" as a single action in 2026 is based on outdated rules — or on deliberate fraud.
The other levers remain available and can be combined. Energy Saving Certificates (CEE — Certificats d'Économies d'Énergie, bonuses funded by energy suppliers) still support wall insulation. The eco-PTZ (zero-rate renovation loan, up to €50,000) funds the out-of-pocket balance. The reduced 5.5% VAT applies to energy renovation works carried out by a company. And all these MaPrimeRénov' subsidies require an RGE-certified company (Reconnu Garant de l'Environnement).
⚠️ Warning: before building your subsidy plan, check your 2026 rating. The reform of the primary-energy conversion coefficient for electricity (the 2.3 factor reduced to 1.9 in the 3CL-DPE methodology, ministerial order of 13 August 2025 published in the Official Journal on 26 August 2025, in force on 1 January 2026) moved many electrically heated dwellings out of classes F and G with no works. If your home is no longer a thermal sieve, the two-class gain required for a full-scale renovation may change in nature.
Case study: an F-rated house with uninsulated walls (illustrative)
To make the trade-off concrete, let's take an illustrative case — the amounts are indicative, within ADEME ranges, and do not replace a quote or a simulation. You own a detached house from the 1970s, rated F, whose 120 m² of external walls are uninsulated. Wall insulation is the priority work item.
Scenario — ITE vs ITI comparison on 120 m² of wall (ADEME orders of magnitude)
| Item | ITE | ITI |
|---|---|---|
| Indicative cost per m² | ~€150/m² | ~€70/m² |
| Wall area treated | 120 m² | 120 m² |
| Cost of works (order of magnitude) | ~€18,000 | ~€8,400 |
| Gross cost gap (illustrative) | ~€9,600 in favour of ITI | |
At first glance, ITI wins by nearly €10,000. But the calculation does not stop there. ITE treats the thermal bridges that ITI leaves in place, which weighs on the class gain actually achieved — decisive for crossing the two-class threshold required by a full-scale renovation. ITI, for its part, reduces the living space: in a house where a square metre is worth several thousand euros, the loss of a few square metres eats into part of the initial saving. Depending on the case, the real gap narrows sharply, or even reverses. This is precisely what an item-by-item simulation must quantify before deciding.
The four mistakes that undermine a wall-insulation project
Mistake no. 1 — Choosing ITI only because it is cheaper
The cost of the works is only part of the equation. ITI leaves thermal bridges in place and reduces living space, two items that erode the initial saving. On a house with no façade constraint, ITE — more expensive to buy — delivers higher performance and preserves the space, a better cost-benefit ratio over ten years in many cases. Compare the expected performance and the space retained, not just the price on the quote.
Mistake no. 2 — Launching ITE without checking planning and the association
ITE alters the façade: it generally requires a prior declaration and may be regulated, or even prohibited, by the local urban plan or in a heritage area. In a homeowners' association, it must be voted at a general meeting. Commissioning a study or signing a quote before these checks risks a late refusal and wasted costs. Check planning at the town hall and the association's approval before any commitment.
Mistake no. 3 — Believing wall insulation is still funded as a single action in 2026
Since 1 January 2026, wall insulation (ITI as well as ITE) has been excluded from the single-action route of MaPrimeRénov' (ministerial order of 12 June 2026). It can only be funded by this grant as part of a full-scale renovation — audit, supervisor, two-class gain. If a company sells you "MaPrimeRénov'" external insulation as a single action, the quote is based on outdated rules or on deception. CEE, the eco-PTZ loan and 5.5% VAT, however, remain available.
⚠️ Warning: never sign a "grant-deducted" quote before ANAH's approval notification. Until the grant is approved, the amount displayed "after MaPrimeRénov'" is guaranteed by no one — and you owe the full cost of the works if the aid is refused.
Mistake no. 4 — Insulating walls without addressing ventilation
Strengthening wall insulation makes the home more airtight. Without suitable ventilation (a working mechanical ventilation system, ideally a high-performance one), the moisture produced indoors no longer escapes: a risk of condensation and mould, especially with ITI where residual thermal bridges create cold spots. A full-scale renovation addresses insulation and ventilation together — a coherent bundle, not an isolated action.
Simulate your insulation and subsidies before choosing
OneDpe 2026 renovation works and subsidies simulator
Compare ITE and ITI on your home: build your works bundle (wall insulation, ventilation, heating), see the estimated DPE class gain item by item, and weigh the built-in 2026 subsidies — MaPrimeRénov' for full-scale renovations, CEE, the eco-PTZ loan — against your out-of-pocket cost based on your ANAH income category.
To go further: the order of energy renovation works and the 5 best-ROI works to gain one DPE grade.
Conclusion
Between ITE and ITI there is no universal winner: there is a choice tuned to your property. ITE is the obvious option whenever the façade allows it — higher performance, thermal bridges treated, space and thermal mass preserved — at the price of a larger budget and permits to obtain. ITI remains the solution when the façade is protected, the homeowners' association opposed to ITE, or the budget tight, accepting residual thermal bridges and a loss of space. In every case, since 2026, MaPrimeRénov' aid runs through full-scale renovation.
Before signing, the OneDpe 2026 renovation works and subsidies simulator lets you cost each scenario item by item, estimate the expected class gain and weigh the real cost of the two techniques against your subsidies — taking into account the new MaPrimeRénov' 2026 rules and grant levels.
