A protected façade, a co-ownership that cannot decide, architectural expression to preserve: when external insulation is not possible, internal wall insulation (IWI) remains the way forward. It is the most demanding insulation technique to design — not because it is complicated to install, but because it changes the physics of the wall. This guide explains when IWI is justified, what happens inside the wall, which systems to choose, and where defects arise.
One reminder, in a sentence, before the subject itself: external wall insulation remains the reference solution whenever it is possible, and in Geneva the building envelope limits are no longer an argument against it — LCI art. 114A provides that the thickness of external insulation counts neither in the calculation of the building envelopeLCI art. 114A para. 3: the thickness of external insulation on an existing building counts neither in the building profile nor in the distances to property boundaries or between buildings.République et canton de Genève — rsGE L 5 05 — Loi sur les constructions et les installations diverses (LCI), art. 114A — Isolation périphérique de constructions existantes nor in that of the distances to boundaries (para. 3). That lever is covered in the thermal bridges dossier.
IWI is therefore not a matter of convenience: it is the option left when the façade cannot change. Four situations, and only those:
An uninsulated old wall is crossed by a constant flow of heat, and this waste has a useful side effect: the wall stays warm and dry. IWI interrupts that flow — that is the point — and simultaneously places the entire masonry on the cold side: a wall that used to sit at 12–15 °C behind the plaster drops close to the outdoor temperature behind the insulation.
The water vapour produced indoors diffuses through the materials, from warm to cold. As long as the temperature inside the wall stays above the dew point, it remains gaseous; but behind an IWI system the temperature drops sharply at the interface between the insulation and the wall, and the vapour that reaches it condenses. This is interstitial condensation — invisible, because it occurs behind the lining.
All of it develops silently: dampening of the masonry, mould on the hidden face of the lining, corrosion of the fixings, frost damage to porous materials — and, in buildings with timber floors, rot in the joist ends.
The Glaser calculation (steady-state diffusion method, EN ISO 13788)ISO 13788:2012 provides simplified methods (monthly balance, steady state) for interstitial condensation and critical surface humidity. Confirmed in 2023.ISO — ISO 13788:2012 — Hygrothermal performance of building components and building elements — Internal surface temperature to avoid critical surface humidity and interstitial condensation — Calculation methods is a pre-sizing tool, never a design justification: it ignores the two phenomena that are decisive for old walls, the driving rain absorbed by the façade and the capillary transport of liquid water. In internal wall insulation, dynamic hygrothermal simulation (such as WUFI, per EN 15026) is the rule and not the exceptionEN 15026:2007 sets the minimum criteria for transient coupled heat-and-moisture transfer software; Fraunhofer IBP states that WUFI meets them (deviation ≤ 2.5 %). The page carries no date.Fraunhofer IBP — WUFI — Benchmark test of EN 15026 (EN 15026:2007) — page non datée: WTA leaflet 6-4 prescribes verification by coupled heat and moisture transport calculation (WTA 6-1, 6-2 and 6-5), hour by hour and over several years. A few days' study, against a lining that has to be removed.
IWI failures tend to look alike: they rarely stem from the material, almost always from a design error or an execution defect at a critical point. The typical cases below recur throughout the technical literature and failure investigations.
| Error | Defect observed | Prevention |
|---|---|---|
| IWI on a damp wall (rising damp, untreated driving rain) | Increasing moisture in the masonry, efflorescence, render delamination, widespread mould behind the lining | Prior moisture diagnosis; treat the cause of the dampness before insulating |
| Vapour barrier punctured or not connected (outlets, conduits, joints, partition bases) | Condensation concentrated at the leaks, localised mould stains, odours — often discovered years later | A drawn-up airtightness plan, a service cavity, inspection before closing up |
| Excess insulation thickness ("while we're at it, let's put 18 cm") | Wall too cold: interstitial condensation, frost damage to the masonry, deterioration of soft stone | Thickness set by the hygrothermal study, not by U-value ambition |
| Untreated reveals and cross walls | Mould on window jambs and at ceiling/façade corners — the cold point has moved, not been eliminated | Systematic insulation returns, thin insulation of the jambs (RCI art. 56A) |
| Water pipes left in the cold wall | Frozen pipes in the first hard winter, water damage in the new lining | Move the pipework to the warm side during the works |
| Ventilation overlooked after the dwelling is sealed up | Rising indoor humidity, condensation on residual cold points, degraded air quality | A ventilation concept at minimum, or balanced mechanical ventilation with heat recovery, in every IWI project |
| Verification limited to a summary Glaser check on an exposed old wall | Wall validated on paper, damaged in reality — driving rain and capillarity were not in the calculation | Dynamic simulation (WUFI, EN 15026) for old walls, embedded timber, exposed façades |
The common thread among these defects: they are invisible at handover and appear in the second or third winter. Hence the importance of design — the contractor cannot fix a wall that was poorly conceived.
Two opposing philosophies: blocking the vapour before it reaches the cold wall (vapour barrier), or letting it in and redistributing it so it gets back out without damage (capillary-active materials). A capillary-active material takes up liquid water, spreads it through its mass and gives it back to the indoor air as that air dries out: it does not merely let vapour through. Two figures make the four families below readable.
In IWI, more insulation is not better: each additional centimetre cools the wall further. Practice converges on 6 to 10 cm, giving a renovated wall around U ≈ 0,25–0,40 W/(m²K). The SIA 380/1:2016 element-by-element limit value is 0,25 W/(m²K) for a wall undergoing conversion (0,17 W/(m²K) for a new element)EN-NE102, April 2023 edition (based on SIA 380/1:2016), table 2 p. 9: opaque element against outside air, U ≤ 0.17 W/(m²K) for new build and 0.25 W/(m²K) for conversion.République et canton de Neuchâtel — Service de l'énergie et de l'environnement — Aide à l'application EN-NE102 « Isolation thermique des bâtiments », édition avril 2023 (base SIA 380/1:2016). RCI art. 56 (the implementing regulation of the LCI, rsGE L 5 05.01)RCI (rsGE L 5 05.01) art. 56: 'The thermal quality of the building envelope must, unless a particular case applies, comply with the SIA 380/1 standard in force.'République et canton de Genève — SILGeneve — Règlement d'application de la loi sur les constructions et les installations diverses (RCI), L 5 05.01, art. 56 requires SIA 380/1 compliance "unless a particular case applies", and the justification can be based on the building's overall performance. The hygrothermal study sets the maximum admissible thickness — not the insulation catalogue.
With EWI, the insulation wraps the building continuously. With IWI, the insulation plane is interrupted by the building's own structure: every cross wall, every slab, every window reveal crosses through the insulation. This is where heat flows — and defects — concentrate.
The Geneva stock affected by IWI largely belongs to the urban ring of the late 19th and early 20th centuries: massive masonry — rubble stone, brick, dressed stone and molasse sandstone —, moulded façades, timber floors. These buildings often fall within protected zones or ensembles under the LCI: review by the CMNS (Commission des monuments, de la nature et des sites), and covering the façades is generally excluded.
Molasse sandstone does not tolerate damp. A soft, porous stone present in many Geneva plinths and surrounds, it is sensitive to frost and wetting cycles, and a poorly designed IWI accelerates its deterioration — on the street side, precisely where it is visible and protected. The study must model the actual stone, not a generic wall.
The legal framework offers some latitude. RCI art. 56A provides for exemptions for protected buildings, on justification, and envelope compliance is assessed "unless a particular case applies" (RCI art. 56). A moderate, well-designed IWI, combined with high-performance windows, an insulated roof and controlled ventilation, makes for a defensible case before both the OCEN — the cantonal energy office — and the CMNS.
The IDC — indice de dépense de chaleur — is the actual heating and hot water consumption read off the building's meters and declared to the canton every year. It is not a theoretical calculation, and the instrument is cantonal. Three thresholds, and only those, decide:
| What it triggers | Threshold | Period |
|---|---|---|
| Mandatory audit | IDC threshold of 125 kWh/m²·an (450 MJ/m²·an) that triggers a mandatory auditREn art. 14: exceeding an IDC of 125 kWh/m²·an (450 MJ/m²·an) requires an audit and improvement measures within 12 months.République et canton de Genève — SILGeneve — Règlement d'application de la loi sur l'énergie (REn), L 2 30.01, art. 14 | at all times |
| Mandatory works significant exceedance, REn art. 14 | 222 kWh/m²·an (800 MJ/m²·an) | until 31.12.2026 |
| 180 kWh/m²·an (650 MJ/m²·an) from 2027 to 2030REn art. 14: significant IDC exceedance at 222 kWh/m²·an (800 MJ) until 31.12.2026, 180 kWh/m²·an (650 MJ) from 2027 to 2030, then 153 kWh/m²·an (550 MJ) from 2031.République et canton de Genève — SILGeneve — Règlement d'application de la loi sur l'énergie (REn), L 2 30.01, art. 14 | 2027–2030 | |
| 153 kWh/m²·an (550 MJ/m²·an) | from 2031 |
When the façade cannot change, IWI is one of the levers to consider for bringing the index down, alongside the roof, windows and systems. See the guide Renovating in Geneva: IDC, standards and strategy →
The cantonal architectural surveys of Vaud, Neuchâtel and Fribourg produce the same constraints on historic centres; in neighbouring France, ABF perimeters lead to the same trade-off.
How much living space will I lose?
Allow 8 to 14 cm of total thickness (insulation + lining) on each façade wall, depending on the system. For a 4 × 5 m room with two external walls, that amounts to roughly 0,8 to 1,2 m². It is real, but should be weighed against the comfort gained: the inner wall surface goes from roughly 14–16 °C to 18–19 °C depending on how hard the winter isISO 6946 calculation (Rsi = 0.13 m²K/W): Tsi = Ti − U·Rsi·(Ti−Te). Wall U = 1.8 at 20/0 °C → 15.3 °C; at 20/−5 °C → 14.2 °C; after IWI U = 0.30 → 19.2 °C.ISO — ISO 6946:2017 — Building components and building elements — Thermal resistance and thermal transmittance — Calculation methods, which removes the cold-wall sensation and often allows the thermostat setpoint to be lowered by a degree.
Can I insulate my apartment alone, without a decision from the co-ownership?
In principle, yes: IWI is carried out within your unit, without touching the common parts — this is one of its advantages. Two caveats: check the condominium regulations (some work that indirectly affects the structure or the façades requires notice or approval), and above all have the wall checked by a hygrothermal study, since a moisture defect in the wall would affect the whole building, not just your unit. In a building subject to authorisation (protected zone, listed building), check with the cantonal office before starting work.
How much does IWI cost?
NRG positive estimate as of June 2026 — no public Swiss cost database covers IWI specifically — supply and installation, finishes included: CHF 150 to 250/m²Internal NRG positive estimate (June 2026): no public Swiss cost database (CRB, KBOB, SIA Bulletin) covers internal insulation specifically. Order of magnitude, not contractual.Source to be established — unverified figure of wall for a mineral wool lining with vapour barrier, CHF 200 to 300/m² for composite panels, CHF 250 to 450/m² for a capillary-active system such as calcium silicate with mineral render. On top of this come the insulation returns, treatment of the reveals and any relocation of pipework — often 20 to 30 % of the budget, and the part that makes the difference between a healthy wall and a failure. The hygrothermal study represents a modest fraction of the total.
Is a hygrothermal study really necessary for a few centimetres of insulation?
Yes — and all the more so when the wall is old. It is precisely the thin layer that needs to be justified: the right IWI is one whose thickness, material and details have been checked for your wall specifically, with its orientation, its exposure to rain and its floors. SIA 180:2014 requires hygrothermal verification of modified walls; for old walls and embedded timber, a Glaser calculation is not enough and a dynamic simulation is required. A few days of study against years of hidden defects: the trade-off is easily made.
Is IWI alone enough to bring my IDC below Geneva's thresholds?
Rarely on its own. Walls account for a significant share of heat loss, but the roof, windows, ventilation and heat generation together weigh more. IWI makes sense within an overall strategy: it is the audit (mandatory above an IDC of 125 kWh/m²·an / 450 MJ/m²·an) that prioritises the measures and their timeline. A renovation planned holistically — even if carried out in stages — is better than a stand-alone IWI decided when repainting.
GenevaProtected zones and ensembles, CMNS opinion, RCI art. 56A for reveals with derogations on justification. Above an IDC of 125 kWh/m²·an, the audit is mandatory (REn art. 14).
SwitzerlandThe architectural inventories of Vaud, Neuchâtel and Fribourg create the same constraints in the old town centres. SIA 180:2014 requires every modified wall to be verified.
Neighbouring FranceWithin ABF perimeters, internal insulation is often the only option. MaPrimeRénov' supports it through the supervised "rénovation d'ampleur" route — not through the parcours par geste.
Every figure and every claim in this dossier links back to its source. Hover or tap a footnote marker to see it.
Moisture diagnosis, hygrothermal simulation of the wall, choice of system and execution details — the pre-works study costs a few days and avoids years of defects. Let's talk about your wall.
Get in touch