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: it accounts only for conduction and vapour diffusion, and 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 15026:2023 (revision of the 2007 edition; BS EN published 31.07.2023): components of a model for transient heat and moisture transfer, and a method for validating software.en-standard.eu (distributeur de normes) — BS EN 15026:2023 — Hygrothermal performance of building components and building elements. Assessment of moisture transfer by numerical simulation (fiche catalogue): WTA leaflet 6-4 makes a moisture verification indispensable and, as a rule, requires it as a coupled heat and moisture transport simulation (WTA 6-1 and 6-2, assessment criteria of WTA 6-5), run over several years until a steady cycle is reached. It admits a simplified verification only under conditions, including: no moisture input from driving rain, normal indoor humidity, a mean annual outdoor temperature of at least 7 °C, and added thermal resistance limited to 2.0 or 2.5 m²K/W depending on how absorbent the substrate isConditions of the WTA 6-4 simplified verification, per G. Scheffler (2018, pp. 4-5): no driving-rain input, existing R ≥ 0.39, normal humidity, annual mean ≥ 7 °C, ΔR ≤ 2.5 (absorbent substrate) or 2.0 m²K/W.G. A. Scheffler, Bausubstanz kompakt (WTA) — Regelwerksübersicht zum Wärme- und Feuchteschutz im Kontext der Innendämmung. This study is a job for a consultancy specialising in building physics.
The defects described below stem from a design error or an execution defect at a critical point. Each follows from the mechanisms set out above: a damp wall, vapour carried by air, a chilled wall, a thermal bridge.
| Error | Defect to expect | 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, insulation of the jambs up to the window frame (at least 20 mm according to Passipedia) |
| 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 may only appear after several winters. 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. Passipedia (Passive House Institute) recommends 4 to 12 cm of insulation at λ 0.035–0.040 for IWI, "normally about 8 cm"Passipedia (PHI): recommended thickness "between 40 and 120 mm […] normally about 8 cm". ISO 6946 calculation: wall U 1.8 (R 0.556) + 8 cm at λ 0.040 → U 0.39; at λ 0.035 → U 0.35.Passipedia — Passive House Institute — Thermal protection using interior insulation: on an old wall at U ≈ 1.8 W/(m²K), 8 cm gives U ≈ 0.35–0.40 W/(m²K) (ISO 6946 calculation), and about 0.55 with a capillary-active board at λ ≈ 0.06. 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 includes 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 may lie in protected zones or belong to the 19th- and early-20th-century ensembles (LCI art. 89 to 93): complete ensembles are maintained and their elements worthy of protection, as a rule, preserved (art. 90 para. 1); permit applications go to the CMNS (Commission des monuments, de la nature et des sites) for an opinion, or to the office du patrimoine et des sites for façade repairs under the accelerated procedure (art. 93)LCI art. 90 para. 1: complete ensembles are "maintained", elements worthy of protection preserved "as a rule". Art. 93: CMNS opinion; accelerated procedure and façade repairs: office du patrimoine et des sites.République et canton de Genève — rsGE L 5 05 — Loi sur les constructions et les installations diverses (LCI), art. 89 à 93 — Ensembles du XIXe siècle et du début du XXe siècle.
Molasse sandstone does not tolerate damp. A soft, porous stone found in many Geneva door and window surrounds and quoins, it weathers under infiltrating water and freeze-thaw cyclesWildi & Decrouez, p. 24: building bases mostly in limestone; quoins and door and window surrounds in molasse; infiltrating rainwater and frost shattering promote its weathering.W. Wildi, D. Decrouez — Université de Genève, Section des sciences de la Terre et de l'environnement — Pierres de Genève — Découvrir la ville par la pierre de ses constructions, 1re éd., p. 24, 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 admits four upgrade variants for the windows of protected buildings (para. 5) and derogations where the requirement is disproportionate (para. 6); LCI art. 90 para. 2 provides for derogations from energy measures where preserving valuable heritage elements requires it; 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 paras 1, 3, 4, 8: 3-year average IDC > 125 kWh/m²·an (450 MJ): audit and measures within 12 months; no audit up to 153 kWh if the measures bring it back below 125.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 →
In Vaud and Neuchâtel the architectural survey, and in Fribourg the survey of immovable cultural property, identify buildings of heritage value, where the same question arises; in neighbouring France, the agreement required from the Architecte des Bâtiments de France around historic monuments leads 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 0.7 to 1.2 m². It is real, but should be weighed against the comfort gained: the inner wall surface goes from roughly 14–15 °C to nearly 19 °C with 0 to −5 °C outdoors (wall U ≈ 1.8, brought down to 0.35–0.40)ISO 6946 calculation (Rsi = 0.13 m²K/W): Tsi = Ti − U·Rsi·(Ti−Te). Wall U = 1.8: 15.3 °C (20/0 °C), 14.2 °C (20/−5 °C). After IWI, U = 0.35–0.40: 18.7–19.1 °C.ISO — ISO 6946:2017 — Building components and building elements — Thermal resistance and thermal transmittance — Calculation methods, which removes the cold-wall sensation and may allow the thermostat setpoint to be lowered slightly.
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?
We do not publish a price range: we have found no public Swiss cost database specific to IWI. The price is read from comparative quotes drawn up for your wall. Ask for the insulation returns, the treatment of the reveals and any relocation of pipework to be priced separately: these are the items that separate a healthy wall from a failure.
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 that no unacceptable moisture accumulates in the wall through diffusion or capillarity (cl. 6.3); as soon as driving rain, capillary-active insulation or embedded timber is involved, a Glaser calculation is not enough and a dynamic simulation is required (WTA 6-4, 6-5 and 6-8). It is a job for a consultancy specialising in building physics.
Is IWI alone enough to bring my IDC below Geneva's thresholds?
It depends on the building: walls are only one item among others, alongside the roof, windows, ventilation and heat generation. IWI makes sense within an overall strategy: it is the audit (ordered above a three-year average IDC of 125 kWh/m²·an / 450 MJ/m²·an, unless, up to 153 kWh/m²·an, improvement measures are enough to bring it back below the threshold) 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 windows, with variants for protected buildings and derogations where the requirement is disproportionate. Above an IDC of 125 kWh/m²·an, an audit and measures are ordered, with no audit required up to 153 kWh/m²·an if the measures suffice (REn art. 14).
SwitzerlandThe cantonal surveys (Vaud, Neuchâtel, Fribourg) identify buildings of heritage value. SIA 180:2014 rules out unacceptable moisture in building components (cl. 6.3).
Neighbouring FranceAround historic monuments, external insulation needs the ABF's agreement; if it is refused, internal insulation is what remains. Since 1 September 2026, MaPrimeRénov' supports it only through the supervised "rénovation d'ampleur" route — no longer 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.
A protected façade, a co-ownership, an old wall: we advise you on the choice of system and on the trade-off between the standards your project is aiming for. The hygrothermal simulation itself is a job for a specialist consultancy. Let's talk about your project.
Get in touch