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.
Let's say it clearly from the outset: external wall insulation (EWI) remains the reference solution whenever it is possible. It keeps the wall warm, addresses thermal bridges continuously, and does not consume living space. Geneva law points in the same direction: the LCI art. 114A specifies that the thickness of external insulation added to an existing building is not counted in the floor area ratio, in the building envelope, or in the distances to property boundaries. In other words, in Geneva, building envelope limits are no longer an argument against EWI.
IWI is therefore not a matter of convenience — it is the option left when the façade cannot change. Four typical situations:
An uninsulated old wall is crossed by a constant flow of heat. This waste has a useful side effect: the wall stays warm and dry. IWI interrupts this flow — that is the point — but it simultaneously places the entire masonry on the cold side. In winter, a wall that used to sit at 12–15 °C behind the plaster can drop close to the outdoor temperature behind the insulation.
Indoor air contains water vapour (cooking, showers, occupants). This vapour diffuses through the materials, from the warm inside to the cold outside. As long as the temperature inside the wall stays above the dew point, the vapour remains gaseous. But behind an IWI system, the temperature drops sharply at the interface between the insulation and the wall: if enough vapour reaches it, it condenses there. This is interstitial condensation — invisible, because it occurs behind the lining.
The consequences develop silently: dampening of the masonry, mould on the hidden face of the lining, corrosion of the fixings, frost damage to porous materials — and, in old buildings with timber floors, rot in the joist ends embedded in the now cold, damp wall.
The minimum check is the Glaser calculation (steady-state diffusion method, EN ISO 13788): it compares, month by month, the amount of vapour entering the wall and the amount that can leave it. It is the basic tool — but it ignores two phenomena that are decisive for old walls: the driving rain absorbed by the façade, and the capillary transport of liquid water within the masonry.
For an exposed old wall, a façade in rubble stone or molasse sandstone, or as soon as timber is embedded in the wall, a dynamic hygrothermal simulation (such as WUFI, per EN 15026) is the only reliable check: it models temperature, humidity, rain and drying hour by hour over several years. It is a few days' study — set against the cost of a lining that has to be removed because of mould.
All IWI systems answer the same question — how to manage water vapour — but according to 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). The choice depends on the wall, its exposure, and the execution reliability that can be guaranteed.
In IWI, more insulation is not better: each additional centimetre cools the wall further and increases the risk of condensation and frost damage. Practice converges on moderate thermal resistances — typically 6 to 10 cm depending on the material — giving a renovated wall around U ≈ 0,25–0,40 W/(m²K). This is often below the SIA 380/1:2016 element-by-element limit value for a renovated wall (≤ 0,20 W/(m²K)) — and the Geneva framework allows for this: RALI art. 56 requires SIA 380/1 compliance "unless a particular case applies", and the justification can be based on the building's overall performance rather than element by element. 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 building stock affected by IWI largely belongs to the urban ring dating from the late 19th and early 20th centuries: buildings in massive masonry — rubble stone, brick, dressed stone and molasse sandstone — with moulded façades, timber floors and generous ceiling heights. These buildings often fall within protected zones or protected ensembles under the LCI; work on them is subject to review by the CMNS (Commission des monuments, de la nature et des sites — Commission for Monuments, Nature and Sites), and covering the façades is generally excluded there.
Molasse sandstone does not tolerate damp. Present in many Geneva plinths and door/window surrounds, molasse is a soft, porous stone, sensitive to frost and wetting cycles. Cooling a molasse wall through a poorly designed IWI accelerates its deterioration — on the street side, precisely where it is visible and protected. The hygrothermal study must model the actual stone, not a generic wall.
Timber floors call for caution. Joist ends embedded at every level, on every façade: the critical detail from point 04 recurs dozens of times in a late-19th-century building. This is the main argument in favour of capillary-active systems in this building stock.
The legal framework offers some latitude. RALI art. 56A provides for exemptions for protected buildings, on justification. Envelope compliance is assessed under SIA 380/1 "unless a particular case applies" (RALI art. 56) — and weighing heritage protection against energy performance is an established practice of the Geneva authorities. A moderate, well-designed IWI, combined with high-performance windows, a well-insulated roof and controlled ventilation, makes for a defensible case before both the OCEN and the CMNS.
In French-speaking Switzerland, cantonal architectural surveys (Vaud, Neuchâtel, Fribourg) produce the same constraints on historic centres — with the same technical responses. In neighbouring France, ABF perimeters around historic monuments lead to the same trade-off: the façade is untouchable, and internal insulation is studied case by case.
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 (RALI 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.
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 12–14 °C to 17–19 °C in winter, 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?
As an order of magnitude, supply and installation, finishes included: CHF 150 to 250/m² 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.
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.
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