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Published 1 November 2026 23 min read · the essentials in 2 min SIA 180:2014 · Condensation Owners · Managing agents Updated October 2026

Mould after replacing windows: the hygrothermal trap

Mould appears in a dwelling in the cold season following a window replacement. The first reflex is usually to blame the occupants. Yet it is the building that has to be examined first. This dossier explains the mechanics of the phenomenon — dew point, thermal bridges, internal insulation — and the three levers for a lasting fix. A rigorous assessment must come before any attribution of blame.

In this dossier
The trap of new windows Dew point, condensation and SIA 180:2014 Vulnerable zones and the internal-insulation case Ventilation, insulation, behaviour Assessment and liability Key questions
01 — The mechanism

The trap of new windows

Replacing old window frames is a common and technically sound intervention. But in a building with no controlled mechanical ventilation, that change can be followed by mould in the cold seasonFOPH guide (August 2023): airtight windows reduce air renewal; if airing does not make up for it, mould can spread on cold walls during the cold season.Office fédéral de la santé publique (OFSP) — Attention aux moisissures — Un guide sur les problèmes d'humidité et de moisissures dans les locaux d'habitation2023-08 · accessed 2026-10-01. A paradox that takes owners, managing agents and occupants by surprise.

Old timber frames, single-glazed sashes and old cellar windows were often far from airtight: air leaked through the gaps in the frame, around the shutters, through the masonry reveals. This unintended infiltration was involuntary — costly in energy terms, uncomfortable — but it provided a continuous air change and removed part of the water vapour produced inside the dwellingUBA mould guide, § 3.2.1: leaky windows carry away part of the moisture produced; airtight windows fitted without insulating walls and reveals lead to moisture problems.Umweltbundesamt (UBA), Innenraumlufthygiene-Kommission — Allemagne — Leitfaden zur Vorbeugung, Erfassung und Sanierung von Schimmelbefall in Gebäuden (Stand April 2024)2024-04 · accessed 2026-10-01.

The new windows created nothing. They removed the escape valve that had been masking a latent problem of ventilation and thermal bridges.

With new, airtight frames, that uncontrolled ventilation disappears. If no effective mechanical or natural ventilation takes over, the water vapour produced by the occupants — cooking, showering, breathing, laundry — builds up in the indoor air. As soon as humidity passes a certain threshold, condensation settles on the coldest surfaces: façade corners, window reveals, areas behind furniture. Mould follows.

This is not a new pathology: it is an existing problem becoming visible. The thermal bridges and the cold zones were already there before the works. The new windows simply revealed them by removing what had been offsetting them.

7.5–12 l
of water vapour produced per day by a household of four, according to the Swiss Federal Office of Public HealthSwiss FOPH guide “Beware of mould”: a four-person household releases “seven and a half litres of water on average, or twelve litres with heavy use” per day.Office fédéral de la santé publique (OFSP) — Attention aux moisissures — Un guide sur les problèmes d'humidité et de moisissures dans les locaux d'habitation2023-08 · accessed 2026-10-01 — 7.5 l on average, up to 12 l with heavy use (cooking, showering, breathing, indoor laundry)
12 °C
dew point in a room at 20 °C and 60 % relative humidity — any colder surface becomes a condensation zone
6 °C
surface temperature in the external corner of a poorly insulated old building, against 15 °C on the bare wall and 11 °C behind a wardrobe — a steady-state example calculated by the German Environment Agency (UBA)UBA, § 3.1.3 and fig. 12 (theoretical, steady state), poorly insulated old building: bare wall 15 °C, 70 % surface humidity; behind a wardrobe 11 °C, 89 %; corner 6 °C, condensation. Room air 20 °C/50 % (recomputed).Umweltbundesamt (UBA), Innenraumlufthygiene-Kommission — Allemagne — Leitfaden zur Vorbeugung, Erfassung und Sanierung von Schimmelbefall in Gebäuden (Stand April 2024)2024-04 · accessed 2026-10-01
80 % RH
relative humidity at the surface above which mould germinates when it is sustained for several daysSwiss FOPH guide: fungal growth begins when humidity at the surface of materials “approaches 80 % for a few days (water activity, aw: 0.8)”.Office fédéral de la santé publique (OFSP) — Attention aux moisissures — Un guide sur les problèmes d'humidité et de moisissures dans les locaux d'habitation2023-08 · accessed 2026-10-01 — that is the 0.8 threshold of EN ISO 13788, as a monthly meanISO 13788:2012 § 5.1: “There is a risk of mould growth when monthly mean surface relative humidities are above a critical relative humidity, φsi,cr, which should be taken as 0,8”.Organisation internationale de normalisation (ISO) — ISO 13788:2012 — Internal surface temperature to avoid critical surface humidity and interstitial condensation (extrait officiel, § 5.1)2012 · accessed 2026-09-02
A typical scenario in Geneva An apartment building from the 1960s or 1970s, with no controlled ventilation, uninsulated façade corners and bare masonry reveals. The owner replaces the windows — sensible in itself, but the Geneva subsidy schedule has no measure for it: window replacement alone triggers no grantThe GEnergie catalogue of measures contains no window-replacement measure. Insulation falls under M-01 (CHF 80.– to 140.–/m²), balanced ventilation under M-09.GEnergie / Office cantonal de l'énergie (OCEN), Genève — GEnergie — Les mesures subventionnées (M-01 à M-18, IP-04 à IP-19)2026 · accessed 2026-09-02. In the following cold season the tenants report mould in the corners of the bedrooms, behind the wardrobes, in the reveals. In this scenario the cause is to be sought in the building before the occupants' behaviour.
02 — The physics, and the standard that measures it

Dew point, surface condensation, and the fRsi of SIA 180:2014

Water vapour is invisible as long as the air stays warm. As soon as that air meets a surface below the dew point, it condenses into liquid water, and if the surface stays damp long enough, mould develops. The dew point depends solely on air temperature and relative humidity: lowering indoor humidity pushes the danger away, but if the surface is cold enough, condensation occurs even at levels commonly considered normal.

Indoor temp. Relative humidity Dew point (Td) Condensation on cold surfaces
20 °C 40 % ≈ 6 °C Any surface below ≈ 6 °C
20 °C 50 % ≈ 9 °C Any surface below ≈ 9 °C
20 °C 60 % ≈ 12 °C Any surface below ≈ 12 °C
20 °C 65 % ≈ 13 °C Any surface below ≈ 13 °C
20 °C 70 % ≈ 14 °C Any surface below ≈ 14 °C
18 °C 60 % ≈ 10 °C Any surface below ≈ 10 °C

Values computed with the Magnus formulaISO 13788:2012 covers the internal surface temperature required to avoid critical surface humidity and interstitial condensation. The table's dew points follow the Magnus relation.Organisation internationale de normalisation (ISO) — ISO 13788:2012 — Performance hygrothermique des composants et parois de bâtiments — Méthodes de calcul2012 · accessed 2026-09-02 (the standard approximation in building physics). In a bedroom held at 18 °C and 60 % relative humidity, any surface below 10 °C is a potential condensation zone.

A hygrometer sitting on the table does not measure the risk in the corners In the UBA example the values correspond to room air at 20 °C and 50 % relative humidity — the top of the 30 to 50 % range recommended by the Swiss Federal Office of Public HealthFOPH guide: as a basic rule the Office recommends a relative humidity between 30 and 50 %; on very cold days it should not exceed 40 %.Office fédéral de la santé publique (OFSP) — Attention aux moisissures — Un guide sur les problèmes d'humidité et de moisissures dans les locaux d'habitation2023-08 · accessed 2026-10-01 — while humidity at the surface reaches 70 % on the bare wall at 15 °C, 89 % behind a wardrobe at 11 °C, and water condenses in the corner at 6 °C. What governs the risk is the surface temperature, not the humidity measured in the middle of the room: a hygrothermal reading is taken in contact with the surfaces concerned.
This is not only a problem of excess humidity. It is also a problem of surfaces that are too cold.

SIA 180:2014: the thermal and moisture protection of buildings

That is exactly what the standard measures. In Switzerland the reference is SIA 180:2014 (“Thermal protection, moisture protection and indoor climate in buildings”). It superseded SIA 180:1999Official extract of SIA 180:2014, title page: "Remplace la norme SIA 180:1999", valid from 1.7.2014; ch. 6 Moisture protection, 6.2 risk of surface condensation and mould.SIA — Société suisse des ingénieurs et des architectes — SIA 180:2014 — Protection thermique, protection contre l'humidité et climat intérieur dans les bâtiments, extrait officiel (page de titre et table des matières)2014 · accessed 2026-10-01; in Geneva the REn cites it by name in art. 12E, in its 2014 edition, alongside SIA 380/1 edition 2016 and SIA 382/1 edition 2025REn art. 12E “Requirements for thermal insulation and solar protection” refers to standards SIA 180 edition 2014, SIA 380/1 edition 2016 and SIA 382/1 edition 2025.République et canton de Genève — rsGE L 2 30.01 — Règlement d'application de la loi sur l'énergie (REn), art. 12E et 12Gaccessed 2026-10-01.

Its indicator for surface condensation, the surface temperature factor fRsi, says what fraction of the indoor-outdoor difference the internal surface sits at: the higher it is, the warmer the surface and the lower the risk of condensation. Thermal bridges — corners, slab edges, untreated reveals — are precisely the points where it collapses. SIA 180:2014 tabulates the minimum fRsi (table 10) for set conditions, with annex E.1 allowing it to be recomputed for othersSIA 180 FAQ (06-2017), F1 and F2: 80 % limit in clause 6.2.1.3; fRsi,min in table 10, recomputable for other conditions via annex E.1. The FAQ gives no single value. Archive copy.SIA — commission de normalisation SIA 180 — Norme SIA 180:2014 — Questions fréquentes et réponses, édition 06-2017 (copie Internet Archive du 23.10.2019 ; le document n'est plus servi par sia.ch)2017-06 · accessed 2026-10-01; its mould criterion is a relative humidity of 80 % at the surface (clause 6.2.1.3).

Application in Geneva: a legal obligation The Geneva REn (art. 12E para. 1) requires compliance with SIA 180:2014 for the thermal insulation and solar protection of buildings, and its art. 12G para. 1 for air supply and ventilation, together with SIA 382/1:2025 and SIA 382/5:2021. RCI art. 56A, for its part, targets the window: in existing buildings, façade openings — glazing, window frames, blind boxes — whose U-value equals or exceeds 3.0 W/(m²·K) had to be brought into compliance by 31 January 2016RCI art. 56A para. 2: façade openings (glazing, window frames, blind boxes) of existing buildings must be brought into compliance when U equals or exceeds 3.0 W/(m²K), by 31 January 2016 at the latest.République et canton de Genève — rsGE L 5 05.01 — Règlement d'application de la LCI (RCI), art. 56Aaccessed 2026-10-01.

The standard covers two kinds of condensation. Surface condensation, visible, is treated by warming the cold surfaces or by lowering indoor humidity. Interstitial condensation, inside the wall, is invisible and potentially destructive: that is the case of internal insulation, dealt with below.

French-speaking Switzerland and neighbouring France: same physics, distinct frameworks In Switzerland the reference is SIA 180:2014; how each cantonal regulation refers to it has to be checked canton by canton. In neighbouring France the calculation methods are those of EN ISO 13788 and EN 15026, and execution falls under the NF DTU standards. Not to be confused: RE2020 places no hygrothermal requirement in the Bbio; summer comfort there falls under a separate indicatorThe ministry presents RE2020 through three distinct objectives: energy sobriety and decarbonisation, reduced carbon impact, and comfort in extreme heat — handled separately from the Bbio.Ministère de la Transition écologique — Réglementation environnementale RE2020accessed 2026-09-02, discomfort degree-hours. Same physics, distinct legal obligations.
03 — The critical points

Vulnerable zones and the special case of internal insulation

Mould settles where the surface is coldest. In existing buildings these are the following zones, particularly after windows have been replaced with no controlled ventilation installed.

Geometric thermal bridge
Façade corners
Two cold walls converging: the internal surface there is markedly colder than the middle of the wall. Warm room air reaches the corner poorly, which lowers the surface temperature further.
Junction thermal bridge
Uninsulated reveals
Jamb, lintel and sill in bare masonry form a thermal bridge of complex geometry — revealed by the change of frames, which removed the unintended infiltration.
Structural thermal bridge
Internal insulation with untreated structural elements
Internal insulation improves the plain wall but leaves slab edges, columns and beams projecting through the insulation — cold points that remain.
Structural thermal bridge
Slab edges (intermediate floors)
The concrete slab crossing the envelope creates a continuous break along the full length of the building, at floor level.
Obstructed air circulation
Areas behind furniture and wardrobes
Furniture set against an external wall cuts the air circulation behind it: the wall is colder there. The Swiss Federal Office of Public Health recommends leaving 10 centimetres between furniture and external wallsFOPH guide, furnishing: leave 10 centimetres between furniture and external walls; no large furniture against external walls or in their corners.Office fédéral de la santé publique (OFSP) — Attention aux moisissures — Un guide sur les problèmes d'humidité et de moisissures dans les locaux d'habitation2023-08 · accessed 2026-10-01.
Plain wall
Middle of the wall (away from corners)
With no geometric or structural thermal bridge the surface is warmer than at the junctions: the problem sits at the junctions and the singularities.

Internal thermal insulation: a good option, with real risks

Internal insulation is chosen when the outside is ruled out: a protected façade, a saturated building envelope, no space on the outside. It improves the plain wall, but shifts the temperature within the wall's thickness and creates risks of its own.

Strengths of internal insulation Feasible on listed or protected façades. Does not affect the external building envelope or the boundary distances. Can be executed dwelling by dwelling, with no scaffolding on the street side. Compatible with façade ornamentation.
Hygrothermal risks Interstitial condensation between insulation and wall if vapour migration is not controlled. Untreated structural thermal bridges (slab edges, columns) — persistently cold zones. The load-bearing wall ends up on the cold side, exposed to more marked freeze-thaw cycles.

The key is the prior hygrothermal verification. The Glaser calculation (steady-state diffusion method, EN ISO 13788ISO 13788:2012 provides simplified steady-state methods (monthly balance) for interstitial condensation and critical surface humidity. Confirmed in 2023.Organisation internationale de normalisation (ISO) — ISO 13788:2012 — Performance hygrothermique des composants et parois de bâtiments — Méthodes de calcul2012 · accessed 2026-09-02) takes only heat conduction and vapour diffusion into account: it ignores driving rain and capillary transport. For internal insulation, the WTA 6-4 leaflet makes the moisture verification indispensable and wants it, as a rule, by simulation of coupled heat and moisture transport (EN 15026, WUFI-type softwareEN 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)2023-07-31 · accessed 2026-10-01; leaflets WTA 6-1 and 6-2, criteria of WTA 6-5WTA 6-4 (10.2016/D), summary: a moisture verification is "indispensable", as a rule by coupled simulation (WTA 6-1, 6-2, criteria in 6-5); a simplified verification is admitted under the conditions it sets.WTA — Wissenschaftlich-Technische Arbeitsgemeinschaft für Bauwerkserhaltung und Denkmalpflege — Merkblatt WTA 6-4 — Innendämmung nach WTA I: Planungsleitfaden (10.2016/D), Kurzfassung2016-10 · accessed 2026-10-01). That simulation is to be entrusted to a specialist building-physics office.

Everything else about internal insulation is covered in its own dossier The four families of systems and how to choose between them, the two quantified criteria to demand from the hygrothermal study, the insulation returns on cross walls and slabs, the embedded joist ends, the thickness and the design errors that produce defects: all of it is set out, in full rather than summarised, in Insulating from the inside without creating defects →
For the professional — the simplified verification of WTA 6-4
The WTA 6-4 leaflet admits a simplified verification, without simulation, 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), leaflet not read in the original: no driving rain, 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 Innendämmung2018-09 · accessed 2026-10-01. Outside that framework, verification goes through simulation of coupled heat and moisture transport to EN 15026, run over several years, with the actual orientation of the façade, its exposure to rain and the embedded timbers of the floor. If a membrane is planned, its position — warm side, uninterrupted — is verified in that same simulation.
04 — The solutions

Ventilation, insulation, behaviour: three levers to combine

Three levers — and their order is half the answer. 1. Ventilation: it is what the window replacement removed, and it acts on every room at once. 2. Insulating the cold surfaces: indispensable in the medium term, because ventilation warms nothing. 3. Use: real, but secondary: it weighs on a building that already has its cold surfaces. None is sufficient on its own, and the third does not come first.

Lever 1 — Priority
Ventilation: removing water vapour at source
Controlled mechanical ventilation continuously extracts damp air from the service rooms and renews the air in the living rooms — which lowers indoor humidity. A balanced system adds heat recovery; an extract-only system removes the air without recovering its heat.

In Geneva: the REn, in its art. 12G “Requirements for air supply and ventilation”, requires thermal energy recovery units on balanced ventilation systems, and on extract systems exceeding 1,000 m³/h and 500 operating hours per yearREn art. 12G “Requirements for air supply and ventilation”: balanced systems are fitted with recovery units; extract systems from 1,000 m³/h and over 500 operating hours per year.République et canton de Genève — rsGE L 2 30.01 — Règlement d'application de la loi sur l'énergie (REn), art. 12E et 12Gaccessed 2026-10-01. On the funding side, balanced ventilation with heat recovery falls under measure M-09 of the GEnergie schedule — heat exchanger with an efficiency above 70 %, CHF 4,800.– per dwellingGEnergie schedule, measure M-09 “Ventilation in dwellings with heat recovery via exchanger (efficiency > 70 %)”: CHF 4,800.– per dwelling unit for balanced systems.GEnergie / Office cantonal de l'énergie (OCEN), Genève — GEnergie — Les mesures subventionnées (M-01 à M-18, IP-04 à IP-19)2026 · accessed 2026-09-02 — and envelope insulation under measure M-01, from CHF 80.– to 140.–/m².
Lever 2 — Indispensable in the medium term
Insulation: raising the temperature of cold surfaces
Ventilation lowers the humidity of the air but warms no surface: on a façade corner at 6 °C, condensation starts at 40 % relative humidity in air at 20 °C (Magnus formula calculation). The temperature of the at-risk surfaces has to be raised, by treating the thermal bridges: reveals, external or internal insulation of the façade, correcting the slab edges. The target is the minimum fRsi that SIA 180:2014 tabulates (table 10) or allows to be recomputed (annex E.1) — and these works are coordinated with the window project from the outset, not afterwards.
Lever 3 — Real but limited
Occupant behaviour: its share of the equation
Drying damp laundry indoors, running a humidifier, never opening a window after a shower, heating very little at night: all of these genuinely worsen the risk. But SIA 180:2014 provides the yardstick: its clause 6.2.1.5 allows the maximum admissible indoor humidity for avoiding mould to be calculated, from the lowest temperature factor of the thermal bridgesSIA 180 FAQ (06-2017), F2: clause 6.2.1.5 allows the maximum admissible indoor humidity for avoiding mould to be calculated, from the lowest surface temperature factor of the thermal bridges.SIA — commission de normalisation SIA 180 — Norme SIA 180:2014 — Questions fréquentes et réponses, édition 06-2017 (copie Internet Archive du 23.10.2019 ; le document n'est plus servi par sia.ch)2017-06 · accessed 2026-10-01. Attributing mould to the occupants therefore presupposes establishing the condition of the building first — hence an assessment. Behaviour is judged against what the building can withstand, not in the absolute.

Regulatory framework: Geneva — French-speaking Switzerland — neighbouring France

Geneva
LEn, REn, LCI, RCI
REn art. 12E (SIA 180:2014 mandatory), art. 12G (ventilation and heat recovery), RCI art. 56A (existing windows from U ≥ 3.0 W/(m²·K)), LCI art. 113 para. 1: the energy needed for a building's function must be used economically and rationallyLCI art. 113 “Principles”, para. 1: “Buildings must be designed and maintained so that the energy needed for their function is used economically and rationally”.République et canton de Genève (silgeneve.ch) — rsGE L 5 05 — Loi sur les constructions et les installations diverses (LCI), art. 113accessed 2026-10-01.
GEnergie schedule: M-09 for balanced ventilation, M-01 for the envelope — nothing for window replacement on its own.
A high IDC and mould in the corners can share one cause: an envelope that loses heat, and a heating system compensating for it.
French-speaking Switzerland
SIA 180:2014, CECB
SIA 180:2014 for thermal and moisture protection, SIA 382/1:2025 and SIA 382/5:2021 for ventilation; how each cantonal regulation refers to these standards has to be checked canton by canton.
CECB: the envelope efficiency class (A to G) places the level of insulation; it is not a damp assessment.
The Buildings Programme co-finances the cantonal subsidies.
Neighbouring France
DTU, RE2020, DPE
EN ISO 13788 and EN 15026 for calculation; NF DTU 68.3 (June 2013) for mechanical ventilation installationsAFNOR Norm'Info: NF DTU 68.3 “Building works — Mechanical ventilation installations”, part 1-1-1 “General rules for calculation, sizing and execution”, June 2013.AFNOR — Norm'Info — NF DTU 68.3 — Travaux de bâtiment : installations de ventilation mécanique2013-06 · accessed 2026-09-02.
The DPE rates the dwelling by its energy use and emissions: it is not a damp assessment. In RE2020, summer comfort falls under a distinct indicator (DH).
The “decent housing” decree: art. 2, adequate ventilation, air renewal and moisture removal suited to normal occupancyDecree no. 2002-120, art. 2 item 6: the dwelling allows adequate ventilation; openings and ventilation devices provide air renewal and moisture removal suited to the needs of normal occupancy.Caisse nationale des allocations familiales (CAF), reproduction du décret — Légifrance étant fermé aux robots — Décret n° 2002-120 du 30 janvier 2002 relatif aux caractéristiques du logement décent (art. 2 et 3)2002-01-30 · accessed 2026-10-01. MaPrimeRénov': since 1 September 2026, neither ventilation nor insulation is fundable under the single-measure routeService-Public: since 1 September 2026 the table of works eligible under the single-measure route no longer lists ventilation or insulation, only heating, hot water and the energy audit.Service-Public.gouv.fr — Direction de l'information légale et administrative — MaPrimeRénov' — travaux retenus au titre du parcours par geste2026 · accessed 2026-09-02 in mainland France.
05 — Before assigning any blame

Assessment: the sequence to follow

The sequence to follow: assess the building first, assign responsibility afterwards. The instrumented surveys described below — surface measurements, thermography, flow-rate measurement — are to be entrusted to specialist providers; NRG positive does not carry them out.

The wrong reflex: blaming the occupants without an assessment “You don't ventilate enough.” Given without an assessment, that answer rests on nothing measured. In Switzerland, the landlord must hand over and maintain the leased object in a condition fit for the agreed use (art. 256 CO)Art. 256 para. 1 CO, cited by the Federal Supreme Court: “The landlord must hand over the object at the agreed date, in a condition fit for the use for which it was let, and maintain it in that condition”.Tribunal fédéral suisse (base de jurisprudence Weblaw) — ATF 136 III 186 — art. 256 al. 1 CO, obligation du bailleur de délivrer et d'entretenir la chose louée2010-04-06 · accessed 2026-09-02. A reader in Annemasse, Gaillard or Ferney comes under a different text: the “logement décent” decree of 30 January 2002, whose art. 2 requires adequate ventilation, and openings and ventilation devices that remove the moisture of normal occupancy. The two texts cannot be cited for one another and are argued before different courts. The Swiss Federal Office of Public Health guide, written with the tenants', owners' and property-industry associations, sets out the procedure: the tenant reports damp or mould without delay, and the landlord or managing agent is from then on responsible for the further investigationsFOPH guide (with the tenants', owners' and property associations): tenants report damp and mould immediately; the landlord or managing agent is from then on responsible for the further investigations.Office fédéral de la santé publique (OFSP) — Attention aux moisissures — Un guide sur les problèmes d'humidité et de moisissures dans les locaux d'habitation2023-08 · accessed 2026-10-01.
Step 1
Site survey: observation and measurements
Precise location of the mould-affected areas; temperature and relative humidity measured at the surface of the affected zones, not in the middle of the room; state of the ventilation system in place.
Step 2
Infrared thermography in cold weather — to be entrusted to a specialist provider
The thermal camera locates thermal bridges and cold surfaces on the building as built. The method is standardised in EN ISO 6781-1:2023, which superseded EN 13187SIS catalogue: SS-EN 13187 (1999), status "Withdrawn", "Replaced by: SS-EN ISO 6781-1:2023". Same entry in the BSI catalogue: BS EN 13187:1999 withdrawn, superseded by BS EN ISO 6781-1:2023.Swedish Institute for Standards (SIS), organisme national de normalisation — SS-EN 13187 — Thermal performance of buildings — Qualitative detection of thermal irregularities in building envelopes — Infrared method (fiche catalogue)accessed 2026-10-01. A public specification applying EN 13187 calls for a temperature difference of at least 10 °C across the building envelopeWalloon CCTB § 03.41.2a, applying EN 13187 (cited without a year): the air temperature difference across the envelope must not be below 10 °C, 24 h before and during; no sun or rain 12 h before.Service public de Wallonie — Cahier des charges type Bâtiments — CCTB — 03.41.2a Thermographies infrarouges, conditions d'essai en application de la norme [NBN EN 13187], citée sans millésimeaccessed 2026-09-04, maintained for the 24 hours preceding the survey, and surfaces sheltered from sunlight and rain for the preceding 12 hours. NRG positive does not carry out this service: it is to be entrusted to a specialist thermographer.
Step 3
Checking the ventilation
Actual flow rates measured, then compared with reference flow rates — SIA 382/1:2025 “Mechanical ventilation in buildings”SIA shop: standard SIA 382/1 “Mechanical ventilation in buildings — bases and requirements”, 2025 edition, in force since 1 February 2025.Société suisse des ingénieurs et des architectes (SIA) — Norme SIA 382/1:2025 — Ventilation mécanique dans les bâtiments : bases et exigences2025 · accessed 2026-09-02 and SIA 382/5:2021 for dwellings in Switzerland, NF DTU 68.3 in France; absence of a system documented. Date of the window replacement versus date the mould appeared: the chronology is documented.
Step 4
Report and plan of measures
Causes identified, at-risk zones quantified, plan of measures prioritised: which intervention first, at what cost, with which subsidies (in Geneva, measures M-01 to M-18 and IP-04 to IP-19 of the GEnergie scheduleThe GEnergie catalogue lists measures under codes M-01 to M-18 and IP-04 to IP-19; M-01 insulation of façade, roof, walls and floor over ground, CHF 80.– to 140.–/m².GEnergie / Office cantonal de l'énergie (OCEN), Genève — GEnergie — Les mesures subventionnées (M-01 à M-18, IP-04 à IP-19)2026 · accessed 2026-09-02, processed by the cantonal energy office). That document serves as a basis in the event of a dispute.
Why call on a qualified third party An assessment carried out by an independent party — neither the owner, nor the tenant, nor the company that fitted the windows — removes the conflict of interest and brings the dispute back to a debate about facts. NRG positive does not carry out that assessment. We advise you: reading the reports drawn up by specialist providers, arbitrating between the standards your project is aiming for, the order of the interventions.
06 — Key questions

What owners and managing agents ask

A tenant reports mould after our window replacement. Who is liable?

Two questions settle it: did the building meet SIA 180:2014 before the windows were replaced, and was the loss of unintended infiltration compensated for? If the thermal bridges already existed and no controlled ventilation was installed, causation lies with the building. A reading by a qualified third party is the first step — before any answer to the tenants.

How can a building problem be told apart from a behaviour problem?

Several signs point the way: mould located in the corners, around the windows or behind furniture (thermal-bridge zones) points to a building problem. Mould present in several dwellings of the same block points to a cause in the building. Infrared thermography and measurement of surface temperature in the affected zones, entrusted to a specialist provider, supply measured evidence.

Our façade is listed. Is internal insulation a viable solution?

Yes, if it is properly designed — that is the subject of the internal insulation dossier. The point that matters for mould: internal insulation does not remove the structural thermal bridges (slab edges, columns), and it does not replace air renewal, even though it improves the plain wall. The two interventions must be coordinated.

Should controlled ventilation always be installed when windows are replaced?

In Geneva, art. 12G REn refers to SIA 180:2014, SIA 382/1:2025 and SIA 382/5:2021 (para. 1) and requires that a ventilation concept drawn up to SIA 180:2014 be attached to the permit application (para. 5)REn art. 12G para. 1: SIA 180 ed. 2014, SIA 382/1 ed. 2025, SIA 382/5 ed. 2021 and SIA 382.715 ed. 2017; para. 5: a ventilation concept to SIA 180:2014 attached to the building permit application.République et canton de Genève — rsGE L 2 30.01 — Règlement d'application de la loi sur l'énergie (REn), art. 12E et 12Gaccessed 2026-10-01. For a frame replacement carried out without an application, we found no obligation there to install controlled ventilation; elsewhere the question goes to the cantonal energy office. SIA 180:2014 devotes its clause 3.2 to the ventilation concept, and the standard's committee states that ventilation — natural or mechanical — must always be controlledSIA 180 FAQ (06-2017), C2, clause 3.2.1: in a correct ventilation concept, ventilation must always be controlled; both natural and mechanical ventilation can and must be controlled.SIA — commission de normalisation SIA 180 — Norme SIA 180:2014 — Questions fréquentes et réponses, édition 06-2017 (copie Internet Archive du 23.10.2019 ; le document n'est plus servi par sia.ch)2017-06 · accessed 2026-10-01. The German Environment Agency, for its part, recommends drawing up a ventilation concept when new airtight windows are fittedUBA, § 3.2.1: "Aus diesem Grund wird beim Einbau neuer dichter Fenster die Erstellung eines Lüftungskonzeptes empfohlen" — a ventilation concept is recommended when new airtight windows are fitted.Umweltbundesamt (UBA), Innenraumlufthygiene-Kommission — Allemagne — Leitfaden zur Vorbeugung, Erfassung und Sanierung von Schimmelbefall in Gebäuden (Stand April 2024)2024-04 · accessed 2026-10-01.

Can occupants be advised to open the windows to solve the problem?

Airing through the windows does remove water vapour and reduces the risk in the short term: the Swiss Federal Office of Public Health recommends cross-ventilating at least three times a day, with all windows and doors openFOPH guide: air "at least three times a day" for five to ten minutes; in old buildings with new windows and poor thermal insulation, air more frequently.Office fédéral de la santé publique (OFSP) — Attention aux moisissures — Un guide sur les problèmes d'humidité et de moisissures dans les locaux d'habitation2023-08 · accessed 2026-10-01, for five to ten minutes. Two limits: the Office itself asks for more frequent airing in old, poorly insulated buildings with new windows, and on a corner at 6 °C condensation starts at 40 % relative humidity in air at 20 °C. Advice accompanies an assessment; it does not replace one.

Key points

What to take away, and in what order to act

  1. Measure before concluding. Surface temperatures at the wall, humidity logged over time, thermography in cold weather by a specialist provider — not one reading in the middle of the room.
  2. Restore air renewal first. That is what the window replacement removed, and this lever acts on every room at once.
  3. Then correct the cold surfaces. Reveals, façade corners, slab edges: they set the threshold beyond which moisture condenses.
  4. Do not install internal insulation without a moisture verification of the wall. WTA 6-4 regards it as indispensable; have it carried out by a specialist office.
  5. Write down what you ask of the occupants, and give them the means to do it. A ventilation instruction without working ventilation rests on their discipline alone.
  6. Date and keep the diagnosis. In a dispute it documents the condition of the building at the time.

GenevaREn art. 12E (SIA 180:2014 mandatory), art. 12G (ventilation and heat recovery), RCI art. 56A (existing windows from U ≥ 3.0 W/(m²·K)). Window replacement on its own triggers no grant.

SwitzerlandSIA 180:2014 is the reference standard; how each cantonal regulation refers to it has to be checked canton by canton. The CECB envelope class places the level of insulation, not the damp risk.

Neighbouring FranceThe "logement décent" decree of 30 January 2002 requires adequate ventilation and moisture removal matched to normal occupancy. The DPE is not a damp assessment.

Sources and references (36)

Every figure and every claim in this dossier links back to its source. Hover or tap a footnote marker to see it.

  1. Office fédéral de la santé publique (OFSP) — Attention aux moisissures — Un guide sur les problèmes d'humidité et de moisissures dans les locaux d'habitation (2023-08 · accessed 2026-10-01)
    FOPH guide (August 2023): airtight windows reduce air renewal; if airing does not make up for it, mould can spread on cold walls during the cold season.
  2. Umweltbundesamt (UBA), Innenraumlufthygiene-Kommission — Allemagne — Leitfaden zur Vorbeugung, Erfassung und Sanierung von Schimmelbefall in Gebäuden (Stand April 2024) (2024-04 · accessed 2026-10-01)
    UBA mould guide, § 3.2.1: leaky windows carry away part of the moisture produced; airtight windows fitted without insulating walls and reveals lead to moisture problems.
  3. Office fédéral de la santé publique (OFSP) — Attention aux moisissures — Un guide sur les problèmes d'humidité et de moisissures dans les locaux d'habitation (2023-08 · accessed 2026-10-01)
    Swiss FOPH guide “Beware of mould”: a four-person household releases “seven and a half litres of water on average, or twelve litres with heavy use” per day.
  4. Umweltbundesamt (UBA), Innenraumlufthygiene-Kommission — Allemagne — Leitfaden zur Vorbeugung, Erfassung und Sanierung von Schimmelbefall in Gebäuden (Stand April 2024) (2024-04 · accessed 2026-10-01)
    UBA, § 3.1.3 and fig. 12 (theoretical, steady state), poorly insulated old building: bare wall 15 °C, 70 % surface humidity; behind a wardrobe 11 °C, 89 %; corner 6 °C, condensation. Room air 20 °C/50 % (recomputed).
  5. Office fédéral de la santé publique (OFSP) — Attention aux moisissures — Un guide sur les problèmes d'humidité et de moisissures dans les locaux d'habitation (2023-08 · accessed 2026-10-01)
    Swiss FOPH guide: fungal growth begins when humidity at the surface of materials “approaches 80 % for a few days (water activity, aw: 0.8)”.
  6. Organisation internationale de normalisation (ISO) — ISO 13788:2012 — Internal surface temperature to avoid critical surface humidity and interstitial condensation (extrait officiel, § 5.1) (2012 · accessed 2026-09-02)
    ISO 13788:2012 § 5.1: “There is a risk of mould growth when monthly mean surface relative humidities are above a critical relative humidity, φsi,cr, which should be taken as 0,8”.
  7. GEnergie / Office cantonal de l'énergie (OCEN), Genève — GEnergie — Les mesures subventionnées (M-01 à M-18, IP-04 à IP-19) (2026 · accessed 2026-09-02)
    The GEnergie catalogue of measures contains no window-replacement measure. Insulation falls under M-01 (CHF 80.– to 140.–/m²), balanced ventilation under M-09.
  8. Organisation internationale de normalisation (ISO) — ISO 13788:2012 — Performance hygrothermique des composants et parois de bâtiments — Méthodes de calcul (2012 · accessed 2026-09-02)
    ISO 13788:2012 covers the internal surface temperature required to avoid critical surface humidity and interstitial condensation. The table's dew points follow the Magnus relation.
  9. Office fédéral de la santé publique (OFSP) — Attention aux moisissures — Un guide sur les problèmes d'humidité et de moisissures dans les locaux d'habitation (2023-08 · accessed 2026-10-01)
    FOPH guide: as a basic rule the Office recommends a relative humidity between 30 and 50 %; on very cold days it should not exceed 40 %.
  10. SIA — Société suisse des ingénieurs et des architectes — SIA 180:2014 — Protection thermique, protection contre l'humidité et climat intérieur dans les bâtiments, extrait officiel (page de titre et table des matières) (2014 · accessed 2026-10-01)
    Official extract of SIA 180:2014, title page: "Remplace la norme SIA 180:1999", valid from 1.7.2014; ch. 6 Moisture protection, 6.2 risk of surface condensation and mould.
  11. République et canton de Genève — rsGE L 2 30.01 — Règlement d'application de la loi sur l'énergie (REn), art. 12E et 12G (accessed 2026-10-01)
    REn art. 12E “Requirements for thermal insulation and solar protection” refers to standards SIA 180 edition 2014, SIA 380/1 edition 2016 and SIA 382/1 edition 2025.
  12. SIA — commission de normalisation SIA 180 — Norme SIA 180:2014 — Questions fréquentes et réponses, édition 06-2017 (copie Internet Archive du 23.10.2019 ; le document n'est plus servi par sia.ch) (2017-06 · accessed 2026-10-01)
    SIA 180 FAQ (06-2017), F1 and F2: 80 % limit in clause 6.2.1.3; fRsi,min in table 10, recomputable for other conditions via annex E.1. The FAQ gives no single value. Archive copy.
  13. République et canton de Genève — rsGE L 5 05.01 — Règlement d'application de la LCI (RCI), art. 56A (accessed 2026-10-01)
    RCI art. 56A para. 2: façade openings (glazing, window frames, blind boxes) of existing buildings must be brought into compliance when U equals or exceeds 3.0 W/(m²K), by 31 January 2016 at the latest.
  14. Ministère de la Transition écologique — Réglementation environnementale RE2020 (accessed 2026-09-02)
    The ministry presents RE2020 through three distinct objectives: energy sobriety and decarbonisation, reduced carbon impact, and comfort in extreme heat — handled separately from the Bbio.
  15. Office fédéral de la santé publique (OFSP) — Attention aux moisissures — Un guide sur les problèmes d'humidité et de moisissures dans les locaux d'habitation (2023-08 · accessed 2026-10-01)
    FOPH guide, furnishing: leave 10 centimetres between furniture and external walls; no large furniture against external walls or in their corners.
  16. Organisation internationale de normalisation (ISO) — ISO 13788:2012 — Performance hygrothermique des composants et parois de bâtiments — Méthodes de calcul (2012 · accessed 2026-09-02)
    ISO 13788:2012 provides simplified steady-state methods (monthly balance) for interstitial condensation and critical surface humidity. Confirmed in 2023.
  17. 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) (2023-07-31 · accessed 2026-10-01)
    EN 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.
  18. WTA — Wissenschaftlich-Technische Arbeitsgemeinschaft für Bauwerkserhaltung und Denkmalpflege — Merkblatt WTA 6-4 — Innendämmung nach WTA I: Planungsleitfaden (10.2016/D), Kurzfassung (2016-10 · accessed 2026-10-01)
    WTA 6-4 (10.2016/D), summary: a moisture verification is "indispensable", as a rule by coupled simulation (WTA 6-1, 6-2, criteria in 6-5); a simplified verification is admitted under the conditions it sets.
  19. G. A. Scheffler, Bausubstanz kompakt (WTA) — Regelwerksübersicht zum Wärme- und Feuchteschutz im Kontext der Innendämmung (2018-09 · accessed 2026-10-01)
    Conditions of the WTA 6-4 simplified verification, per G. Scheffler (2018, pp. 4-5), leaflet not read in the original: no driving rain, normal humidity, annual mean ≥ 7 °C, ΔR ≤ 2.5 (absorbent substrate) or 2.0 m²K/W.
  20. République et canton de Genève — rsGE L 2 30.01 — Règlement d'application de la loi sur l'énergie (REn), art. 12E et 12G (accessed 2026-10-01)
    REn art. 12G “Requirements for air supply and ventilation”: balanced systems are fitted with recovery units; extract systems from 1,000 m³/h and over 500 operating hours per year.
  21. GEnergie / Office cantonal de l'énergie (OCEN), Genève — GEnergie — Les mesures subventionnées (M-01 à M-18, IP-04 à IP-19) (2026 · accessed 2026-09-02)
    GEnergie schedule, measure M-09 “Ventilation in dwellings with heat recovery via exchanger (efficiency > 70 %)”: CHF 4,800.– per dwelling unit for balanced systems.
  22. SIA — commission de normalisation SIA 180 — Norme SIA 180:2014 — Questions fréquentes et réponses, édition 06-2017 (copie Internet Archive du 23.10.2019 ; le document n'est plus servi par sia.ch) (2017-06 · accessed 2026-10-01)
    SIA 180 FAQ (06-2017), F2: clause 6.2.1.5 allows the maximum admissible indoor humidity for avoiding mould to be calculated, from the lowest surface temperature factor of the thermal bridges.
  23. République et canton de Genève (silgeneve.ch) — rsGE L 5 05 — Loi sur les constructions et les installations diverses (LCI), art. 113 (accessed 2026-10-01)
    LCI art. 113 “Principles”, para. 1: “Buildings must be designed and maintained so that the energy needed for their function is used economically and rationally”.
  24. AFNOR — Norm'Info — NF DTU 68.3 — Travaux de bâtiment : installations de ventilation mécanique (2013-06 · accessed 2026-09-02)
    AFNOR Norm'Info: NF DTU 68.3 “Building works — Mechanical ventilation installations”, part 1-1-1 “General rules for calculation, sizing and execution”, June 2013.
  25. Caisse nationale des allocations familiales (CAF), reproduction du décret — Légifrance étant fermé aux robots — Décret n° 2002-120 du 30 janvier 2002 relatif aux caractéristiques du logement décent (art. 2 et 3) (2002-01-30 · accessed 2026-10-01)
    Decree no. 2002-120, art. 2 item 6: the dwelling allows adequate ventilation; openings and ventilation devices provide air renewal and moisture removal suited to the needs of normal occupancy.
  26. Service-Public.gouv.fr — Direction de l'information légale et administrative — MaPrimeRénov' — travaux retenus au titre du parcours par geste (2026 · accessed 2026-09-02)
    Service-Public: since 1 September 2026 the table of works eligible under the single-measure route no longer lists ventilation or insulation, only heating, hot water and the energy audit.
  27. Tribunal fédéral suisse (base de jurisprudence Weblaw) — ATF 136 III 186 — art. 256 al. 1 CO, obligation du bailleur de délivrer et d'entretenir la chose louée (2010-04-06 · accessed 2026-09-02)
    Art. 256 para. 1 CO, cited by the Federal Supreme Court: “The landlord must hand over the object at the agreed date, in a condition fit for the use for which it was let, and maintain it in that condition”.
  28. Office fédéral de la santé publique (OFSP) — Attention aux moisissures — Un guide sur les problèmes d'humidité et de moisissures dans les locaux d'habitation (2023-08 · accessed 2026-10-01)
    FOPH guide (with the tenants', owners' and property associations): tenants report damp and mould immediately; the landlord or managing agent is from then on responsible for the further investigations.
  29. Swedish Institute for Standards (SIS), organisme national de normalisation — SS-EN 13187 — Thermal performance of buildings — Qualitative detection of thermal irregularities in building envelopes — Infrared method (fiche catalogue) (accessed 2026-10-01)
    SIS catalogue: SS-EN 13187 (1999), status "Withdrawn", "Replaced by: SS-EN ISO 6781-1:2023". Same entry in the BSI catalogue: BS EN 13187:1999 withdrawn, superseded by BS EN ISO 6781-1:2023.
  30. Service public de Wallonie — Cahier des charges type Bâtiments — CCTB — 03.41.2a Thermographies infrarouges, conditions d'essai en application de la norme [NBN EN 13187], citée sans millésime (accessed 2026-09-04)
    Walloon CCTB § 03.41.2a, applying EN 13187 (cited without a year): the air temperature difference across the envelope must not be below 10 °C, 24 h before and during; no sun or rain 12 h before.
  31. Société suisse des ingénieurs et des architectes (SIA) — Norme SIA 382/1:2025 — Ventilation mécanique dans les bâtiments : bases et exigences (2025 · accessed 2026-09-02)
    SIA shop: standard SIA 382/1 “Mechanical ventilation in buildings — bases and requirements”, 2025 edition, in force since 1 February 2025.
  32. GEnergie / Office cantonal de l'énergie (OCEN), Genève — GEnergie — Les mesures subventionnées (M-01 à M-18, IP-04 à IP-19) (2026 · accessed 2026-09-02)
    The GEnergie catalogue lists measures under codes M-01 to M-18 and IP-04 to IP-19; M-01 insulation of façade, roof, walls and floor over ground, CHF 80.– to 140.–/m².
  33. République et canton de Genève — rsGE L 2 30.01 — Règlement d'application de la loi sur l'énergie (REn), art. 12E et 12G (accessed 2026-10-01)
    REn art. 12G para. 1: SIA 180 ed. 2014, SIA 382/1 ed. 2025, SIA 382/5 ed. 2021 and SIA 382.715 ed. 2017; para. 5: a ventilation concept to SIA 180:2014 attached to the building permit application.
  34. SIA — commission de normalisation SIA 180 — Norme SIA 180:2014 — Questions fréquentes et réponses, édition 06-2017 (copie Internet Archive du 23.10.2019 ; le document n'est plus servi par sia.ch) (2017-06 · accessed 2026-10-01)
    SIA 180 FAQ (06-2017), C2, clause 3.2.1: in a correct ventilation concept, ventilation must always be controlled; both natural and mechanical ventilation can and must be controlled.
  35. Umweltbundesamt (UBA), Innenraumlufthygiene-Kommission — Allemagne — Leitfaden zur Vorbeugung, Erfassung und Sanierung von Schimmelbefall in Gebäuden (Stand April 2024) (2024-04 · accessed 2026-10-01)
    UBA, § 3.2.1: "Aus diesem Grund wird beim Einbau neuer dichter Fenster die Erstellung eines Lüftungskonzeptes empfohlen" — a ventilation concept is recommended when new airtight windows are fitted.
  36. Office fédéral de la santé publique (OFSP) — Attention aux moisissures — Un guide sur les problèmes d'humidité et de moisissures dans les locaux d'habitation (2023-08 · accessed 2026-10-01)
    FOPH guide: air "at least three times a day" for five to ten minutes; in old buildings with new windows and poor thermal insulation, air more frequently.

Further reading

All dossiers →

First understand what the building is saying

Mould after window replacement, a tenancy dispute, an internal insulation project or a whole-building renovation: we advise you on the order of the interventions — ventilation before envelope — and on arbitrating between the standards your project is aiming for: HPE and THPE, the two levels of high and very high energy performance written into Geneva law, Minergie, EnerPHit. Surface measurements, thermography and hygrothermal simulation are to be entrusted to specialist providers. Let's talk about your project.

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