Published on June 15, 202610 min readEnerPHit · Minergie-P RetrofitBuilding envelopeUpdated June 2026
Passive retrofit in practice
Fixed geometry, structural thermal bridges, occupants in place: a retrofit to passive level is won in the execution details. This dossier covers implementation — the envelope component by component, airtightness, and building systems. The Geneva regulatory framework (IDC — heating expenditure index), the choice of standard and the project approach are covered in the strategy dossier.
Retrofitting to passive level requires working with the existing geometry, structural constraints, and often occupants in place. These constraints generate technical challenges that new construction does not have — but they also create opportunities for ingenuity.
Major
Structural thermal bridges
Cantilevered slabs, balconies cast integrally with the structure, structural cross-walls running through the envelope — all interruptions in the insulation that cannot be removed without major structural intervention. In new construction they are eliminated at the design stage; in a retrofit, they must be cut, wrapped, or accepted in the energy balance.
Major
Airtightness of an existing envelope
A new envelope is designed to be airtight from the outset. In a retrofit, every existing leak must be identified and addressed within a structure that is often complex: timber floor junctions, historic service penetrations, roller shutter boxes, window reveals. The target of n₅₀ ≤ 1,0 h⁻¹ is achievable, but requires rigour and intermediate testing.
In an occupied retrofit, certain interventions (airtightness, ventilation) can only be carried out if the occupants cooperate. Phasing the works, communicating in advance, and training users on the new ventilation system are key factors that are often underestimated.
Manageable
Phasing and step-by-step retrofit
Not every owner can finance everything at once. EnerPHit allows a step-by-step retrofit on the basis of an EnerPHit Retrofit Plan (ERP): pre-certification is issued once the ERP has been submitted, the first step completed in accordance with it, and primary energy demand reduced by at least 20%PHI v10c §3.3.1: pre-certification once the ERP is submitted, the 1st step is compliant and primary energy (PER or PE) is cut by at least 20%. No maximum interval between steps.Passive House Institute (PHI), Darmstadt — Criteria for the Passive House, EnerPHit and PHI Low Energy Building Standards, Version 10c — §3.3 Pre-certification for staged retrofits2023-01-20 · accessed 2026-08-20. The PHI criteria set no maximum duration between steps — what is checked is the coherence of the plan, not the schedule. In Switzerland, Minergie offers a simplified verification procedure for retrofitted residential buildings, the Renovation Models; Minergie "modules", by contrast, are certified products (Window Module, Monitoring Module)Minergie label regulations 2026.1, ch. 16: the "Renovation Models" are a simplified verification procedure for retrofitted residential buildings (annex H).Association Minergie — Règlement des labels MINERGIE / MINERGIE-P / MINERGIE-A, version 2026.1, ch. 16 « Modèles de rénovation »2026-01-01 · accessed 2026-08-20 and not construction stages.
02 — Envelope strategy
Component by component: targets and solutions
The thermal envelope is the core of the passive retrofit. Each component has its own specific constraints and preferred solutions. The target values below are those of Table 2 of the PHI criteria — the EnerPHit component method, for the cool-temperate climate zone to which Geneva belongs. That route and the heating demand route (Table 3) are two equivalent alternatives: complying with either one is enoughPHI v10c §2.2: "The EnerPHit Standard can be attained by complying with the criteria in the component criteria method (Table 2) or alternatively … the energy demand method (Table 3)."Passive House Institute (PHI), Darmstadt — Criteria for the Passive House, EnerPHit and PHI Low Energy Building Standards, Version 10c — §2.2, Tables 2 et 32023-01-20 · accessed 2026-08-20, and in both cases the general criteria of Table 4 apply on top. Wherever we aim higher than the standard, it is flagged as a project target and never presented as a PHI requirement.
External insulation (ITE) : the reference solution — continuous, free of thermal bridges, protects the structure. ETICS system (render on polystyrene or mineral wool), ventilated timber cladding, or insulation under cladding. Footprint impact +20–30 cm per face.
Internal insulation (ITI) : when the exterior is protected (heritage status, footprint constraints) — but it reduces usable floor area and creates thermal bridges at the floor slabs. Requires a careful hygrothermal calculation.
Flat roof : inverted roof insulation — extruded polystyrene (XPS) only, laid above the waterproofingCSFE professional rules (in force 30 June 2021): the scope covers "panneaux plans en mousse de polystyrène extrudé (XPS) conformes à la norme NF EN 13164+A1" — XPS only.Fédération Française du Bâtiment / Chambre Syndicale Française de l'Étanchéité — Règles professionnelles — Isolation inversée de toiture-terrasse (CSFE)2021-06-30 · accessed 2026-08-20, because the insulation is permanently exposed to water there and only XPS to EN 13164 has the corresponding scope of use — or warm roof construction, with the insulation beneath the waterproofing, where PIR, PUR and mineral wool belong. Substantial thicknesses, but feasible without major footprint constraints.
Habitable attic : insulation between and beneath the rafters, or roof covering replacement with continuous insulation. Unheated attics allow a thick layer of blown-in cellulose (λ ≈ 0,040CAUE 31 technical sheet (03/2016): loose-fill / blown cellulose λ = 0.035 to 0.040 W/(m·K) depending on density. 0.040 is the upper, hence conservative, figure.CAUE 31 / Les CAUE d'Occitanie — Fiche technique — Isolants d'origine végétale et animale : ouate de cellulose2016-03 · accessed 2026-08-20, low cost, no thermal bridge).
Floor over crawl space or slab-on-grade : insulation from below (accessible crawl space) or installation of a new insulated floor on top. The perimeter thermal bridge at the wall/slab junction is the critical point: it must be addressed at the same time as the wall insulation.
Heated vs unheated basement : if the basement is outside the thermal envelope, the basement ceiling is the component to insulate (U ≤ 0,15 W/m²·K), and heating/DHW pipes in the basement must be insulated.
Triple glazing mandatory to reach EnerPHit level. Uw ≤ 0,85 W/m²·K corresponds to triple glazing with a high-performance frame (timber, timber-aluminium, high-performance PVC). The value to be met is the installed Uw, including installation thermal bridges — not the manufacturer's catalogue figure; 0,85 applies to vertical windows, 1,00 to inclined onesPHI v10c, Table 2: in cool-temperate, UW,installed ≤ 0.85 W/(m²K) vertical, 1.00 inclined, 1.10 horizontal. INSTALLED value, installation bridges included.Passive House Institute (PHI), Darmstadt — Criteria for the Passive House, EnerPHit and PHI Low Energy Building Standards, Version 10c — Table 2, colonne Windows (UD/W,installed)2023-01-20 · accessed 2026-08-20 and 1,10 to horizontal ones.
Position of the window within the wall : in a retrofit, the window must be repositioned within the insulation plane (external face-fixed installation if ITE). The reveals, lintels and sills must be insulated without creating new thermal bridges. This is often where several kWh/m²·an are lost.
Thermal bridges
No Ψ threshold in EnerPHit
Ψ ≤ 0,01 W/m·K on external dimensions is the Passive House definitionPassipedia: "thermal bridge free" = Ψ ≤ 0.01 W/(mK) on external dimensions. PHI v10c, note 1: "it is not always possible to eliminate thermal bridges with reasonable expense".Passipedia — Passive House Institute — What defines thermal bridge free design?accessed 2026-08-20 of a "thermal bridge free" detail — a design target for new build. PHI acknowledges that in a retrofit thermal bridges cannot always be eliminated at reasonable expense, and requires them to be minimised as far as long-term cost-effectiveness justifies. Existing bridges = major constraint
Balconies : cut the existing slabs and rebuild them as a lightweight structure (timber, steel with thermal break) — or wrap the whole element in insulation, accepting a residual bridge documented in the PHPP calculation.
Cross-walls and interior slabs : if the insulation is external, cross-walls perpendicular to the façade do not create a thermal bridge at the façade — they remain within the heated volume. Only the perimeter slab/wall junction needs to be addressed, often with an insulated slab edge. Thermal bridges integral to the component's structure — cladding ties, timber studs — are not handled separately: PHI requires them to be included in the mean U-value of the assembly.
The other route: heating demand
When a component cannot reach its target — a façade on the property boundary, a heritage-imposed window — the component route hits a wall. PHI then opens a second, fully equivalent route: a heating demand ≤ 25 kWh/m²·a in the cool-temperate climate zonePHI v10c, Table 3: maximum heating demand 25 kWh/(m²a) in cool-temperate — 35 arctic, 30 cold, 20 warm-temperate, 15 warm. An alternative to Table 2.Passive House Institute (PHI), Darmstadt — Criteria for the Passive House, EnerPHit and PHI Low Energy Building Standards, Version 10c — Table 3 : EnerPHit energy demand criteria2023-01-20 · accessed 2026-08-20 (Table 3 of the criteria), whatever the individual U-values. On a constrained building this is often the only workable route, because it allows a weak component to be offset by a better one elsewhere. Only one of the two routes has to be met; in both cases the general criteria of Table 4 apply on top — airtightness and primary energy.
Internal insulation: the 25% threshold and the "+i" designation
If more than 25% of the opaque external wall area is insulated internally, the certificate carries the EnerPHit+i designationPHI v10c §2.2: "if more than 25% of the opaque exterior wall area is insulated on the inside, then EnerPHit+i … will be used". Table 2: U ≤ 0.35 W/(m²K) for internal insulation.Passive House Institute (PHI), Darmstadt — Criteria for the Passive House, EnerPHit and PHI Low Energy Building Standards, Version 10c — §2.2 et Table 2, note 2 « Interior insulation »2023-01-20 · accessed 2026-08-20. The requirement on those walls is then no longer 0,15 but U ≤ 0,35 W/m²·K in the cool-temperate zone — a level that can be reached without putting the wall's moisture balance at risk. In a protected zone in Geneva, this is the piece of information that changes the conversation: certification remains within reach, with a requirement matched to the heritage constraint. Note: for roofs, basement ceilings or ground floors insulated on the inside, the external-insulation requirement continues to apply.
03 — Airtightness
The critical point of any passive retrofit
In new construction, airtightness is designed into the drawings from the outset and installed "cleanly". In a retrofit, an airtight envelope has to be reconstructed within a building that, by definition, has hundreds of leaks. This is often the criterion that separates a successful passive retrofit from an aborted attempt.
In Switzerland, Minergie does not count in n₅₀
Minergie-P and Minergie-A require an airtightness test and set the limit as qE50, the leakage flow per square metre of envelope area: 1,6 m³/(h·m²) for retrofits, 0,8 for new buildMinergie regulations 2026.1, ch. 6.2: for Minergie-P and Minergie-A, qE50 ≤ 0.8 m³/(h·m²) for new build and 1.6 for retrofits, measured to SIA 180:2014.Association Minergie — Règlement des labels MINERGIE / MINERGIE-P / MINERGIE-A, version 2026.1, ch. 6.2 « Étanchéité de l'enveloppe »2026-01-01 · accessed 2026-08-20, measured to SIA 180:2014. The two indicators cannot be converted into one another: n₅₀ relates the flow to the internal volume, qE50 to the envelope area — the ratio depends on the compactness of the building. A project aiming at both standards has to demonstrate both values separately. On top of that, Minergie-P retrofits must meet a heating demand Qh ≤ 90% of the MoPEC 2025 limit value for new buildMinergie regulations 2026.1, ch. 6.1: Minergie-P retrofit Qh ≤ 90% of the MoPEC 2025 new-build limit value (70% for new build), calculated to SIA 380/1:2016.Association Minergie — Règlement des labels MINERGIE / MINERGIE-P / MINERGIE-A, version 2026.1, ch. 6.1 « Besoins en chauffage »2026-01-01 · accessed 2026-08-20.
Identifying the airtightness plane
The first step is to clearly define the airtightness plane on the drawings: a continuous line that encloses the heated volume, without interruption. In a retrofit, this plane often follows the existing structure (interior wall faces, slab soffits) and must remain continuous at every penetration (pipes, cables, frames).
Critical detail points
Historic timber junctions
Old timber floors, with their slag or ash infill, are often very permeable. The junction between the ceiling/floor and the walls must be resealed with specialised adhesive tape or an airtightness coating.
Roller shutter boxes
One of the worst leakage points in a retrofit. Solution: external boxes (outside the envelope) or replacement with sealed integrated systems fitted with an accessible maintenance hatch and carefully sealed joints.
Pipe and cable penetrations
Every hole in the envelope is a potential leak. Airtight sleeves, approved sealing foams, airtight electrical boxes. Must be planned before finishing works: once the ceilings are closed, it is too late.
Electrical panels and suspended ceilings
Electrical panels recessed into exterior walls, recessed spotlights in ceilings under the roof — all of these are point penetrations of the airtightness plane. They must be relocated or systematically sealed.
Test airtightness before the ceilings are closed
Timing matters as much as the result. A fan pressurisation test carried out before the ceilings are closed and the finishes applied makes it possible to locate leaks while they are still accessible; PHI requires it in any case for every pre-certification and for every n₅₀ between 0,6 and 1,0 h⁻¹, in the form of a documented and signed leak detectionPHI v10c §3.2.10: for EnerPHit between 0.6 and 1.0 h⁻¹ and for any pre-certification, "extensive leak detection must be carried out during the pressurisation test", confirmed in writing and signed.Passive House Institute (PHI), Darmstadt — Criteria for the Passive House, EnerPHit and PHI Low Energy Building Standards, Version 10c — §3.2.10 Airtightness of the building envelope2023-01-20 · accessed 2026-08-20. Once the renders are applied and the ceilings closed, any remedial work means opening up the finishes: its cost bears no relation to that of a strip of tape applied at the right moment. To be written into the relevant works package at design stage.
04 — Building systems
Ventilation, heating and domestic hot water
A very high-performance envelope profoundly changes the requirements for building systems: heating capacities drop, air renewal can no longer rely on infiltration, and domestic hot water becomes proportionally more significant in the energy balance.
Mechanical ventilation with heat recovery (heat recovery ventilation)
With n₅₀ ≤ 1,0 h⁻¹, natural ventilation by infiltration is no longer sufficient. Heat recovery ventilation is essential — it ensures air quality, recovers heat from the stale air, and prevents indoor humidity problems. The EnerPHit threshold is 75% under the PHI method, and it applies to the ventilation installation as a wholePHI v10c, note 7: "The limit applies to the entire ventilation system as a whole (not simply the ventilation unit) … including the heat losses of the ventilation ducts."Passive House Institute (PHI), Darmstadt — Criteria for the Passive House, EnerPHit and PHI Low Energy Building Standards, Version 10c — Table 2, note 7 « Ventilation, minimum heat recovery efficiency »2023-01-20 · accessed 2026-08-20, including the heat losses of the ducts between the thermal envelope and the unit — not to the unit alone.
Benefits of heat recovery ventilation in a retrofit
Recovery of 75 to 90% of the heat from the extracted air, measured under the PHI method, which accounts for the electrical power of the fans and the casing losses; efficiencies quoted to EN 13141-7 are appreciably higherPHI ventilation component criteria: "effective dry heat recovery efficiency" > 75% in cool-temperate climate, computed with the Pel/(ṁ·cp) term; EN 13141-7 tests "with additional measurement technology".Passive House Institute (PHI), Darmstadt — Criteria for the certification of Passive House components: Ventilation systems with heat recovery (capacity < 600 m³/h)accessed 2026-08-20 and are not comparable with these figures. Filtration of pollen and particulates. Control of indoor humidity. No cold draughts. Adjustable flow rate according to occupancy. Reduced risk of mould in humid rooms.
Constraints specific to retrofit projects
Ductwork to be integrated into an existing building volume (suspended ceilings, service ducts). The air handling unit is often located in the attic or utility room. Higher installation cost than in new construction. Occupant training is essential: do not block the vents, do not leave windows permanently open in winter.
Heating: matching capacity to the new reality
After a passive retrofit, heating capacity requirements drop drastically: the load is now counted in watts per square metre rather than in tens of kilowatts — at 15 W/m², a 200 m² treated floor area house needs 3 kW. The old high-temperature radiators become unnecessary. Heating integrated into the ventilation system (post-heating coil), a low-temperature underfloor heating system, or oversized water radiators fed by an air/water heat pump are the most consistent solutions.
Do not oversize heat generation
The classic mistake is installing a boiler or heat pump sized for the old requirements. An oversized system operates in an on/off cycling regime, which reduces its efficiency and increases wear. After an EnerPHit retrofit, recalculate the heating load of the retrofitted building before choosing any generation system. A useful benchmark, but not to be confused: 10 W/m² is the alternative criterion of the new-build Passive House — EnerPHit sets no heating load requirementPHI v10c, Table 1: "Heating load ≤ 10 W/m²" is the ALTERNATIVE criterion for a new Passive House. EnerPHit Tables 2, 3 and 4 contain no heating-load criterion.Passive House Institute (PHI), Darmstadt — Criteria for the Passive House, EnerPHit and PHI Low Energy Building Standards, Version 10c — Table 1, critère alternatif « Heating load »2023-01-20 · accessed 2026-08-20, and a retrofit commonly sits above that benchmark.
Domestic hot water (DHW)
In a renovated passive building, DHW stops being a secondary item: once the heating demand has been brought down to passive level, DHW can exceed it. A heat pump water heater (air/DHW heat pump) is the most efficient solution; its COP only means something alongside its test condition — ask for the value declared to EN 16147, with the tapping profile and the air source temperature, never a bare figure. Watch out for distribution losses: carefully insulate all DHW pipes in unheated volumes.
05 — Key questions
Frequently asked technical questions
My building is listed or in a protected zone — can we still aim for a good level of performance?
Yes, but with adaptations. Internal insulation (ITI) is often the only option on protected façades — with the necessary hygrothermal precautions. Minergie, by contrast, provides no procedure dedicated to protected buildingsThe 2026.1 label regulations mention no "heritage" procedure. Only the simplified Renovation Models procedure exists (ch. 16, annex H).Association Minergie — Règlement des labels MINERGIE / MINERGIE-P / MINERGIE-A, version 2026.1, version 2026.1, entrée en vigueur 01.01.20262026-01-01 · accessed 2026-08-20: the label regulations contain none, and any relaxation is a matter for the cantonal heritage authority, not for the label. What Minergie does offer for retrofitted residential buildings is a simplified verification procedure, the Renovation Models. EnerPHit can be approached via the component-based route, documenting the constraints in the PHPP calculation. In all cases, early collaboration with the cantonal heritage authority is essential.
Is heat recovery ventilation mandatory?
It depends on the standard. Under EnerPHit via the component route, yes: Table 2 requires at least 75% heat recovery across the ventilation installation. Under Minergie-P, the regulations require an automatic outdoor air supply but specify "with or without heat recovery"Minergie regulations 2026.1, ch. 10.1: an automatic outdoor air supply "with or without heat recovery"; uncontrolled manual window airing does not satisfy the label.Association Minergie — Règlement des labels MINERGIE / MINERGIE-P / MINERGIE-A, version 2026.1, ch. 10.1 « Aération — Principes »2026-01-01 · accessed 2026-08-20 — manual window airing, on the other hand, is never sufficient; it is the heating demand requirement for retrofits that makes heat recovery ventilation all but unavoidable in practice. For a high-performance retrofit that is not certified, it is strongly recommended: without controlled air renewal in an improved envelope, indoor humidity levels rise and the risks of mould and poor air quality appear. Heat recovery ventilation is the "comfort" investment that is immediately noticeable day to day.
Sources and references (27)
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Structural junctions, the airtightness plane, ventilation sizing: these decisions are made at the design stage, not on site. Let's talk about it beforehand.