Published on August 1, 2026
10 min read
EnerPHit · Minergie-P Retrofit
Building envelope
Updated 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.
01 — Retrofit specifics
The challenges new construction doesn't have
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.
Significant
Insulation thickness and building footprint
Insulating from the outside to reach U ≤ 0,15 W/m²·K requires 20 to 30 cm of insulation. This changes the building's overall dimensions (affecting setback distances), its architectural appearance, and can be incompatible with municipal building regulations or heritage protection rules.
Significant
Moisture and wall physics
Changing the thermal behaviour of an existing wall alters its moisture balance. The dew point shifts. A wall that has historically been "breathable" can develop interstitial condensation if the insulation is poorly positioned. A diffusion calculation (Glaser / WUFI) is essential before choosing any system.
Manageable
Coordination with occupants
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. Component-based certification (EnerPHit) or module-based certification (Minergie) allows the works to be spread over 5 to 10 years — provided the overall logic is planned from the outset to avoid future incompatibilities.
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 correspond to the EnerPHit component-based requirements (an alternative approach to the overall energy balance).
Opaque walls — façades
U ≤ 0,15 W/m²·K
Required thickness: 20–30 cm depending on insulation λ
Typical existing value: 0,8–2,0 W/m²·K
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.
Roof and attic
U ≤ 0,12 W/m²·K
Thickness: 30–40 cm
Often the first component to be retrofitted
Flat roof : inverted roof insulation (XPS or PIR above the waterproofing) or warm roof insulation beneath the waterproofing. 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 ≈ 0,040, low cost, no thermal bridge).
Ground slab / lower floor
U ≤ 0,15 W/m²·K
Clear-height constraint
Avoid perimeter thermal bridges
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.
Windows and doors
Uw ≤ 0,85 W/m²·K
Ug ≤ 0,70 W/m²·K (triple glazing)
Face-fixed installation: critical positioning
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).
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
Ψ ≤ 0,01 W/m·K
EnerPHit target: "acceptable"
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.
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.
Target reminder
EnerPHit: n₅₀ ≤ 1,0 h⁻¹ (Blower Door measurement per SN EN ISO 9972, which replaces EN 13829). By comparison, an unrenovated building typically shows n₅₀ between 5 and 15 h⁻¹. A new passive building reaches ≤ 0,6 h⁻¹.
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.
Intermediate Blower Door test: essential
Do not wait until the end of the works to test airtightness. An intermediate test — before the ceilings are closed and finishes applied — makes it possible to identify and correct accessible leaks. Once the renders are applied and the ceilings closed, remedial work costs 3 to 5 times more.
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 with a high-efficiency heat exchanger (η ≥ 75 %) is essential — it ensures air quality, recovers heat from the stale air, and prevents indoor humidity problems.
Benefits of heat recovery ventilation in a retrofit
Recovery of 75–90% of the heat from the extracted air. 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 — often from 15–20 kW to 3–5 kW for a single-family home. 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 required capacity (often < 10 W/m²) before choosing any generation system.
Domestic hot water (DHW)
In a renovated passive building, DHW can account for 40 to 60% of the remaining total energy consumption. A heat pump water heater (air/DHW heat pump) is the most efficient solution — COP of 2,5 to 4 depending on conditions. 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-P certification has a specific procedure for heritage buildings. 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?
For a retrofit certified to EnerPHit or Minergie-P, yes — the required level of airtightness makes natural ventilation insufficient. 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.
The details will make the performance.
Structural junctions, the airtightness plane, ventilation sizing: these decisions are made at the design stage, not on site. Let's talk about it beforehand.
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