Getting below 15 kWh/m² a year of heating demand in a retrofit means holding three thresholds together: measured airtightness under 0.6 ach at the n50 test, a roof thermal resistance above 10 m²·K/W and walls above 7 m²·K/W. The threshold of 15 kWh/m² a year is a calculation result and not a promise on the bill: orientation, compactness, ceiling height and heritage constraints can put it out of reach on an existing building without heavy work. The order of execution counts as much as the thicknesses, the continuous envelope and the airtightness being dealt with before any generator is sized, and the heat recovery ventilation being commissioned on measured flow rates. The blower-door test is planned during the works and not at handover, because a leak found after the linings are up is only put right by taking the finishes down.
Aiming for the 15 kWh passive threshold: what it really means on site
Understanding the 15 kWh indicator and its limits with existing buildings
The “15” threshold corresponds to a very low annual heating need, expressed in kWh per m² per year. It's a calculation target, not a promise on the bill. In retrofits, the existing building sets the rules: orientation, compactness, ceiling height, heritage constraints. All of this can make passive performance more expensive, or simply out of reach without major works.
Identifying the items that drag down heating performance: envelope, thermal bridges, airtightness
On site, you gain kWh wherever air and heat escape. First the envelope, with continuous insulation. Then the junctions, which create thermal bridges. Finally airtightness, to be checked with a blower-door test. Without these three points, even a good heat pump won't make up for the drift.
Choosing your target: a “near-passive” home or strict passive
Set a realistic goal. Strict passive aims for a tightly controlled performance level, often with certification. “Near-passive” keeps the same logic but targets the most cost-effective items first. What matters is aligning the study, execution details and checks. An audit and a thermal simulation keep you from aiming too high, or not high enough.
Passive strategy: the order of works to reach < 15 kWh/m²/year
Tackle the envelope first: continuous insulation and eliminating air leaks
In a passive retrofit, you start with the envelope. The goal: continuous insulation on walls, roof and floors, with no “forgotten” zones, then careful airtightness. Sealing, membranes, tapes. And a blower-door test to check, before sizing the heating.
Managing moisture and ventilation: the right mechanical ventilation system, flow rates, settings
A very airtight house needs to breathe differently. Mechanical ventilation becomes the engine of comfort. Choose a suitable solution (often heat-recovery ventilation for ambitious retrofits). Set the right flow rates, check the air inlets, and plan for maintenance. This limits mould, odours, and unnecessary over-ventilation.
Securing the execution details: junctions, penetrations, joinery
kWh get lost in the details. Treat the junctions (wall-roof, wall-floor), network penetrations, shutter boxes. Take care with window and door installation, with continuous sealing. A check during the works avoids costly rework.
Sizing the heating system in a passive retrofit: small system, big comfort
Calculate precisely: avoid oversizing the heat generator
In a passive building, needs are low. Sizing must start from the actual heat loss, room by room, and a simple usage scenario. An oversized generator short-cycles, uses more energy and wears out faster. Aim for a precise calculation, then keep a small margin for very cold days. That calculation is set out in a sizing report compliant with NF EN 12831-1, produced from the technical visit readings.
Comparing retrofit solutions: heat pump, electric backup, stove, existing network
In retrofits, several paths work. A small air-to-water heat pump can be enough if the network is compatible with low-temperature operation. Electric backup heating is simple if needs stay occasional. A stove can add comfort, provided power and heat distribution are well managed. Keeping an existing network is possible if you lower the flow temperatures.
Control and distribution: thermal mass, low-temperature emitters, zoning
Comfort mainly comes from the control system. In a passive building, thermal mass and internal gains quickly raise the temperature. Favour low-temperature emitters, a stable heating curve and limited zoning. A reliable measurement in the living area avoids heating for nothing. For more on this, see our article on temperature control levers.
Proving performance in 2026: calculation methods and checks to plan
Energy audit and thermal study: who does what, and when
The energy audit assesses the existing building, compares work scenarios and estimates the gains. It's carried out by a qualified auditor, upstream of the quote. The thermal study, led by a design office, turns the chosen scenario into sizing calculations (insulation, ventilation, heat pump) and helps secure the thermal bridges, especially if you're aiming for a near-passive level. To scope the content, deliverables and budget, you can rely on a thermal study.
Blower-door test and checks: when to schedule them to avoid redoing work
Schedule an interim blower-door test as soon as airtightness is continuous, before closing up the linings. You correct issues at the right time, without breaking anything open. A final check, sometimes supplemented by thermal imaging, is done at handover with a report to keep on file. Good timing, fewer reworks.
Site file: photos, product data sheets, traceability of sensitive points
Build a simple, complete site file. Dated photos of the insulation before closing up, product data sheets (CE marking, BBA certificate where available), batch references and proof of thickness. Note the sensitive points (junctions, joinery, network penetrations) and keep the signed certificates the scheme requires.
Making the passive strategy pay off: subsidies, trades, and team organisation
ECO4 and the Great British Insulation Scheme in 2026: points to watch for high-performance retrofits
In 2026, secure the financing from the pre-project stage. Check eligibility (the route and its performance requirements), the requirement to use installers registered for the measure, and the documents to provide (assessment or supporting evidence, detailed quotes). What a measure attracts depends on the obligated supplier and on the measure specification. Get the sign-off before signing, and lock in the combination of routes to aim for a passive level without a cash-flow gap.
Coordinating trades: layout planning, schedule, interfaces
Profitability is decided at the interfaces. A clear insulation layout plan, a tight schedule, and a dedicated “airtightness” lead avoid rework. Anticipate the sensitive points: window/door surrounds, sills, network penetrations, vapour-barrier continuity. Get every trade to sign off against an up-to-date model or sketch.
Client pitch: comfort, bills, home value, without promising the impossible
Talk concrete results: winter and summer comfort, healthier air, less noise, and better-controlled bills. Give ranges, not miracles. The gains depend on usage, weather, and system settings. On value, stay factual: a leaner, better-rated home generally sells more easily. You reassure, you move forward.
