Understanding wall adjacency and its impact on the DPE
What adjacency covers: a wall facing outside, a heated volume, or an unheated space
In a DPE (diagnostic de performance énergétique — France's energy performance certificate, calculated with the 3CL method), adjacency describes what's on the other side of a wall. Three cases come up. A wall facing the exterior (air, ground, roof). A wall facing a heated volume, so very low losses. A wall facing an unheated space (garage, cellar, unconverted loft), with an in-between temperature.
Why adjacency changes heat loss: basic rules and common mistakes
Adjacency changes the temperature difference used in the calculation and the coefficients applied. Get it wrong, and the DPE can end up too pessimistic or too flattering. Common mistakes: an adjoining garage entered as "exterior." A cold stairwell counted as heated. A ventilated loft treated as a buffer space.
Real-world cases: loft, garage, stairwell, cellar, conservatory
- Loft. Converted and heated: a heated volume. Unconverted and ventilated: an unheated space.
- Garage or cellar. Usually unheated. Pay particular attention to the connecting door.
- Conservatory. No heat emitter and not very airtight. Usually close to exterior conditions.
Correctly identifying each type of adjacency on site
Wall facing outside: façades, roof, top floor and sensitive points
When a wall's adjacency is with outside air, losses add up fast. Check that insulation is continuous across the façades, the rafters or loft, and the top floor under the roof. Track down thermal bridges at wall-roof junctions, window reveals, balconies, and loft hatches. Good airtightness keeps the insulation from "working in the wind."
Wall facing inside: separations between heated volumes and unheated zones
Here, adjacency usually involves a garage, a cellar, a shared corridor, or an unheated loft. Treat these separations as a partial envelope. Insulate the wall, take care of doors and hatches, and limit air leaks. Think buffer zone to prioritise the coldest walls.
Wall on the ground and party wall: ground floors, separating walls, terraced housing
For ground adjacency, distinguish between a slab-on-grade, a crawl space, and a basement. Insulation must stay continuous around the perimeter, at the edges and thresholds. With a party wall, losses are usually lower, but the junctions stay sensitive. In terraced housing, gable walls need close attention at the sensitive points.
Adjacency and modelling: translating reality into the tool without getting it wrong
Choosing the right adjacency category in the model: a step-by-step method
For each wall, start from what's on site. What's on the other side, and is it heated? Exterior, ground, unheated space, crawl space, cold loft, party wall. Also note whether the volume is ventilated. This simple reading avoids overestimated or underestimated heat loss. If in doubt, take photos, cross-check against the plans, then decide room by room rather than "by feel."
Handling "hybrid" zones: partial basements, half-levels, extensions and successive renovations
Break the home down into consistent volumes. A partial basement often creates two adjacencies on the same floor. A recent extension may be better insulated and sometimes heated differently. Model these pieces separately, then check the junctions. That's where errors hide.
Consistency of the description: surface areas, orientations, thicknesses, insulation continuity
Before validating, run a quick check. Surface areas should match the plans. Orientations should follow the façade, not the angle of the room. Insulation thicknesses and materials should be continuous or clearly interrupted. Miss one detail, and a "perfect" adjacency stops compensating for it.
Avoiding the pitfalls that distort the DPE and the renovation proposal
Common mix-ups: party wall vs exterior, ground vs unheated space, interior vs crawl space
Errors usually come from the thermal boundary. A party wall is not a wall facing the exterior. A floor can sit on a slab-on-grade, over a cellar, or over a garage. And an "interior floor" can actually be adjacent to a ventilated crawl space. These mix-ups artificially inflate or shrink the heat loss figures.
Quick on-site checks: visual clues, simple measurements, useful photos
On site, look for simple clues. Ventilation grilles, hatches, basement windows, damp marks, clearance under the floor, insulation continuity. An infrared thermometer and a tape measure are usually enough to settle a doubt. Take 3 well-framed photos: access to the crawl space or cellar, the wall-floor junction, and the street-facing façade to confirm the party wall.
Consequences in 2026: DPE rating gaps, sizing, and justifying scenarios
In 2026, getting the "wall type" wrong can tip a DPE rating and complicate renting the property out. On the works side, the error shows up as an oversized heat pump sizing, overestimated insulation, or scenarios that are hard to justify. It's worth locking down these points before pricing and defending your options.
How Argile helps RGE tradespeople get adjacency right and speed up quoting
Fast energy diagnosis: structuring wall adjacency in a few minutes
On site or back at the office, you set out the framework of the home and Argile helps you describe the adjacency of each wall. Exterior, lofts, crawl space, unheated space, party walls. This clear reading avoids "by feel" surface areas and secures the heat-loss figures. You move forward with a more reliable base for insulation and systems.
AI and open-data feasibility analysis: spotting site constraints (party walls, building type, exposure)
Argile combines AI with public data to spot early on what often derails a quote. Likely building type, exposure, nearby obstructions, party walls, climate zone. You save time on checks and prepare a better-targeted technical visit.
Scenarios and grant-inclusive quotes: linking accurate modelling to pre-quotes and MaPrimeRénov'/CEE estimates
A properly set adjacency lets you compare consistent scenarios and price them without re-entering data. Argile generates pre-quotes, then calculates an estimate of MaPrimeRénov' and CEE grants under the rules in force. You present a clearer remaining cost, without promising the impossible (and limiting bad surprises thanks to common mistakes RGE tradespeople make with financial support).
Key figures
dedicated Ue method
Wall on ground
heat loss = 0
Party wall
Frequently asked questions
Treat it as an "unheated space" as soon as it's an enclosed adjoining volume (garage, cellar, unconverted loft), even a cold one, because its temperature sits somewhere in between. Switch to "exterior" only if the volume is heavily ventilated or open to the wind (a very draughty roof space, a leaky conservatory) or if the wall opens directly onto outside air. On site, check above all for a connecting door and the level of ventilation (vents, daylight under the door, lack of airtightness).
Louis Meneteau
CPO of Argile
