Understanding the Ubât and its role in controlling heat loss
Ubât: simple definition and differences from Uparoi, Ufen and thermal bridge Ψ
The Ubât (W/m².K) sums up the "average" transmission loss of the envelope. It aggregates walls, roofs, floors, glazing and the effect of thermal bridges. By contrast, Uparoi describes a specific wall or roof element, Ufen describes a window, and thermal bridges are handled instead with a linear coefficient (often noted ψ) linked to the junctions.
What the Ubât really tells you about the envelope: conduction, thermal bridges, joinery
The lower the Ubât, the less heat escapes through conduction. It also "sees" the details that can sink a job — window reveals, floor-wall junctions, internal load-bearing walls, and joinery quality. In other words, it's not just about insulation thickness — it's about overall consistency.
Why this indicator speaks to tradespeople: comfort, bills, system sizing
On the ground, a controlled Ubât improves comfort, limits cold surfaces and secures energy savings. It also helps avoid oversized systems, particularly for heat pumps, and makes it easy to argue your case to the client. It's a good indicator for prioritizing work items and framing a whole-house renovation.
How the Ubât is calculated: the envelope elements that weigh most
Opaque walls: walls, roofs, floors (thermal resistance and insulation continuity)
The Ubât is a weighted average of the U-coefficients of the walls/roofs/floors, relative to the heat-loss surfaces. Walls, loft, roof slopes and floors matter mainly through their surface area and their U-value, hence through the thermal resistance R of the insulation (U = 1/R). A discontinuity, an insulation junction or an uncontrolled air gap pushes up the Ubât, even with a good product on paper.
Glazed surfaces and air permeability: when airtightness changes everything
Windows carry double weight. Their Uw is higher than an insulated wall and there are often many of them. Add installation, shutter boxes, and seals. Air leaks do not change the regulatory Ubât, but they drive up actual heat loss. Careful airtightness work secures the result and comfort.
Thermal bridges: the impact of junctions (floor/wall, load-bearing walls, balconies) on the Ubât
Thermal bridges are added via the junctions, with Ψ values in W/m.K multiplied by lengths. Intermediate floor on external wall, internal load-bearing walls, balconies, window sills. A single poorly treated detail can cancel out part of the insulation gains. Thermal breaks and insulation returns are often the most cost-effective.
Ubât on site: diagnosing, prioritizing and avoiding work mistakes
Recording useful data during the visit: surfaces, materials, thicknesses, condition of the walls
On site, aim for measurements that are simple but accurate. Note the usable surface area per wall, the nature of the materials (brick, concrete block, stone, timber frame), the thickness of existing insulation, the condition (cracks, saltpeter, mold) and hard-to-reach areas. Take labeled photos and also note the ventilation system in place. Within Ubât, you gather this information to price the job without any gaps.
Prioritizing work on the envelope to reduce heat loss: roof, walls, floors, joinery
The rule on site stays the same. Cut the leaks first. Generally, start with the roof and loft, then the walls, ground floors, and finally the joinery, keeping continuous insulation and good airtightness. This sequencing avoids oversizing a heat pump and secures the announced performance.
Common pitfalls: insulating without treating thermal bridges, moisture risks and ventilation to adapt
Jobs that go wrong often follow the same scenario. Insulation installed, but thermal bridges forgotten. Result: cold surfaces, condensation, discomfort. Check the junctions (slab edges, window reveals) and adapt the ventilation. If you make the home more airtight, a properly adjusted mechanical ventilation system becomes non-negotiable to limit moisture and protect the insulation.
Ubât, subsidies and requirements in 2026: what to anticipate for your files
Ubât and technical consistency of scenarios: justifying the reduction in envelope heat loss
The Ubât serves as a through-line to show that the envelope loses less heat after the work. To avoid an "on-paper only" scenario, link each gain to a real action. Continuous insulation, thermal bridges, joinery, airtightness and ventilation should tell the same story. Keep a simple calculation note, with surfaces, thicknesses and resistances.
Link with MaPrimeRénov' and CEE: expected documents, consistency of assumptions and traceability
In 2026, files succeed or fail on consistency between the audit, the quote and the invoices. Prepare solid supporting documents. Product data sheets, certificates (ACERMI or equivalent), exact references, performance figures, dated photos, and CEE certificates. Align your assumptions. Same surfaces, same R-values, same work items, otherwise processing or inspection gets stuck. To limit back-and-forth, rely on best practices to avoid MaPrimeRénov' file rejections.
RGE and inspections: securing your choice of materials, thicknesses and installation
RGE is not just a label on the quote. Check that the qualification covers the specific work item, and that the materials installed are indeed the ones declared. On site, your thicknesses, continuity and critical points must be visible and traceable. Clean installation work means fewer client callbacks and less risk during an inspection.
Saving time on the Ubât with Argile: from diagnosis to quote, without losing rigor
Quick energy diagnosis: estimating the envelope's condition and comparing scenarios in a few minutes
You enter the home's key information, and Argile helps you estimate the condition of the envelope, including a first approach to the Ubât. In a few clicks, you compare consistent scenarios (insulation, ventilation, heating), with assumptions that are visible and easy to justify to the client.
Feasibility analysis: spotting technical constraints at the address (Open Data) to make your assumptions more reliable
Before the visit, Argile cross-references public data at the address to surface key constraints. Access, climate zone, risks, heritage context, building morphology. You arrive on site with a clear checklist, and you avoid quotes based on guesswork.
Quotes with integrated subsidies: pricing the work, integrating MaPrimeRénov'/CEE and producing a stronger file
You move from scenario to pricing, then to the quote, without juggling between files. Integrated subsidies (MaPrimeRénov' and CEE) are taken into account to present a clearer remaining cost to the client. The result: a better-framed file, easier to defend, and fewer administrative setbacks.
Key figures
1.5 to 2.5 W/m²·K
Ubât 1970s house
0.3 to 0.5 W/m²·K
Ubât RE2020 house
Ubât < 0.6
Renovation target
Frequently asked questions
In energy renovation, aim for the lowest possible Ubât, but above all check that each work item's performance reaches the subsidy thresholds (MaPrimeRénov', CEE). In practice, requirements often set minimum thermal resistances: R ≥ 3.7 m².K/W (walls), R ≥ 6 to 7 (loft/roof depending on configuration), and joinery with a Uw generally ≤ 1.3 to 1.6 W/m².K depending on the scheme. Have the exact values validated by an up-to-date subsidy simulator and the DPE/study, since thresholds change.
Louis Airy
COO of Argile
