The Ubât expresses the average transmission loss of the whole envelope, in W/m²·K, aggregating walls, roof, floors, glazing and thermal bridges against the heat-loss surface area. An unrenovated 1970s house sits between 1.5 and 2.5, a new build between 0.3 and 0.5, and the usual target for a deep retrofit is to get under 0.6. The Ubât is what tells you whether the envelope holds together before a generator is sized, since a high value mechanically leads to an oversized heat pump and a degraded seasonal COP.
Understanding the Ubât and its role in controlling heat loss
Ubât: simple definition and differences from Uparoi, Ufen and thermal bridge Ψ
Four indicators sit side by side in a study, and they do not describe the same thing.
| Indicator | What it describes |
|---|---|
| Ubât (W/m².K) | The "average" transmission loss of the envelope, aggregating walls, roofs, floors, glazing and the effect of thermal bridges |
| Uparoi | A specific wall or roof element |
| Ufen | A window |
| Linear coefficient (often noted ψ) | The junctions, handled separately from the elements |
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.
What gets recorded element by element
- 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, mould) and hard-to-reach areas.
- Labelled photos and 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.
The order of the work items
- The roof and the loft.
- The walls.
- The ground floors.
- The joinery, last.
- As a through-line, 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
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 the funding: expected documents, consistency of assumptions and traceability
In 2026, files succeed or fail on consistency between the assessment, the quote and the invoices. Prepare solid supporting documents.
- Product data sheets.
- Certificates (BBA or equivalent).
- Exact references and performance figures.
- Dated photos.
- The declarations the scheme requires.
- Same surfaces, same U-values, same work items from one document to the next, otherwise processing or inspection gets stuck.
To limit back-and-forth, rely on best practices to avoid file rejections.
Registration and inspections: securing your choice of materials, thicknesses and installation
Registration is not just a line on the quote. Check that it 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 databases at the address to reconstruct the building and 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 funding: pricing the work, folding in what the measures attract and producing a stronger file
You move from scenario to pricing, then to the quote, without juggling between files. The funding is 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.
