Coefficient b weights the heat loss of a wall that faces not the outside but an unheated space, on a scale of 0 to 1. In the French 3CL-DPE 2021 method it is 1 for a crawl space, an unheated basement or a slab on ground, whose effect goes through the floor calculation. For a garage, an attic or a corridor it depends on the type of space, the ratio between wall areas and their insulation: from 0.25 to 1 for a garage depending on the configuration. Counting such a wall as though it gave directly onto the outside oversizes the heating output, often by several kilowatts on a house with an attached garage.
Understanding coefficient b and its direct link to heat loss
What coefficient b is for in your envelope and heating calculations
Coefficient b is a weighting factor that adjusts the heat loss of a wall when it does not face directly outside, but instead borders an adjacent space. It avoids oversizing heating capacity by capturing a simple reality. If the neighbouring space is milder than outdoor air, the "useful" loss is smaller.
Buffer space: how an "unheated" zone changes the heat loss you account for
A garage, a cellar, an unconverted attic or an airlock often act as a buffer space. In that case, only a fraction of the wall's losses is counted, via coefficient b (between 0 and 1). The more airtight and minimally insulated the buffer space is, the higher its temperature rises and the lower the calculated heat loss.
Common site mistakes: heated-volume boundaries, party walls and adjacent spaces
Mistakes mostly come from a poorly defined heated-volume boundary. A party wall shared with a heated dwelling is not treated the same way as a wall against a garage. Another pitfall: treating a wall onto an unheated stairwell as "outdoor," or forgetting the impact of doors and hatches. On site, clarify actual room-by-room use before fixing coefficient b. This classification is made wall by wall, according to each wall's adjacency: exterior, interior, ground or party wall.
Identifying and assessing the buffer space on site without getting it wrong
Common cases in single-family homes: garage, attic, conservatory, cellar and hallway
In a house, the buffer space is often an unheated volume between the inside and the outside. An attached garage, an unconverted attic, a "cold" conservatory, a cellar, a hallway leading to a workshop. On site, check whether the volume is genuinely unheated and how it "touches" the dwelling. This is what determines coefficient b in the heat-loss calculation.
Checklist for the technical site visit: ventilation, insulation, doors, continuity of airtightness
- Ventilation. Presence of vents, air inlets, frequently opened windows, occasional extraction.
- Insulation. Whether separating walls are insulated, continuity with rafters and floors.
- Doors. Door type, seals, threshold, door closer, routing of networks.
- Airtightness. Address ceiling-wall junctions, hatches, ducts, and "return" zones.
2026 points of attention: consistency between photos, site notes and expected supporting documents
In 2026, document checks come down to the details. Take clear photos of separating walls, access points and specific details. Cross-check them against your site notes (areas, thicknesses, opening direction) and the expected supporting documents (product references, performance data, invoice, proof of registration). A simple inconsistency can block the file.
Using a coefficient-b table to make your heat-loss figures reliable
How to read a table: buffer-space typology, walls concerned and conditions of application
A coefficient-b table is used to correct transmission losses when a wall does not face directly outside, but onto a buffer space (garage, unconverted attic, cellar, unheated hallway). You identify the type of buffer volume, then the wall concerned (wall, floor, ceiling), and apply the stated condition (unheated space, degree of opening to the outside, ventilation). The heat loss is then calculated by multiplying the "fully outdoor" loss by b, with b between 0 and 1.
Values of b in the 3CL method
For an unheated space, the 3CL reads b from three inputs:
- The type of space, which sets its heat-loss coefficient to the outside, Uv,ue. The more ventilated the space, the closer it is to the outside.
- The Aiu/Aue ratio: Aiu is the area of the walls between the dwelling and the space, Aue the area of the walls between the space and the outside. A small garage attached along a large wall has a high ratio and a low b.
- The insulation of the walls on the dwelling side and of the walls of the space.
Wall between dwelling and space uninsulated, space uninsulated:
| Type of space | Aiu/Aue ≤ 0.25 | 0.25 to 0.50 | 0.75 to 1 | 2 to 2.5 | 4 to 6 |
|---|---|---|---|---|---|
| Strongly ventilated attic | 0.95 | 0.90 | 0.80 | 0.65 | 0.40 |
| Garage, storeroom, weakly ventilated attic, other outbuildings | 0.90 | 0.80 | 0.70 | 0.45 | 0.25 |
| Very weakly ventilated attic, corridor opening directly outside, hall with automatic closer | 0.85 | 0.75 | 0.55 | 0.35 | 0.20 |
| Corridor with no direct opening to the outside | 0.80 | 0.65 | 0.50 | 0.30 | 0.15 |
Wall between dwelling and space insulated, space uninsulated:
| Type of space | Aiu/Aue ≤ 0.25 | 0.25 to 0.50 | 0.75 to 1 | 2 to 2.5 | 4 to 6 |
|---|---|---|---|---|---|
| Strongly ventilated attic | 1.00 | 1.00 | 0.95 | 0.90 | 0.85 |
| Garage, storeroom, weakly ventilated attic, other outbuildings | 1.00 | 0.95 | 0.95 | 0.85 | 0.70 |
| Very weakly ventilated attic, corridor opening directly outside, hall with automatic closer | 0.95 | 0.95 | 0.90 | 0.80 | 0.60 |
| Corridor with no direct opening to the outside | 0.95 | 0.95 | 0.85 | 0.75 | 0.55 |
Insulating the wall on the dwelling side raises b: the space, warmed less by the dwelling, moves closer to the outside temperature. Heat loss still falls, because the wall's U drops far more than b rises.
For a conservatory or an enclosed loggia, b depends on orientation: 0.85 to 0.95 facing north, 0.53 to 0.63 facing east or west, 0.48 to 0.58 facing south, depending on the climate zone and the insulation of the wall. The other cases are fixed: 1 for a crawl space, an unheated basement or a slab on ground, 0.95 for an inaccessible unheated space, 0.2 for premises used for something other than housing, 0 for a heated neighbouring dwelling.
Example: an uninsulated 15 m² wall (U = 2.5 W/m².K) separates the living room from an uninsulated garage whose outside walls total 50 m². Aiu/Aue is 0.30, so b = 0.80. The wall's heat loss is 15 × 2.5 × 0.80 = 30 W/K, against 37.5 W/K if it faced the outside.
Adapting the value to reality: partial insulation, interior doors, hatches and thermal bridges
On site, the buffer space is never "textbook." If a zone is only partially insulated, if an interior door lets air through, or if an attic hatch leaks, choose a more cautious, usually higher, value. You can also split the wall into zones (e.g. insulated garage wall, then uninsulated) to avoid smoothing over the defects. Don't forget thermal bridges, they still need to be treated separately.
Quick checks: verifying that the final heat loss stays consistent with the existing building
Three simple checks. 1. A wall against an unheated space should not end up "performing better" than an insulated wall against the outside. 2. If b sharply reduces the losses, check the buffer space's actual ventilation. 3. Compare the order of magnitude with the existing heating need and any comfort complaints. If it doesn't add up, the table is fine, but the category you chose isn't.
Saving time with Argile: coefficient b, heat loss and scenarios in a few clicks
Fast energy assessment: estimate performance and compare scenarios in under 5 minutes
You start from the address and a few field details. Argile estimates the heat loss, factors in coefficient b, then proposes several scenarios. Insulation, ventilation, heat pump. You quickly compare the gains and the order of magnitude of consumption, without redoing the calculations by hand, and the same heat loss calculation then comes out as a sizing note compliant with BS EN 12831-1.
Feasibility analysis: spotting constraints at the address (open data) and prioritising the work
Before pitching a project, you see what could get in the way. Climate zone, altitude, overheating risk, heritage constraints, access, available networks. The open-data analysis highlights points of attention and helps you choose a realistic work plan, in the right order.
Pre-costing and quotes: factoring in the funding while justifying your assumptions
Argile prepares pre-costing and a quote with the funding folded in. Your assumptions stay clear. Areas, insulation levels, equipment performance, settings. You save time on admin work, and you keep a consistent file to explain the project. To go further, here's how to make the most of the certificate schemes on your job sites.
Turning the calculation into a decision: proposing the right renovation to the right client
When coefficient b changes the strategy: insulation, joinery, ventilation, heating
Coefficient b adjusts the impact of a wall based on what's behind it. A wall against a garage or cellar doesn't "weigh" the same as a wall against the outside. As a result, you target the right item. Insulation when transmission dominates. Joinery when glazed areas skew the balance. Ventilation when fresh air and infiltration eat into the gains. Heating only once the envelope stops leaking.
Whole-house renovation: presenting clear gains and an understandable "before/after" heat loss
To win buy-in, show a simple before and after by item. Heat loss, kWh and comfort. A diagram that goes from a draughty house to a home that holds its heat. Then work out the funding with visible assumptions. The client understands what they're paying for, and why.
Securing your file: traceability of choices, a documented technical visit and consistent measurements
Keep clear evidence. Site notes, photos, plans, insulated areas, product references, outputs, and consistency between measurements and quotes. A well-documented technical visit limits back-and-forth, secures eligibility for incentives, and avoids surprises during installation.
