Ubat (heat loss coefficient)

The Ubat (heat loss coefficient) expresses the average thermal transmittance of a building's envelope (walls, roofs, floors, windows and doors), including thermal bridges, in W/m²·K. The lower the Ubat value, the less heat the building loses for every 1 Kelvin difference in temperature between the inside and outside; heating requirements decrease and wall comfort improves. This "envelope" indicator is independent of the heating and cooling systems (heat pump, boiler) and serves as a guide from the initial design phase to ensure a robust and energy-efficient design.

Ubat: operational definition and useful calculation

Ubat is a weighted surface average of the U-values ​​of the walls and the contributions of thermal bridges:

- we aggregate each wall with its U (W/m²·K) multiplied by its exchanged surface area;

- we add the linear bridges (Ψ) multiplied by their length and, if relevant, the point bridges (χ);

- we relate everything to the total exchange surface of the envelope.

In practical terms, lowering Ubat involves:

- Reinforce the insulation of opaque walls (thickness, conductivity, continuity),

- Choose high-performance windows and doors (low Uw) with suitable Sw/TL values

- Address thermal bridges (Ψ, χ): EWI, balcony thermal breaks, insulation returns in window and door frames, carefully designed sills/flashes,

- ensure airtight installation and details conforming to Technical Approvals.

A reduced Ubat lowers the Bbio and stabilizes indoor temperatures ; it then facilitates the achievement of energy/carbon objectives without oversizing systems.

Ubat project: advantages, limitations and points of attention

Interests

• Reduced heat loss → reduced heating needs and power requirements.
• Superior wall comfort: fewer cold surfaces and drafts.
• Robustness in the face of usage hazards (instructions, schedules) and energy prices.
• Leverage effect on other indicators (Bbio, DH via adequate sun protection).

Boundaries

• Costs and constraints of thickness, weight or interface (existing facades).
• Reduced solar gain if poorly chosen glazing (Sw too low on the South side).
• Theoretical gains if the execution (sealing, thermal bridges) is not maintained.

Points to consider

• Prioritize EWI to cut floor edges and lower Ψ.
• Joinery: aim for low Uw and consistent Sw by orientation (South/West ≠ North).
• Specific details: paintings, supports, acroteria, facade bases, fixings (χ).
• Coordination between architect/structure/HVAC: avoid late “construction bridges”.
• Quality control: leak testing, thermography, verification of thicknesses and continuity.
• Simulation (STD/SED): compare Ubat variants + solar protections to balance winter/summer.

Anecdote — “A tamed Ubat on the Butte Sainte-Anne (Nantes)”

In Nantes, a 1970s-era apartment building was struggling with cold walls and a boiler that was overloaded during heating cycles. The study focused on Ubat: external rock wool insulation on the facades, thermal breaks on the new balcony brackets, insulation returns in the window and door frames, and windows with a Uw value of approximately 1.2 on the north side. The following winter: reduced heating capacity, a sharp drop in complaints, and a heat pump sizing that became possible without additional cost. The building manager smiled: "We haven't changed the Nantes winter, we've changed the way it enters."