Bbio (Bioclimatic Need)

The Bbio (Bioclimatic Need) measures a building's intrinsic heating, cooling, and lighting requirements, independent of the systems used (heat pump, boiler, etc.). The lower the Bbio, the more energy-efficient the design: a continuous insulated envelope, well-managed solar gain, optimized natural light, useful thermal inertia/phase shift, and appropriate compactness. Under the RE2020 regulations, the project must have a Bbiomax value ≤ (a threshold that varies depending on climate, altitude, use, and surface area). The Bbio is complemented by energy indicators (Cep/Cep,nr) and summer comfort (DH).

Bbio: definition and design levers

The Bbio aggregates, according to a reference weather and conventional uses, the heating, cooling and lighting needs related to the architecture and the envelope: Ubat (insulation, Ψ/χ bridges), air tightness, orientation/compactness (S/V), glazing (Uw, g, TL), external solar protections, inertia/phase shift, daylight factor, near/far shading.

Typical levers include: continuous EWI, treatment of thermal bridges, selective glazing (g/TL) adjusted by facade, controlled adjustable sunshades (BMS), light colors on roofs/facades in very sunny areas, exposed slabs + night ventilation in commercial buildings, and plans promoting compactness without losing daytime quality.

Advantages, limitations and points of attention of Bbio

Interests

• Simplicity “through design”: fewer needs even before choosing the machines.
• Robustness in the face of energy prices and usage excesses.
• Improved comfort (warmer walls, controlled glare, better summer performance).
• Facilitates the achievement of the Cep/Cep,nr and DH targets.

Boundaries

• Conventional indicator: does not always reflect actual usage (times/density).
• May penalize certain architectural constraints (glass facades, heritage).
• Theoretical gains cancelled if the execution (sealing/Ψ) is not up to standard.

Points to consider

• Orientation & glazed ratio: higher g to the South (with external protections), lower g to the West; maximize TL without glare.
• Insulation continuity: aim for low Ubat, treat Ψ/χ, take care of frames/supports.
• Inertia & night-cooling: combining internal mass + night-time ventilation.
• Compactness/plan: limit S/V, share buffer zones (circulation, technical rooms).
• STD/SED simulation: compare variants (g/TL, BSO, inertia) and verify DH.
• Installation quality: airtightness, thermal breaks, tints/lights, cladding/plaster details.

Anecdote — “The Bbio that convinced Angers”

In Angers, a highly glazed office building was struggling to meet the Bbiomax standard. Rather than blindly thickening the insulation, the team rebalanced the facades: TL ~60% / g ~0.35 on the west side with motorized external blinds, a higher g on the south side under canopies, continuous external thermal insulation over slab bridges, and exposed concrete slabs with nighttime ventilation. Recalculation: Bbio compliant, DH reduced, while preserving the natural light that defined the project. The client summarized: "We didn't turn off the sun, we harmonized it with the building."