Complete heat-loss survey: methodology for tradespeople (2026)

Scoping the site before the heat-loss survey: gathering the right data

Recording building information: areas, volumes, orientation, walls and openings

Before the heat-loss survey, start from a simple baseline. Record the heated floor area, the ceiling height, the volumes, the orientation of the façades and any shading (trees, buildings). Describe the walls, floors and roof, noting material, thickness, existing insulation and condition. For openings, note the type of glazing, the frame, the shutters, and take photos. The goal: reliable data that you can compare.

Identifying usage and scenarios: heating, hot water, ventilation, setpoints

A good diagnosis isn't limited to the building fabric. List the systems in place (heat generator, emitters, controls), domestic hot water, ventilation, and the actual setpoints room by room. Gather at least 12 months of energy bills and note the occupancy pattern (number of occupants, daytime presence). This gives you a realistic usage scenario, without overestimating free gains.

Spotting weak points on site: thermal bridges, moisture, air infiltration

On site, look for what's "leaking" energy. Spot thermal bridges at junctions (floor slabs, load-bearing walls, balconies), signs of damp, condensation, mould, and air infiltration around openings, hatches and service penetrations. A smoke test or a thermal camera used under suitable conditions helps make this objective. These signals guide your on-site priorities.

Calculation methodology for a heat-loss survey: from survey to heat loss

Calculating heat loss by component: walls, roof, floors, glazing, air renewal

For a reliable heat-loss survey, start by recording the areas, thicknesses and nature of each wall, floor and roof component. Then calculate item by item. Keep the base simple. Measure everything. Transmission heat loss: Φ = U × A × ΔT. For air: Φ = 0.34 × Qv × ΔT. Add thermal bridges when they're significant, otherwise you'll underestimate the heating need.

Setting 2026 assumptions: thermal resistances, U-values, local climate conditions

In 2026, avoid rule-of-thumb guesses. Use R and U values taken from product data sheets or standardised calculations. Check the continuity of the insulation, the actual state of air gaps, and the joinery as installed. Consistent U-values. For climate data, use a base outdoor temperature and degree-days suited to the specific town, not the wider region.

Checking result consistency: orders of magnitude, cross-checks, common pitfalls

Check the order of magnitude. For a house, the total typically sits around a few kW at the base temperature. Cross-check against the DPE, energy bills and feedback from site. Common sense. Classic pitfalls: double-counting ventilation and infiltration, forgetting a sloped ceiling, overestimating settled insulation, or mixing internal and external dimensions.

Producing a complete heat-loss survey: tools, formats and evidence to keep

Choosing the right tool: spreadsheet, software, or support from a design office

For a useful heat-loss survey, choose the tool to match the objective. A spreadsheet is enough to compare variants and produce consistent quantities. Software becomes preferable as soon as you need to produce a report, estimate savings, or pre-size a heat pump. For a whole-house retrofit, support from a design office (bureau d'études) provides a robust framework and de-risks the calculations.

Structuring your file: photos, sketches, quantities, product sheets, calculation evidence

Bring everything together in a single file, dated and signed, with a PDF version.

• Before, during and after photos, labelled by room.
• Dimensioned sketches, areas, volumes, insulation thicknesses.
• Technical data sheets, technical approvals (avis techniques), RGE proof, calculation notes.

Documenting your assumptions: ventilation, airtightness, temperatures, intermittency

Write your assumptions down in black and white. This is what makes your results defensible in the event of an inspection or a client discussion. To settle on the temperature figures, you can refer to setpoint temperatures.

• Type of ventilation and the flow rates used.
• Infiltration level and airtightness treatments.
• Setpoint temperatures and occupancy periods.
• Heating intermittency and internal gains.

Turning the heat-loss survey into a works plan: priorities and sizing

Prioritising actions: envelope first, then systems (heat pump, boiler, mechanical ventilation)

The heat-loss survey acts as a map of where energy is lost. Start with the envelope: loft, walls, ground floor, airtightness. You reduce the heating demand before touching the equipment. Only then do you decide on the system: heat pump, boiler, then a mechanical ventilation system suited to managing humidity after insulation work.

Sizing accurately: heating output after works and safety margin

After the insulation work, redo the heat-loss calculation. This is what gives you the heating output you need. An oversized heat pump consumes more and wears out faster. Keep a reasonable margin for cold snaps, without "inflating" the unit. Also factor in hot water and the control mode.

Costing and phasing: work packages, site constraints, coordination with other trades

Translate the plan into work packages and stages. The aim is to avoid doing anything twice.

• Insulation and airtightness, then inspection and adjustments.
• Ventilation, before finishes, to run ductwork cleanly.
• Heating and electrics. Then commissioning and balancing.

Securing the compliance and value of the heat-loss survey in 2026

Combining a heat-loss survey and an audit énergétique: when one is enough, when the other is required

A heat-loss survey is used to understand where energy is lost and to size a solution, for example before installing a heat pump or insulating a single component. As soon as the project targets a whole-house retrofit, the sale of a highly energy-intensive home, or a grant that requires it, the audit énergétique (statutory energy audit) becomes the reference document. It follows a regulated method and proposes coherent work scenarios.

Anticipating grant requirements: MaPrimeRénov', CEE, and what inspections expect

For MaPrimeRénov' and CEE, compliance comes down to three points: RGE qualification, proof of performance, and traceability. Keep detailed quotes and invoices, product references (ACERMI, ETAS, COP), sizing notes, before-and-after photos, and sworn statements. During an inspection, a single missing document can hold up payment.

Explaining clearly to the client: reading the results, expected savings, limits of the calculation

Present the results as orders of magnitude and restate your assumptions. Savings depend on usage patterns, climate and settings. Give a range, not a promise. Mention the limits of the calculation (thermal bridges, infiltration, occupant behaviour) and what will be verified after the works.