Rooftop insulation calculator
Calculate the insulation thickness needed to reach the thermal resistance required by the grants (R from 4.5 to 7 m²·K/W), compare ten insulants at equal performance, get the quantities to order and the estimated yearly heating savings. A roof loses around 30 % of a house heat, and R 6 means roughly 21 cm of glass wool, 23 cm of cellulose or 14 cm of polyurethane. The result is a private figure, worked out on your device.
- 100% local
- Free · no account
- Instant result
This tool runs entirely in your browser — nothing is sent to a server.
The thermal performance of a roof is expressed by its U value (W/m²·K): the lower it is, the less heat escapes. Adding an insulation layer always improves it, but with diminishing returns — going from U = 1.8 to U = 0.5 is worth far more than going from U = 0.4 to U = 0.33.
This calculator computes the U value of your roof, then that of the roof with a target layer added, and translates the difference into estimated annual savings (Belgium) or a CO₂ estimate (France). Targets are based on official benchmarks: RPE 2022 in Belgium, DPE requirements in France.
How it works
Type the figures or drop the file you want to process — the tool checks them as you go.
Everything is computed directly in your browser. Nothing is sent to a server.
The result appears instantly, with the detailed breakdown ready to copy, download or keep.
Why use this tool
No account, no download, no limit: the result appears as soon as you enter your data.
Every rule, algorithm and check reflects what we use ourselves every day at Flexina.
The calculations follow the official standards, and the step-by-step detail lets you double-check the outcome.
What does running it locally bring you?
Files, figures and documents never leave your device: nothing is stored, nothing is sent.
Once the page is loaded, you can cut the network: the tool keeps working.
No upload, no account, no third-party server in between: results stay fast and fully private.
Frequently asked questions
How much insulation should I add to a roof?
For a sloping roof, 20 cm of mineral wool typically brings the U value close to 0.4. Beyond 30 cm, the improvement becomes modest compared to the cost. The tool allows you to compare thicknesses before deciding.
Why does the calculation only apply to sloping roofs?
The formulas and air layers differ between flat roofs (which present condensation risks) and sloping roofs. For a flat roof, consult a professional: the subject is delicate.
How precise are the energy savings?
They are an order of magnitude: they depend on your actual heating, the indoor temperature, and the local climate. Use them to compare solutions, not to budget the bill to the euro.