Understanding your ground floor before choosing an insulation technique
Identifying the substrate: slab on grade, crawl space, basement, cellar
Before any insulation work, look at what's under your feet. A slab on grade is often insulated from above during a floor renovation. A crawl space or a cellar instead allows installation from underneath. In a basement, you can aim for continuity with the walls, to avoid thermal bridges.
Identifying site constraints: ceiling height, access, moisture, services
Measure the available height. A few centimeters of insulation can block a door or a step. Check access (hatch, crawl space, clutter), moisture condition (odors, efflorescence, condensation), and ventilation. Also identify the water, gas, drainage, and cable runs. They dictate the thickness, the fixing method, and the protection needed.
Clarifying the goal: thermal comfort, reduced heat loss, acoustics
Set your direction. For comfort, you want a less cold floor and no "icy" zones. For performance, you aim for continuous insulation with careful junctions. For acoustics, favor decoupled solutions and suitable underlays.
Insulating from below: the most common technique when you have access
Installation: panels under the slab, insulation between joists, fixing, and airtightness
When the ground floor is accessible, the insulation goes on the cold side. Under a slab, rigid panels are installed (often in 2 crossed layers) with anchors or rails, then the joints are treated for good continuity. On a timber floor, the insulation is placed between the joists with mechanical support, ideally supplemented by a continuous layer under the joists to limit thermal bridges. Airtightness mainly comes down to the connections: joints, hatches, service penetrations.
Points to watch: thermal bridges at the perimeter, vapor barrier, condensation risks
Losses hide at the perimeter: wall-floor connections, joist ends, insulation returns. On the water vapor side, don't install a membrane at random. Depending on the humidity of the crawl space or cellar, the wrong choice can trap water and create internal condensation. If in doubt, a hygrothermal check and adherence to technical approvals avoid bad surprises.
Practical cases: accessible crawl space, unheated basement, vaulted cellar
Accessible crawl space: rigid panels under the slab, sealed joints, ventilation preserved. Unheated basement: ceiling insulation with mineral wool, a protective facing if needed. Vaulted cellar: moisture-tolerant materials, reversible installation, and effective airing kept in place to let the walls breathe.
Insulating from above: the right option during a floor renovation
Installation: insulation + screed, floating floor, hydraulic underfloor heating
When you're redoing a floor, insulating from above is often a quick win. Rigid insulating panels are installed, then a screed or dry boards. A common alternative, a floating floor over an underlay, limits losses and improves comfort. With hydraulic underfloor heating, the insulation goes under the pipes, then a screed encases the whole assembly, to spread the heat without it escaping into the slab.
Constraints: raised floor level, door thresholds, stairs, allowable loads
The key point is the finished height. A few extra centimeters can block a door, shrink a step, or require redoing the skirting boards. Also think about loads. Screed, flooring, and furniture add up. In renovation, check the substrate's load-bearing capacity before adding thickness.
Details that make the difference: perimeter strips, decoupling, insulation continuity
A perimeter strip prevents cracks and acoustic transmission. Decoupling the floor from the walls helps the screed work without stress. Finally, hunt down thermal bridges. The insulation must stay continuous at partitions, hatches, and services to keep a floor that's genuinely comfortable.
Comparing the two insulation techniques: cost, performance, timelines, site impact
Actual performance: insulation continuity, thermal bridge treatment, everyday comfort
In practice, exterior insulation gives a more continuous envelope. It better addresses thermal bridges and preserves the walls' thermal mass, for more stable comfort in both summer and winter. Interior insulation is effective too, but requires more care at the floor connections, load-bearing partitions, and reveals, and can reduce living space.
Site organization: phasing, co-activity, disruption, returning rooms to use
On the site side, exterior work limits interventions inside the rooms. You often keep the dwelling occupied, with disruption mainly outside. Watch out for weather constraints and scaffolding. Interior work is managed room by room, but requires protection, moving furniture, removing radiators, and redoing service connections.
Budget: materials, labor, finishing rework, and contingencies
The overall budget is often lower for interior work on the pure insulation line item. But you need to factor in finishes, window reveal returns, linings, and contingencies related to moisture. Exterior work costs more (scaffolding, render, or cladding), but reduces certain interior rework. Plan for a 5 to 10% contingency margin.
Choosing the right insulation technique in 2026: rules, subsidies, and evidence to provide
Regulatory benchmarks and common requirements: target thermal resistance and supporting evidence
In practice, eligible insulation targets a minimum thermal resistance. Commonly required benchmarks: unconverted loft R ≥ 7, rafters R ≥ 6, walls R ≥ 3.7, ground floors R ≥ 3 (in m².K/W). On the quote and invoice, show the R, area, thickness, lambda, product reference, and if possible certification (ACERMI or equivalent).
2026 subsidies: MaPrimeRénov' and CEE, RGE conditions, documents to give the client
For MaPrimeRénov' and CEE certificates, the rule stays simple: the application is made before the work starts, and the company must be RGE-certified in the right scope (walls, lofts, floors). Give the client a clear file: a dated quote, a detailed invoice, the RGE mention, technical data sheets, and the CEE sworn statement to sign.
Checks and quality: photos, technical data sheets, insulation traceability, job-site sign-off
Audits do happen, especially on the CEE side. Secure your job with before, during, and after photos (tricky junctions, thickness), batch traceability (labels, batch numbers), and a signed sign-off report. Keep this evidence for at least the duration of the review period. Visible quality means smoother subsidies.
Key figures
€25 to €45/m²
Underside insulation price
3.0 to 4.0 m²·K/W
Target R
7 to 10%
Floor heat loss
Frequently asked questions
For subsidies, in practice target a thermal resistance R ≥ 3.0 m².K/W for a ground floor (a value commonly required depending on the operation data sheet). Keep the evidence: the product's technical data sheet (R, λ, thickness), a detailed invoice, and if needed a test report/technical approval. Get the exact eligibility validated before quoting, since criteria can change.
Louis Meneteau
CPO of Argile
