Insulating your
ground floor.

The ground floor is the floor of the lowest heated level of the house. Underneath, there may be an unheated basement (cellar, garage, boiler room), a crawl space, an open passage — a porch, stilts —, or the ground itself: this is then called a slab-on-ground floor.

ADEME's guide Isoler sa maison breaks down the heat loss of an uninsulated house as follows: the roof 25 to 30%, the walls 20 to 25%, renewed air and leaks 20 to 25%, the windows 10 to 15%, ground floors 7 to 10%, thermal bridges 5 to 10%. Let us be frank: the ground floor is the smallest share of the table. If your loft is not insulated, that is where to start — we explain it in our guide to insulating unconverted lofts.

Three situations, however, move the ground floor up the list. When the roof and walls have already been dealt with, it becomes one of the last cold surfaces of the house. When an unheated basement runs under the whole living area, the surface in contact with the cold is as large as the house itself. And when the discomfort is first and foremost physical: icy tiles, cold feet, ground-floor rooms that can never be made pleasant. ADEME also lists the junction between the floor and the walls among the main thermal bridges of a house.

Before talking about insulation materials or thickness, you have to go down and look. The technique, the cost and even access to grants follow from this single answer.

An unheated basement, a cellar or a garage with full standing height. This is the most favourable case. The work is done from the underside, without touching the living rooms, and everything is visible: condition of the floor, services, bearings on the walls.

A crawl space. Everything depends on its height and access. A crawl space you can move around in allows insulation from below, in more constrained working conditions. If it is too low to work in properly, the job has to be done from above. In both cases, we check that it remains ventilated and dry.

An open passage — porch, projection on posts, house on stilts. The floor is exposed to the outside air: the insulation fixed to the underside must be protected from the weather, from impacts and sometimes from view, with a suitable facing.

A slab on ground. The slab rests directly on the ground: there is no space underneath, so only one route, from above. This means raising the floor level or removing the floor covering, or even the existing screed. It is a heavy job, justified mainly when the floors are being redone anyway, for example during a full renovation or a barn conversion into a home.

A timber floor on joists, common in older buildings, above a cellar or a former cowshed. The insulation can then be fitted between the joists, working from below or after lifting the floorboards.

ADEME distinguishes three ways of insulating a ground floor, the choice depending in particular on the height available between the floor and the ceiling below.

From below. The insulation is fixed to the underside of the floor. According to ADEME, this is the simplest technique. Flexible insulation is used on an irregular substrate, rigid boards on a flat surface. The insulation can be left bare or given a finish — boards or render — if it remains visible or needs protection, which is the rule in a garage or a frequently used room. The decisive advantage: nothing changes in the living rooms.

From above. The insulation is laid on the floor, then covered with a surface you can walk on — a floating screed or a timber floor. This is the solution for a slab on ground and an inaccessible crawl space. It has a cost that does not always appear on the quote: the floor rises by the thickness of the insulation and the covering. Doors then have to be trimmed, thresholds reworked, and the height of the first stair step and the remaining ceiling height checked.

Between structural members. On a joisted floor, the insulation fills the space between the timbers. The technique uses no height at all, but requires access from below or lifting the floorboards, and particular care so that the insulation stays in place and tightly fitted.

As for materials, ADEME's guide lists for floors both petrochemical insulation — expanded or extruded polystyrene, notably on slabs on ground and under screeds, polyurethane — and mineral wool, wood fibre, cork or cellulose wadding. The right choice depends on the technique, the humidity of the space and the room available. We only make it after seeing the substrate.

The performance of an insulation material is measured by its thermal resistance R, in m².K/W: the higher it is, the better it insulates. For a ground floor, the official energy savings certificate sheet (sheet BAR-EN-103, « Isolation d'un plancher ») requires a thermal resistance of at least 3 m².K/W for the insulation installed. An important point: insulation already in place is not taken into account in this calculation.

The thickness follows from the thermal conductivity of the insulation, written λ (lambda): thickness = R × λ. Common insulation materials have a λ between 0.025 and 0.05 W/m.K. To reach R = 3, you therefore need:

• about 7.5 cm of an insulation at λ = 0.025;
• about 9 cm at λ = 0.030;
• about 10.5 cm at λ = 0.035;
• about 12 cm at λ = 0.040;
• up to 15 cm for the least efficient materials, at λ = 0.05.

These orders of magnitude matter most when space is short: under the ceiling of a garage whose door already brushes the underside, or from above when every centimetre saved on the floor height avoids trimming a door. In these cases, a more efficient and therefore thinner insulation may cost more per square metre and still be the best choice. The exact thickness is read on the data sheet of the chosen product, not from an average.

1. Boards that do not touch the floor. ADEME is explicit: to avoid thermal bridges, the boards must be tightly butted, laid continuously and pressed against the floor. A ventilated air gap between the insulation and the slab cancels out much of the benefit.

2. Forgotten edges. The junction between the floor and the walls is one of the main thermal bridges in the house. The floor bearings and the corner walls must also be insulated, with a return of insulation along the basement walls. It is the line most often missing from a cheap quote.

3. Insulating a damp substrate. A seeping cellar or a poorly ventilated crawl space is dealt with before insulating, never after. When necessary, a vapour barrier or equivalent device protects the insulation from moisture transfer: the official sheet expressly provides for it. In the cellars of old stone houses, this damp diagnosis comes before everything else.

4. Pipes left on the cold side. Once the floor is insulated, the heat of the home no longer warms the basement or the crawl space, which become colder. The water pipes running through them must be protected from frost, or placed on the warm side of the insulation.

5. Insulation left unprotected. An insulation system, as defined by the official sheet, combines the insulation with fixing devices and protection against moisture, impacts or fire. In a garage or a room people walk through, bare insulation on the underside does not last.

6. Poorly reinstated services and light fittings. Conduits, junction boxes, light points and ducts fixed to the underside must be removed and refitted, or relocated. This work is planned in the quote, not at the moment the electrician discovers there is no longer access to anything.

Buildings in the two Savoies raise the issue in two ways. On the one hand, many detached houses stand on a semi-buried basement housing the garage, cellar and boiler room: a large floor area above an unheated space, accessible and often quite flat, the ideal case for insulation from below.

On the other hand, old farmhouses, barns and village houses stand on vaulted cellars or stone cowsheds, with timber floors or masonry vaults. There, damp is the first issue: a stone wall must be able to dry out, and you do not seal up a cellar without understanding how water moves through it. We go into these precautions in our guide to insulating walls from the inside, which devotes a whole section to older buildings.

For landlords, the ground floor is also one of the levers for gaining a DPE energy rating band, alongside the roof, the walls and the windows and doors — a concrete issue as the rental deadlines approach, detailed in our article on the F rating and the 2028 letting ban.

Energy savings certificates (CEE) are the first lever for a single measure. Sheet BAR-EN-103 applies to residential buildings existing for more than two years, for a ground floor located between a heated volume and an unheated basement, a crawl space or an open passage — slab-on-ground floors are not included. Its conditions: a thermal resistance of at least 3 m².K/W, a vapour barrier where necessary, installation by a professional holding an RGE quality mark, and a technical visit of the building no later than before the quote is drawn up, during which the professional also checks whether the existing insulation can be kept — otherwise, it is restored or removed. A minimum period of seven clear days must separate acceptance of the quote from the start of installation. The current version of the sheet itself provides for its repeal on 1^st May 2027: the conditions may therefore change.

VAT at 5.5% applies to work improving energy performance in a home completed more than two years ago. Since 1^st March 2025, a simple statement on the quote or invoice, by which the client certifies that the conditions are met, replaces the former certificate.

MaPrimeRénov' has changed its scope. The user guide published by Anah in September 2026 no longer includes insulation in the flat rates of the per-measure pathway: insulating only your ground floor no longer qualifies. On the other hand, the supported renovation pathway, which requires at least two insulation measures among walls, windows and joinery, floors or roof, and a minimum gain of two DPE rating bands, can include ground floor insulation in a wider renovation project. Details of rates and caps are in our MaPrimeRénov' 2026 guide for Savoie.

Our RGE CertiRénov certification (no. CR-2026-73-0095) gives access to these schemes for insulation work. Scales change from one year to the next: we check the conditions applicable at the time of your project, and we invite you to cross-check them on france-renov.gouv.fr. A France Rénov' adviser or an Accompagnateur Rénov', third parties independent of the company carrying out the work, can also confirm them for you.

You will not find a price per square metre here: no official source publishes one for this measure, and the same floor can be priced very differently depending on access, flatness of the substrate, services to be moved and the expected finish. A comparable quote, on the other hand, can be recognised by its content. The official sheet lists what the proof of completion must state; it is a good reading grid:

• the mention of a ground floor thermal insulation;
• the brand and reference of the insulation, its thickness and the area installed;
• its thermal resistance R, assessed according to current standards;
• the necessary arrangements, including the vapour barrier where required;
• the date of the prior technical visit.

Add to that what the sheet does not say but the job demands: treatment of the edges and bearings, protection or facing of the insulation, reinstatement of services and light fittings, and, when working from above, the doors, thresholds and steps to be reworked. A quote showing only a price per square metre lets you neither compare nor check your eligibility for grants.