Expanded polystyrene
or extruded?

Expanded polystyrene, or EPS, begins as small beads that steam inflates before they are fused in a mould. The resulting board is very light, easy to cut and inexpensive. It comes in white and grey: the grey, known as graphite, contains particles that slow radiation and improve its insulating power.

Extruded polystyrene, or XPS, comes out continuously from a die, like a paste pushed through an opening. Its cells are closed and tightly packed: the foam is far more resistant to crushing, and water hardly penetrates it. It can be recognised by its smooth skin and by its colour, which varies by manufacturer.

Each has its own standard: NF EN 13163 for expanded, EN 13164 for extruded. But a family does not tell the whole story: it is the data sheet of the product named on your quote that counts, with its ACERMI certificate where it has one. Our guide Thermal insulation: which one to choose? places both polystyrenes among the ten insulation families.

Rather than unverifiable averages, we compared two specific products: a white expanded board intended for façades under render, and an extruded board intended for flat roofs, floors and foundations. All the values below come from their data sheets.

What the table shows. On pure insulation, the gap is slight: to reach R = 3.7, the level targeted for a wall, you need 140 mm of this white expanded product and slightly less of this extruded one. The real differences lie elsewhere: the extruded product withstands seven and a half times more load before being crushed by 10%, it absorbs very little water and its data sheet declares its frost resistance. These three properties are what justify its uses.

The case of the grey. On one of our 2026 quotes, a 130 mm graphite expanded product reaches R = 4.20 m²·K/W, which corresponds to a conductivity of about 0.031. With the white expanded product in the table, you need 160 mm for the same resistance. On a façade, those 3 cm matter for window sills and roof overhangs.

On a façade, under render: expanded. This is its main territory. The boards are bonded and plugged to the wall, then covered with a render reinforced with a mesh, strengthened at the base of the façade against impacts. These systems are installed in accordance with the Technical Assessment (Avis Technique) or the Technical Application Document of the render manufacturer. The data sheet of the extruded product we consulted does not list direct render among its applications, only ventilated façades. For the full budget, see our guide to the price of external insulation per m².

Under a screed: expanded, chosen for this use. The floor board is laid under the screed in accordance with standard NF DTU 52.10, and its data sheet states its compressibility class. The one we consulted, classed SC2 a2, is limited to lightly loaded rooms, without floor drains, excluding level-access showers. For a ground floor, our guides aim for R ≥ 3: 120 mm of this board gives R = 3.15. The details of screeds and their price are in our guide to the price of a screed per m².

Under a slab or a floor over a crawl space. In our quotes for new houses and extensions, expanded polystyrene is placed under the structural topping of a beam-and-block floor. Our guide to the price of a concrete slab per m² distinguishes this floor from a ground-bearing slab, and our guide to insulating your ground floor explains where to place the insulation depending on the existing floor.

In contact with soil and water: extruded. Plinths, buried walls, beneath foundations, inverted flat roofs where the insulation stays exposed to water under its protection: this is where its low absorption, frost resistance and crushing resistance make the difference. Its data sheet lists foundations, subfloors and inverted roofs among its applications. Polyurethane, which insulates even better at equal thickness, will have its own guide in this series.

Fire. Both polystyrenes we consulted are classed E: they burn. They are therefore never left exposed: the render protects them on a façade, the screed on a floor. The façade board we consulted is fire-retardant and its data sheet approves it for individual houses, that is, dwellings of the 1^st and 2^nd families. For taller buildings, it refers to the ETICS-EPS recommendations guide of September 2020, which governs the fire protection of these façades. On some of our quotes, the render is in fact specified over a mixed substrate, polystyrene and rock wool.

Water vapour. With a μ factor of 30 to 70 for the expanded product we consulted and 150 for the extruded one, polystyrene strongly restricts vapour. That is an asset under a slab or against a buried wall. It is a flaw on an old stone, rammed-earth or rubble wall, which must keep drying out: there we fit a vapour-open insulation instead, such as wood fibre.

Summer heat. Polystyrene is a light foam, which stores little heat before letting it through. That is not a problem on a façade or under a floor, but under a south-facing roof, we do not choose it for summer comfort. The extruded product's data sheet gives a specific heat of 1,450 J/(kg·K), but neither data sheet gives a density: we therefore publish no heat-storage figure for them.

The environment. Both come from petrochemicals. The data sheet of the expanded product we consulted declares, according to its verified environmental data sheet, 6.14 kg of CO₂ per m² at 140 mm over its whole life cycle; the extruded product's data sheet gives no value. To compare insulation materials on this criterion, the public INIES database collects the environmental data sheets of products.

We recorded every line of our 2024-2026 quotes in Savoie and Haute-Savoie that mentions polystyrene. The result: expanded everywhere, extruded nowhere. Polystyrene is never sold on its own there: it forms part of a complete piece of work, and it is the price of that work that makes sense.

The insulation's share. On a 2026 quote for a house in the Aix area, the 130 mm graphite expanded boards, bonded and plugged, account for €93 excl. VAT per m², and the sprayed render on mesh for €67. For the screeds, two pairs of quotes repeat the same specification at the same price: we count them separately, but in practice the range rests on four different specifications.

These prices are exclusive of VAT. VAT depends on the project: 5.5% for the insulation of a home completed more than two years ago, subject to conditions, 10 or 20% in other cases. Our guide to VAT on renovation work sets out the rules.

Façade of a house in sound masonry: expanded under render, white or grey depending on the thickness available. Floor under a screed or floor over a crawl space: floor-grade expanded, whose class matches the load of the room. Plinth, buried wall, inverted roof or heavily loaded floor: extruded, for its resistance to water and crushing. Old wall that must dry out, or a roof exposed in summer: neither.

In every case, it is our salaried façade renderers, plasterboard fitters and masons who fit the insulation themselves and the work that protects it, whether render, screed or slab. One and the same team therefore handles the board layout, the plugs, the mesh at the base of the façade and the upstands around windows and doors.