On a a construction site for individual houses, the choice of structural material sets a significant portion of the budget even before discussing finishes. Comparing the price of a house made of polystyrene (insulating formwork blocks) to that of a concrete block construction requires looking beyond the simple cost per square meter: it is necessary to include insulation, labor, and current regulatory constraints.
Additional insulation cost of concrete blocks since RE2020
Since January 1, 2022, RE2020 imposes a thermal resistance for walls that is significantly higher than the previous RT2012. We are talking about a target around R ≈ 5 m²·K/W, compared to about 3.7 before. This tightening changes the game for concrete blocks.
A bare concrete block wall provides almost no insulation. To meet regulatory thresholds, an insulating layer must be added either from the inside (ITI) or from the outside (ITE), with thicker layers than before. This additional item leads to an estimated extra cost of between 5 and 10% compared to a RT2012 construction.
Comparing the price of a polystyrene house to the cost of an externally insulated concrete block yields a tighter result than one might imagine, because the formwork block already integrates insulation into its structure. The thickness increases from 12 cm to about 16 cm of expanded polystyrene in the wall to comply with RE2020, but this thickness is part of the block itself: no second trade dedicated to the installation of a lining.
With concrete blocks, each added layer multiplies the interfaces and the risks of thermal bridges at the level of floors, window frames, and corners. Treating thermal bridges often costs more than the insulation itself on a concrete block house compliant with RE2020.
Labor costs: formwork blocks versus traditional masonry
It is often said that polystyrene is cheaper in raw materials. This is true for the block alone, but the real leverage lies in the construction time.
Formwork blocks fit together by stacking, somewhat like a modular assembly system. An experienced mason builds the walls faster than with traditional concrete blocks because there are no mortar joints between each row. The concrete is poured in a single operation into the cells.
- A concrete block site requires a mason, a rebar worker, and then a plasterer or facade worker for insulation, which means at least two distinct interventions after the structural work.
- A formwork block site combines structure and insulation in a single phase, reducing the overall duration by several weeks on an average-sized house.
- The reduction in construction time also limits ancillary costs: equipment rental, water consumption, site supervision.
Feedback on this point varies by region and the availability of installers trained in formwork blocks. In areas where few companies master this technique, installation rates may rise and eat into the theoretical savings.
Mechanical resistance and durability: concrete block or insulating polystyrene
The concrete block has a strong argument: its compressive strength is known, standardized, and all engineering offices can calculate it with their eyes closed. It is a masonry material that has been proven for decades.
The insulating formwork block in polystyrene is not a foam wall. Once the concrete is poured inside, the load-bearing wall is made of reinforced concrete, not polystyrene. The EPS serves only as a permanent formwork and insulation. Therefore, the final mechanical resistance depends on the concrete and the reinforcement, just like for a traditional concrete wall.
The nuance lies in the surface impact resistance. The expanded polystyrene on the exterior cladding requires a reinforced coating (fiberglass mesh + undercoat) capable of withstanding impacts. Without this properly applied coating, the facade remains vulnerable to mechanical damage, which is not the case for a traditionally coated concrete block wall.
Fire behavior
Expanded polystyrene is classified Euroclass E without treatment, making it combustible. The formwork blocks used in construction incorporate a fire-retardant treatment and are protected by the exterior coating and the interior plasterboard. The concrete poured inside constitutes the real fire barrier, but the implementation must be flawless to avoid any direct exposure of the EPS in case of fire.
Comparative table: decisive criteria for choosing
| Criterion | Polystyrene house (formwork block) | Concrete block house |
|---|---|---|
| Integrated insulation | Yes, in the block | No, added insulation required |
| Construction duration | Shorter (one structure + insulation phase) | Longer (masonry then insulation) |
| RE2020 compliance | Facilitated by integrated thickness | Extra cost of 5 to 10% to meet thresholds |
| Mechanical resistance of the finished wall | Reinforced concrete (identical to a concrete wall) | Hollow concrete blocks + mortar |
| Availability of companies | Variable by region | Very broad, any qualified mason |
| Facade impact resistance | Reinforced coating essential | Classic coating sufficient |
Overall budget: where the real difference lies
The price per square meter often favors concrete blocks. Hollow concrete blocks can be found at a modest price in any building materials store. The formwork block in polystyrene costs more per unit.
The shift occurs when adding up all the items. For a house compliant with RE2020, the total cost of structure + insulation is comparable between the two systems, because the concrete block regains its material advantage by losing it on added insulation, additional labor, and thermal bridge treatment.
The choice often hinges on two non-financial factors: the local availability of companies trained in formwork blocks, and the preference of the project owner for a construction system they are familiar with. A poorly insulated concrete block will cost more in energy over twenty years than a properly implemented formwork block, but a formwork block installed by an inexperienced team can lead to costly defects that need to be rectified.