Raw earth construction, a low-carbon solution to extreme heat

Under the harsh August sun, workers collect the earth stored under a tarpaulin and introduce it into a machine from which it will emerge in the form of large solid blocks. Here, the municipality of Montanay, in the suburbs of Lyon, is renovating and expanding an old adobe farm to make it a media library.

And unlike the workers who sweat profusely, future readers will be protected from the heat.

“Thanks to the rammed earth technique, we are in a building which will itself guarantee freshness and interesting comfort,” explains Camille Announ, director of the Le Pisé company, which is carrying out the masonry for the work, to AFP. .

This "ancestral" technique, very widespread in Rhônes-Alpes until the beginning of the 20th century due to the composition of the soil, consists of building well-packed raw earth walls with successive layers of around ten centimeters in height, which will dry in the sun.

The thickness of the walls - around 50 cm - and the thermal inertia of the material allow a "buffering effect in the event of high heat", explains Camille Announ, so that it will be as cool in summer in an adobe building as in a stone house.

But “all raw earth constructions” benefit from another advantage: “hygrothermal regulation”, explains the mason. Concretely, raw earth walls, this "living material", have the power to capture excess humidity in winter and evaporate it in summer, and therefore to lower the temperature felt: "It's like air conditioning natural!”

Low carbon

Rammed earth in Rhône-Alpes, cob in Brittany, cob in the East or in Normandy: the French regions have an “important heritage” of raw earth construction, but “we are very little aware of it”, affirms Mathieu Lecaille, researcher at ENTPE, specialist in sustainable construction.

In Montanay for example, the land used for the media library was recovered from two construction sites located less than 3 km away. “Our raw material is super local, we can’t do better in terms of carbon footprint,” says Julie Benezet, collaborator of the Zeppelin architects firm in charge of the project.

Making rammed earth requires “only earth and water,” she adds, as well as a little electricity. The architect shows the block prefabrication machine: the earth is simply filtered from large stones, then compressed using an electric rammer, before the blocks are unmolded and placed in their final location, on a wooden base. concrete protecting against rising damp.

A process which does not require transformation at high temperature, which emits very much CO2, "unlike cement", underlines Mr. Lecaille, also praising the "indefinite" recyclability of rammed earth and the absence of waste.

“Change of times”

With the “awareness of a necessary ecological transition, we are at a change of era”: “there is a real demand which is exploding”, confirms Camille Announ.

At Le Pisé, it has been “increased tenfold since 2017”, mainly driven by public constructions, which the RE2020 law now requires not to exceed a certain threshold of CO2 emissions over their lifespan.

But rammed earth has certain limitations.

The height of buildings is generally limited to three levels. It is also a “more challenging” material than others, believes Julie Benezet, whose firm does not only work on rammed earth. You have to think well in advance of the construction site "for the joinery, the floors, make an opening", because "we cannot make holes everywhere like in concrete!"

The price can also be a barrier. Despite widely available and inexpensive raw materials, specialized labor and the duration of the project increase the investment cost.

Building with rammed earth "takes time", explains the architect: construction sites can only be done between April and October, and the earth can take several months to completely dry, depending on the weather. But building ecologically is “a game of patience” anyway.