Hemp and straw, the houses of tomorrow?

In the heart of the Saint-Jean district, in Clermont-Ferrand (Puy-de-Dôme), a new high school in straw will open its doors in September 2022. The 16,500 m school building2 consists of a wooden frame, filled with 17,000 bales of straw ordered from a farmer in Saint-Laure. To date, it is the largest “eco-constructed” public building project in France. And he created an attraction for these types of projects. In this same district, located near the CHU, the city announced the delivery for 2023 of a vast associative and sports complex of 4,279 m², its first positive energy equipment, based on the same wooden load-bearing structure. of straw bales.

In fact, recently, buildings with low carbon impact and high environmental performance are gaining ground, from Brittany to Occitania, via Loiret – which in 2021 opened two new straw colleges. Coralie Garcia, R&D project manager at the French straw construction network, is delighted to “The acceleration of eco-construction projects in this material – factories, social housing, nurseries or head offices – of medium size and now on a larger scale, such as this vast storage building for L’Oréal, extension of its Caudry plant in the North.

The State and local communities are on the front line, on the lookout for responsible consumption and production methods. A decree of December 2012, specifying the conditions for awarding the “Biobased building” label, defines this resource as “A material derived from plant or animal biomass which can be used as a raw material in construction products”, excluding “Materials of geological or fossil formation”. Wood, straw, hemp, reed, rapeseed, corn or sunflower residues therefore fall into this category and prove to be valuable assets for working towards carbon neutrality in construction.

It is still necessary to structure this stammering sector. A report, published in the spring of 2020 by the consultancy firm Carbone 4 and the French Institute for the performance of buildings (Ifpeb) thus questions the place of these materials in construction by 2030: “Are the sectors sufficiently mature and developed to respond to the expected trajectory? Will we be able to make room for these (not so new) materials in a coherent cost-carbon equation? “ However, according to Carbone 4, straw or hemp – still very marginal in construction – have intrinsic qualities and many advantages.

“At the scientific level, these materials have excellent thermal properties and water performance”, underlines Naima Belayachi Belaiche, teacher-researcher at the University of Orleans (Loiret). The latter has been testing, for a dozen years, in Polytech laboratories, the resistance of new insulating blocks based on plant aggregates (cereal straw, rapeseed, sunflower). In straw, the hygrothermal behavior promotes good management of moisture transfer through the walls, and ensures thermal comfort in each season. Cooler in summer and very waterproof in winter.

This comfort has effects on the wallet: for the user, the energy savings are reflected in the bills. In the HLM Jules-Ferry residence in Saint-Dié-des-Vosges (Vosges), tenants pay less than € 10 per month for heating. And that’s not all: renewable, recyclable and recoverable in essence, these plant materials also have the capacity to absorb CO2 from their growth phase and over their entire lifespan in the building. “These materials are potentially very interesting carbon sinks, that is to say they are partly composed of carbon atoms pumped into the atmosphere”, underlines Natan Leverrier, consultant of the “building” division at Carbone 4. Their carbon footprint is therefore excellent, compared to conventional construction materials.

Still, to take a real step forward, the question of volumes arises. How to produce on a large scale to meet the needs? The French straw construction network is reassuring: it believes that the building currently takes only a tiny part of the amount of wheat produced each year – the straw is also a plant residue. What makes Coralie Garcia say that “If we insulated all new buildings with straw, we would use 5 to 10% of existing agricultural production”.

Natan Leverrier is more careful. And warns against possible conflicts of use in the event of rapid development of the sector. The risk of these material productions encroaching on agricultural land “Should not be taken lightly”, according to the Carbon 4 consultant. In general, its deployment involves “To be very vigilant about the use of the soil”, he warns, adding: “There are physical limits and choices to be made. We will not be able to do everything. The construction and energy sectors (to produce biomethanes) will want to source biobased materials at the same time. It is therefore very important to think about the chaining of uses. “

On paper, this necessary arbitration turns to the advantage of straw and any other transformable agricultural residue, inexpensive and abundantly available throughout the territory. (read the benchmarks). Especially since the recycling of these resources at the time of the demolition of buildings is “Simple and virtuous”, assures Coralie Garcia. “Just put the straw back in a field”. A concrete application of the principle of the circular economy.


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