Study shows that hemp concrete reduces energy consumption in homes

Hemp concrete is a construction material developed in France since the early 1990s, obtained by mixing hemp particles (hemp), a binder, water and possible additives. It makes it possible to carry out eco-construction of buildings as a filling material for a load-bearing structure.

We now know that hemp concrete makes it possible to store carbon sustainably: a complex wood-infill hemp concrete structure 26 cm thick can store up to more than 35 kg of CO2 per square meter of wall installed. implemented over a lifetime of the enclosure (ie 50 years or even 100 years). This characteristic will be greatly enhanced in the years to come with the implementation of RE2020.

On the other hand, the thermal performance of hemp concrete is currently still underestimated. Indeed, the exchanges of water vapor contained in the air with the hemp concrete make it possible to dampen, very significantly and passively, the daily variations in temperature and hygrometry in a building. The consequence of these exchanges is to reduce energy consumption and greatly improve thermal and water comfort in homes. These internal phenomena of change of state of the water (vaporization and condensation) are not taken into account in the current regulatory calculation engine, the thickness of hemp concrete implemented to comply with RT 2012 is then greater than that effectively necessary to target a given energy consumption: this therefore leads to additional construction costs and, consequently, to a lower competitiveness of hemp concrete vis-à-vis other materials.

As part of the study carried out by Cerema, on behalf of Construire en Chanvre Ile de France, the simulations carried out using the WUFI® Plus software on a fictitious building of 100 m2 (ground floor and one floor) whose walls are insulated by 30 cm of hemp concrete, have shown that the coupled transfers of heat and humidity within the hemp concrete walls make it possible to reduce the need for energy by 20 kWh / m2 / year. heating of the building when it is well insulated elsewhere (slab, roof, etc.). Thus, the 30 cm of hemp concrete then behaves "energetically" like 22 cm of hemp wool. This can represent up to 70% of heating savings in the case of a building with high thermal performance.

On the other hand, if the building is less well insulated, the relative influence of the hemp concrete in the wall decreases because the “thermal losses” occur through the less well insulated elements.

The important point to remember from this exploratory study is that the hygrothermal transfers within the hemp concrete significantly modify the building's heating needs. However, although the physics of the phenomena involved at the scale of hemp concrete is now well known and mastered, it appears that their translation in terms of the energy savings generated constitutes a scientific obstacle that is rather difficult to remove. , in particular on the issue of cooling needs which remains unresolved at the end of this study.

Additional work using WUFI® Plus software and another software will be carried out in the coming months by Cerema to compare the results and broaden the hypotheses of the case studies.

Field monitoring-type readings will be carried out in particular to have data available to support these simulations.