Craft and design for a hempcrete extension

With hempcrete you can build and restore. Here, the project was an extension, a meeting of old and new. The idea was to create an additional room using state-of-the-art eco-friendly materials and an eco-friendly approach.

Entreprise Durand Bâtiment 1/3
Interview with Fabien Pautrel on 17 May 2018: Weber Tradical®

Weber: The timber framework is the first thing that strikes you!

Fabien Pautrel: Yes, on this project, the framework stands out with its unusual thick timber sections playfully arranged in a contrasting chevron and cross design between closely spaced load-bearing posts.

Weber: What’s the configuration for this extension?

FP: It covers a floor area of 30 to 40 m², with a gable wall on one side belonging to a house made of granite near St Malo. Hempcrete was used for the floor slab.

The plinth is made of local stone retrieved from a demolition. It was laid using Tradical® formulated lime and the interior was lined with pumice stone masonry. The whole plinth was topped with a levelling course on which the timber structure rests.

Weber: And what about the insulation?

FP: The owner assigned the design to a main contractor specialised in thermally insulating eco-friendly materials. So naturally, Tradical® hempcrete became a part of the project. We manually cast the hempcrete between shuttering panels, wall by wall. We cast in 50-cm-high layers, from the ground floor to under the eaves.

The framework design is visible from the inside. The hempcrete was flush to begin with, but a 10 mm layer was scraped off after removal of the shuttering formwork while the hempcrete was still fresh. A lime and sand finish was then applied, flush with the timber work.

Woodwool was placed over the wood and hempcrete wall on the outside, and was cladded with timber.

Weber: How did you handle the conduit and pipework?

FP: The conduits run through the draining layer and are embedded in the hempcrete.

Weber: Can this extension be rated as high-end?

FP: No, in terms of size, but yes when it comes to the technical choices made. One thing’s for sure, all the different materials used for this project made progress more complicated, with extensive phasing.

It was a project spearheaded by high-quality materials and expert knowledge. The timber sections were fastened together using traditional mortise and tenon joints, and the pegs were made of wood too.

Project profile

  • Project owner: Private individual
  • Location: St Malo
  • General contractor: Durand Bâtiment – La Ville Joie, 35540 Le Tronchet, France |
  • Products: Tradical® Hempcrete
  • Wall application: Tradical®PF 70 + Chanvribat®.
    Tradical® PF 70 is a building lime complying with standard NF EN 459

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