Hemp block

What is a hemp block?

The hemp block is a non-loadbearing thin-joint masonry unit. It is made of hemp shiv, the chips from the woody core of the hemp stem, bound with air lime and hydraulic lime. It is laid like a brick, except that the insulation is distributed through its mass rather than added to the wall.

The hemp shiv is supplied by La Chanvrière de l'Aube, in France, and the lime comes from Saint-Astier. The mix is moulded, pressed, then cured and dried in the open air, without firing or added heat. This very low-energy process counts in favour of the product's carbon footprint.

Its density, around 340 kg per cubic metre, gives it useful thermal mass. It contributes to summer comfort by damping outdoor temperature swings, which lightweight insulation does not offer.

Vapour-permeable, it regulates indoor humidity and allows an old stone or rammed earth wall to be lined from the inside without sealing it in, the main difficulty in renovating traditional buildings.

It replaces the calcium silicate block where an insulating wall is wanted without an added lining, the insulating wall then being built in a single operation.

Installation & uses

The blocks are laid as thin-joint masonry, with 3 mm joints and a lime-based adhesive mortar supplied by IsoHemp. They are cut with a saw and remain light enough to handle without lifting equipment. To line an existing wall, ties or angle brackets provide the mechanical connection.

IsoHemp offers seven build-up configurations, covering internal or external lining, timber frame infill, partitions in housing or offices, and new build; depending on the thickness chosen, the wall can reach low-energy, very-low-energy and passive standards. The manufacturer nonetheless offers a self-supporting system, Syst'HEMP Pro, in which P and U blocks take a reinforced concrete insert that makes the wall build-up self-supporting.

As for finishes, the hemp-lime block can take a clay or lime plaster, or the manufacturer's PCS plaster. Outside, the finish is either a render applied directly to the masonry, or timber or brick cladding with a 2 cm air gap.

A few precautions govern installation. The masonry must be protected from rising damp and laid at least 20 cm above ground level outside, and it is built at temperatures between 5 and 30 °C, and not in rain or frost.