Building with hemp
Hemp has many uses within the construction sector;
The strong bast fibres of the plant can be used in lightweight hemp-fibre insulation, or mixed into
lime plaster to provide a strengthening element to help resist cracking. Hemp oil wood treatments
have recently become available, and hemp fibres and oil are now increasingly used in composite
products which may also have applications in the construction industry.
In its most widespread use in construction, the chopped woody hemp stem (‘shiv’ or ‘hurds’) is wet-
mixed with a lime binder to produce ‘hempcrete’; a medium density material which can be used to
create breathable walls, floor and roof insulation. The material is naturally fire, pest and rot resistant
without need for any chemical treatment, and gives an exceptional thermal and acoustic
performance as a result of hempcrete’s combination of insulation and thermal mass. Once the
surface of the hempcrete is plastered, exceptional airtightness values are achieved.
Although hempcrete has a very high tensile strength, it isn’t sufficiently strong in compression to be
load-bearing, and so is always cast around a structural frame – usually of timber. Hempcrete can be
wet-mixed and applied on site, or pre-cast into blocks and panels, which are then assembled on site
to create the building. The former gives better thermal performance since the whole thermal
envelope of the building is cast in one piece, but the later avoids waiting for the hempcrete to dry on
site.
Hempcrete has big benefits for the health of the building, its occupants and the planet. Its natural
breathability protects the building fabric from rot and decay, and reduces humidity condensation
and the growth of damp spores, viruses and bacteria inside the building. What’s more, hempcrete is
a better-than-zero-carbon material: more atmospheric CO2 is locked away in hempcrete for the
lifetime of the building than was emitted in its construction.
Hempcrete walls can take a range of finishes, as long as these remain “breathable” (vapour
permeable). Typical finishes include lime or clay plasters (applied directly to the face of the
hempcrete wall), timber cladding (with a breather membrane below this and vented air gap), or
masonry cladding using lime mortars. Paints for hempcrete walls should also be breathable; e.g. a
lime, clay or silicate paint.
Construction using hempcrete is suitable for both eco-friendly new build and renovation of historic
buildings, and has become widespread in Europe over the last 30 years, with an increasing number
of buildings now springing up across Canada and the USA, South Africa, Australia and New Zealand.
(© Alex Sparrow, Author of The Hempcrete Book; Designing and building with hemp-lime)
http://www.ukhempcrete.com/hempcrete-book