Hemp in construction - The Hempcrete Topic: Designing and Building with Hemp-lime - page 41

heating the building when it is first occupied, as the hempcrete continues to dry out. 1 In
other words, depending on the skills of the contractor, the energy saved by not pre-dry-
ing the panels in the factory might instead be expended on heating the finished cast-in-situ
building until the hempcrete has completely dried. This is a difficult question to settle, as
detailed comparisons between the energy consumption in the two processes are required,
and the type of binder or finishes used may have an effect on drying times. Further re-
search, with full lifecycle analysis (LCA - a technique used to assess the environmental
impacts associated with all stages of a product's life) of the processes involved, would be
of great benefit.
Other uses of hemp in construction
Hempcrete is not the only material made from hemp in the construction industry. Hemp
shiv is also used as a reinforcing and insulating addition to coarse lime plasters and
renders, and the bast fibres, from the bark of the stem, are used in the same way to create
fine finishing lime plasters. The bast fibres are also used to make natural quilt-type insula-
tion.
A few years ago, hemp fibres were being used in the manufacture of a strong breathable
hemp fibreboard for the construction industry, but at the time of writing no such board is
available from natural building suppliers in the UK.
Lime-hemp plasters
Over recent years several natural and heritage building material suppliers in the UK have
been supplying lime plasters that contain hemp shiv or bast fibres (see Resources ) . The
main benefit of the hemp is to add a reinforcing structure to the plaster, in the same way
that animal hair or wheat straw (and probably also hemp!) did when it was added to lime
plasters historically. Tests have shown that the addition of hemp fibres to lime plaster in-
creases its strength by more than 300 per cent. 2
The addition of hemp also gives the plaster an increased insulation value, as a result of
the air trapped in the hemp, although with suggested coverings of 20-50mm (depending on
the manufacturer, product and substrate), the thermal performance is a lot lower than in a
hempcrete wall. Since both the hemp and the lime within the plaster are vapour-permeable
materials, these plasters preserve the breathability of traditional solid-wall structures.