Hemp in construction - The Hempcrete Topic: Designing and Building with Hemp-lime - page 20

time, similar standards for the nature and quality of the product can be agreed for the UK
industry.
The processing of hemp shiv for use as a building aggregate (once all the leaves, seeds and
bast fibres have been removed) involves breaking it up into small pieces and removing any
remaining fibre and dust. Hemp shiv for building should be as dry and clean as possible,
with a minimum of fines (small pieces of bast fibre) and dust present.
Loose hemp shiv against a new hempcrete wall.
The length of the pieces should be between about 10mm and 25mm, but this is not abso-
lutely critical: successful walls have been made with shiv that contains shorter pieces (es-
pecially when spray-applying, see page 29 ), but it is generally acknowledged that pieces
of this length produce a good matrix structure within the wall, which is beneficial for its
thermal performance (a material's 'success' in conserving heat and power in a building)
and vapour permeability (the degree to which a material allows water vapour through it).
Walls are also regularly built with hemp shiv that contains a certain amount of fines, al-
though the proportion must be low, otherwise the fines can soak up too much water and
potentially affect the setting of the binder. The absence of dust from the shiv is far more
important, since excessive dust can have an even more significant impact on the structural
integrity of the wall - in extreme cases leading to collapse (see Chapter 5, page 76 ) . This
is because the dust soaks up a very high proportion of the water added at the mixing stage,