Shuttering - The Hempcrete Topic: Designing and Building with Hemp-lime - page 303

frames are a different matter, and the speed of shuttering is also a convincing argument for
using a permanent shuttering board internally.
Permanent shuttering
As described in earlier chapters, permanent shuttering can replace temporary shuttering on
one side of the wall. It cannot be applied to both sides, since at least one side of the wall
must be left open for a number of weeks after casting to allow drying of the hempcrete (see
Chapter 16, page 215 ). Permanent shuttering has to be fixed to an exposed frame, flush
with the wall's face, and is generally used internally.
In order for the hempcrete wall to retain its vapour permeability, permanent shuttering
boards should, of course, be breathable themselves. These boards need to be able to carry
whatever finish is desired, and the more open-structured boards on the market actually
provide the ideal key for a lime plaster or render. It is unusual for permanent shuttering
to be used externally, since a render finish can be applied quite easily to a normal hemp-
crete wall, without the added risk associated with having the structural timbers so close to
the external face of the wall. At the time of writing, two choices of permanent shuttering
are available: wood wool board - made from compressed and bound wood fibres, which is
very breathable due to its open structure; and magnesium silicate board - a dense, smooth
board that resembles gypsum plasterboard, but is white in colour.
The open structure and the strength of wood wool board makes it an ideal surface to carry
lime and clay plasters. At the time of writing, several choices of wood wool board are
available on the UK market, for example Heraklith, Troldtekt and Anutone, which are
comparable products in terms of their physical properties. However, different brands may
vary in terms of the binders used, the process and site of production (usually in Europe)
and the level of certification provided with regard to the board's embodied energy, includ-
ing whether proper lifecycle analyses have been carried out.