Indicative detailing - The Hempcrete Topic: Designing and Building with Hemp-lime - page 547

ing of the hempcrete. These were presumably caused by the moisture passing very slowly
out through the board. Nor are these boards able to carry traditional lime plasters, due to
their very smooth surface, and this can significantly reduce finishing costs on a hempcrete
build. To our knowledge, at the time of writing magnesium silicate boards are all produced
in India and China and so may have a high carbon footprint owing to being transported
halfway around the globe. Very little information is available about exactly what goes into
the boards, the quality standards of the products, or working conditions in the factories.
Wood wool boards have a very open surface structure, similar to that of hempcrete. Be-
cause of this the hempcrete keys very well to the board, leaving no voids. However, the
open, rough surface of the wood wool board means that at least two skims or a basecoat
and a topcoat will be required to finish the board. An alkali-resistant plasterer's mesh is
usually incorporated into the basecoat to stop cracking at joins between boards. Some
wood wool boards have the advantage of being external grade and so can be used intern-
ally or externally to carry the render. At the end of the day, the deciding factor in choice of
carrier boards may be budget, with magnesium silicate board at the time of writing costing
approximately four times as much as the most expensive wood wool boards.
Diagonal timber bracing
This is the usual option for providing racking strength to the frame, and for those who
value the simplicity of a monolithic hempcrete wall, with finishes on both sides applied
directly to the surface of the hempcrete, it is probably the best. These bracing timbers usu-
ally consist of pieces of timber 50mm x 100mm or 50mm x 150mm in section, or 150mm-
wide strips of 25mm thick ply, screwed to one side of the frame diagonally. The size, type,
quantity and spacing of the timbers should be specified by a suitably competent person.
Diagonal timber bracing is sometimes used in conjunction with timber gussets: triangular
pieces of plywood fixed to the junctions of the studs with the sole or wall plates. They add
extra racking strength and also provide an excellent fix between the studs and the wall or
sole plate, as an alternative to angle brackets.