Indicative detailing - The Hempcrete Topic: Designing and Building with Hemp-lime - page 546

This is not to say that OSB can't be used within a hempcrete wall for other applications.
For example, it is used as the through-wall connecting element in a double-frame structure
(see Chapter 13, page 160 ) . The difference between this and using it as a racking board is
that when used as a racking board on one side of the hempcrete, the OSB forms a complete
barrier against the lateral flow of vapour, forcing any moisture to travel through it. When
used as the joining plate within a double frame, it is positioned in line with the flow of va-
pour and so does not form a barrier to it. Moisture will take the easiest route from one side
of the wall to the other, which will be through the far-more-permeable hempcrete rather
than the OSB.
Viable methods for providing additional racking strength to the softwood frame within a
hempcrete wall are: a vapour-permeable plaster carrier board, diagonal timber bracing, di-
agonal stainless-steel straps, or any combination of the three. These methods are discussed
below. It is important to stress that the total racking strength required is dependent on
many factors, including prevailing wind speeds, the loading of the building, the shape of
the building, whether there are structural internal walls, etc. The strength required, and the
means used to achieve it, should be calculated, given knowledge of the building design and
the declared racking strength of the materials to be used, by a suitably competent person
such as a structural engineer.
Vapour-permeable plaster carrier boards
The two options available at the time of writing are wood wool board and magnesium
silicate board, as described in Chapter 14 . Magnesium silicate board provides the higher
racking strength of the two, and, having a very smooth surface, it requires only a skim
of topcoat plaster to finish it. However, since this will usually be a gypsum skim it is not
the ideal finish for a breathable hempcrete wall. The smooth surface of these boards also
means they do not key to the hempcrete very well, and in our experience they seem to de-
laminate from the hempcrete as it dries. This is corroborated by a 2013 evaluation of the
Renewable House Programme, which notes that “a further factor reported by builders was
that the hemp and lime may have de-bonded from the Resistant [magnesium silicate] board
permanent shuttering”. 3
Magnesium silicate boards are also made up of a very dense material which, while sold
as vapour permeable, doesn't seem to have the same clear breathability characteristics in
use as more open-structured boards do. We have noted large patches of damp and dis-
colouration occurring on these boards, which remained for several weeks after the cast-