Placing hempcrete - The Hempcrete Topic: Designing and Building with Hemp-lime - page 349

The finished material should be closer to the 'low density' end of the continuum - achiev-
ing a high enough density that it will support its own weight, resist damage from things
knocking against it, and carry the render and plaster finishes, while remaining as low dens-
ity as possible in order to maximize insulation and minimize material cost.
The density of the finished material is partly defined by the amount of binder in the
mix, which varies depending on the application, for example floors, walls or roof insula-
tion, with different ratios specified to achieve different densities for specific applications.
Another factor that can affect the density is the amount of water: as described in the previ-
ous chapter, excess moisture in the mix can increase the density of the finished cast mater-
ial.
Assuming the correct ratio of binder to hemp is used, and the mixing technique is good and
hasn't allowed excess water to enter the mix, then the only thing that can affect the density
of the finished material is placing technique.
Apart from density, the other important quality of the finished material is the open structure
of the surface of the hempcrete wall. The flat surface formed by the shuttering board is
covered with small pockets or openings formed by the individual interlocking pieces of
hemp shiv in the mix. This open texture is useful for three reasons: it provides the ideal
key for lime renders and plasters, it improves the breathability of the finished wall, and it
speeds up the drying of the freshly cast hempcrete by connecting the outside air with the
many small air-filled channels inside the cast material.
The degree to which the surface is open (lots of relatively large holes) or closed (fewer
and smaller holes) depends on how much the hempcrete that lies against the shuttering is
compacted during the placement process. More compaction at the edges means a less open
structure.
The significance of this is that there is a balance to be struck: a very open surface, while
desirable for quicker drying times and better breathability of the finished wall, will also
swallow up a lot of basecoat when you come to apply the finishes. This can significantly
increase the material and labour costs in the application of finishes.
In general, it is probably best to err on the side of the surface being more open than closed,
since the benefits of shorter drying times and good breathability probably outweigh the ex-
tra expense in the finishes. However, it is important to keep this balance in mind when per-
fecting your technique, and to demonstrate to all members of the placing team what level
of compaction is required at the edges, and explain the thinking behind it.