Placing hempcrete - The Hempcrete Topic: Designing and Building with Hemp-lime - page 365

The more important factor to consider, from a builder's point of view, is how much drying
needs to happen before finishes can be applied. The surface of hempcrete quite quickly
forms a dry crust, and there is often scheduling pressure to apply finishes as soon as the
wall 'looks' dry. This is especially the case with external renders, stemming from a wish
to take down scaffolding at the earliest opportunity to reduce costs.
Although finishes for hempcrete are always breathable coatings, and must remain so in or-
der for the wall to work properly, they do slow down the drying of water out of the wall
once applied. This is because the finish covers up the open structure of the bare hempcrete,
reducing ventilation to the surface. In the case of plasters and renders, the open structure
of the hempcrete surface is closed up by the finish, reducing the overall surface area from
which the water can evaporate. A cladding finish with a well-ventilated cavity allows faster
drying of the hempcrete than does a standard render finish.
It is important that finishes are not applied too early, for two reasons:
First, because the drying out process can be slowed significantly when there is still a
very high water content in the wall, there is an increased risk of the hemp shiv rotting,
although in reality this is probably a fairly unlikely outcome.
Second, and much more likely, is the risk of staining on the plaster or render finishes
(see Chapter 5, page 74 ) . The reason for this is that tannins present in the hemp shiv
can be carried through in water droplets and deposited on the surface of the render as
the water in the wall evaporates. These unsightly stains can be painted out when the
wall has dried sufficiently, but if the reason for rendering early was to reduce scaffold-
ing costs, you will be disappointed to find that it is even more expensive to have scaf-
folding reinstalled so the walls can be painted again.